Recycled Smelter Slags for In Situ and Ex Situ Water and Wastewater Treatment—Current Knowledge and Opportunities

Abstract

Slags from the ferrous and nonferrous metallurgical industries have been used to treat toxic contaminants in water and wastewater. Using slag as a recycling or renewable resource rather than a waste product has environmental and economic benefits. Recycled smelter slags can be used in both in situ and ex situ treatment. However, their application has some limitations. One of the challenges is how to handle spent slag adsorbents, as they contain the accumulation of solid waste loaded with high concentrations of toxic contaminants. These challenges can be overcome by regeneration, recycling, reuse, and immobilization treatment of spent slag adsorbents. The present paper explored the scientific and technical information about the composition, reaction mechanisms, adsorption capacity, and opportunities of recycled slags while adsorbing toxic compounds from contaminated water. It comprehensively reviewed the current state of the art for using smelting slags as sustainable adsorbents for water and wastewater. The study revealed that ferrous slags are more effective in removing a wide range of toxic chemicals than nonferrous smelter slags. It investigated the necessary improved approach through the 5Rs (i.e., reduce, reuse, recycle, remove, and recover) using smelter slags as reactive materials in ex situ and in situ treatment.

1. Introduction

Slag, a byproduct of metallurgical smelting processes, is a mixture of metal oxides, silicon dioxide, and different compounds [1,2,3,4,5]. It is produced from raw ore combustion or the extraction of specific minerals from smelting materials [1,5]. Huge quantities of slags (1868.8 million tonnes of global steel production in 2019–2020) are produced around the world, and their disposal into nature in an unplanned way or without proper treatment causes severe contamination [4]. Iron and steel slags, nickel smelter slags, metallurgical slags, copper slags, lead and zinc-containing slags, granulated phosphorus slags, vanadium slags, salt slags (a waste material generated by secondary aluminium processing), waste incineration slags, etc. are commonly produced as waste products by different types of industries located in different parts of the world [1,5,6]. Their management and handling are very challenging tasks, as these waste products sometimes release toxic heavy metals and other toxic substances. Due to the rapid growth of industrialization, urbanization, and infrastructure development, the availability of free land for solid waste disposal is decreasing [5]. The disposal and handling costs increase dramatically [6,7]. It is also important to note that any landfill is a major source of soil, groundwater, and water contamination, which can have a major impact on human and ecosystem health [5,6,7]. In recent years, engineers and scientists have been looking for ways to avoid the risk of slag handling and land disposal activities [8,9]. Slags’ recycling and sustainable use in different activities have been appealing to many of them. The options can also eliminate the disposal cost, limit the damage to the environment, make it easier to get resources back and enhance the opportunity for resource recovery [4,8].

Most slag particles contain significant amounts of iron oxide compounds, which can adsorb heavy metals from different water media [3,5,10,11]. The major sources of these toxic metals’ release (namely, lead (Pb), copper (Cu), cadmium (Cd), zinc (Zn), arsenic (As), and nickel (Ni)) are from different industrial activities such as mining, ore smelting, painting, leather tanning, pesticide, and pharmaceutical manufacturing industries, etc. [3,11,12,13]. Over the last century, the release of heavy metals into the environment has increased to a level that exceeds their natural capacity, thus accelerating surface water and subsurface contamination [11,13,14,15]. Moreover, the major health risks from these metals are their different degrees of toxicity, persistence or inability to decay, and less reactive affinity to geo-composition or biological organisms in the environment, which can cause many diseases in human bodies [13,14]. According to the United States Environmental Protection Agency (USEPA) and the World Health Organization (WHO), many metals are well known as “Type A” carcinogens. These toxicants enter the human body by ingestion, inhalation, and skin contact. Their presence in certain concentrations can cause cancer and other diseases, such as hemochromatosis and brass chills [13,14]. To reduce metal-related water and wastewater contamination, slag can be utilized as an adsorbent since slag particles have a high affinity for heavy metals [7,10,11,12,13,16,17]. According to Chowdhury [11], Zhou and Haynes [18]; and Feng et al. [19]; the composition of minerals (e.g., TiO₂, Al₂O_3, Fe₂O₃, FeO, Fe₃O₄, BaO, MgO, CaO, MnO, P₂O₅, SiO₂, etc.) in slag materials has a high adsorption capacity for different contaminants from aqueous solutions. Thus, the justification for investigating the recycled smelter slag as an adsorbent for the removal of heavy metals and other contaminants (e.g., phosphorus (P), nitrogen-NH₄, trichloroethylene (TCE), and dichloroethylene (DCE)) from water and wastewater is very rational and practical. Table 1 shows the review articles concerning slag materials in water and wastewater treatment published in the years 2014–2022. The research in the manuscripts (listed in Table 1) included information about the production, characterization, useful applications, performance, circular economy prospects, problems, and opportunities, as well as the sustainability of different smelter slags. However, to date, no study has evaluated the comparison of both ferrous and nonferrous smelter slags’ performance in removing toxic contaminants at different capacities. According to Holappa et al. [4], Manchisi et al. [5], Chowdhury and Yanful [7], Mehmood et al. [10], and Gisi et al. [13], recycled smelter slags, which are inexpensive and have improved treatment sustainability, are particularly attractive for use as low-cost adsorbents.

Slags with both amorphous and crystalline phases have a higher adsorptive capacity for various ions from water solutions [13,20,21,22,23]. It is obvious that physical (e.g., internal pore volume, porosity, and internal surface area), chemical composition, and phase chemical properties, as well as chemical and geometric heterogeneity, control the performance of slag-based adsorbents [5,13]. Slag surface heterogeneity has a substantial impact on the equilibrium and kinetics of adsorption. Heterogeneous slag materials have multiple adsorption sites wherein the targeted compounds can adsorb, with each site having a different rate of adsorption capacity. In addition, the crystalline phases of slags have a significant impact on adsorption capacity [5,22]. The physical and chemical properties of smelter slags are affected by the degree of crystallization achieved during the solidification process. According to Manchisi et al. [5], adsorption takes place both inside the crystal lattice and the crystal space of slag materials.

Around 450 million tons of these smelter slags are produced every year in different parts of the world [24,25,26]. Most of them are disposed of in landfills, with the remainder handled in an unplanned way [4,11]. Despite the fact that smelter waste products are not universally considered waste in different parts of the world, the annual data (as shown in the study) highlight the significant environmental concerns from improper slag management and final disposal. Thus, proper utilization of smelter slag in different engineering applications can reduce the negative impact. The purpose of this study was to investigate the scientific and technological advantages (such as composition, reaction processes, and opportunities with sustainable and circular aspects) of recycled slags when adsorbing hazardous contaminants from various waste streams. The field-scale application and the gaps for implementing in situ treatment have not been conducted comprehensively. To date, there is no manuscript that has comprehensively reviewed the current state of the art for using smelting slags as sustainable adsorbents for water and wastewater, as well as the necessary approach and crucial challenges for using smelter slags as reactive materials in treatment processes. The present study reviewed ferrous and nonferrous smelter slags’ applications in contaminated water treatment. It investigated the necessary improved approach through the 5Rs (i.e., reduce, reuse, recycle, remove, and recover) using smelter slags as reactive materials in ex situ and in situ treatment. It covers the treatment of slags for heavy metals, phosphorus (P), nitrogen-NH₄⁺, chlorinated solvents such as trichloroethylene (TCE), and dichloroethylene (DCE), toxic mining effluent, etc., and also discusses the cost of reactive slags and field application.

Table 1. Review articles concerning slag materials in water and wastewater treatment published in the years 2014–2022.

Number	References	Manuscripts	Year
1	Holappa et al. [4]	A review of circular economy prospects for stainless steel making slags	2021
2	Manchisi, et al. [5]	Ironmaking and steelmaking slags as sustainable adsorbents for industrial eluents and wastewater treatment: a critical review of properties, performance, challenges and opportunities.	2020
3	Mehmood et al. [21]	Mno and P2O5 removal mechanisms of slag against potentially toxic elements in soil and plants for sustainable agriculture development: a critical review. sustainability	2021
4	Gisi et al. [13]	Characteristics and adsorption capacities of low-cost sorbents for wastewater treatment: a review	2016
5	Yu et al. [27]	An overview of resource utilization of steel slag as absorbent material for wastewater treatment.	2020
6	Matinde et al. [16]	Mining and metallurgical wastes: a review of recycling and re-use practices.	2018
7	Chowdhury and Yanful [7]	Application of recycling waste products for ex situ and in situ water treatment methods	2020
8	Phiri et al. [8]	The potential for copper slag waste as a resource for a circular economy	2021
9	Gabasiane et al. [28]	Environmental and socioeconomic impact of copper slag—a review	2021
10	Ping et al. [29]	The removal of phosphate and ammonia nitrogen from wastewater using steel slag	2015
11	Reddy et al. [30]	Critical review of applications of iron and steel slags for carbon sequestration and environmental remediation	2019

Table 1. Review articles concerning slag materials in water and wastewater treatment published in the years 2014–2022.

Number	References	Manuscripts	Year
1	Holappa et al. [4]	A review of circular economy prospects for stainless steel making slags	2021
2	Manchisi, et al. [5]	Ironmaking and steelmaking slags as sustainable adsorbents for industrial eluents and wastewater treatment: a critical review of properties, performance, challenges and opportunities.	2020
3	Mehmood et al. [21]	Mno and P2O5 removal mechanisms of slag against potentially toxic elements in soil and plants for sustainable agriculture development: a critical review. sustainability	2021
4	Gisi et al. [13]	Characteristics and adsorption capacities of low-cost sorbents for wastewater treatment: a review	2016
5	Yu et al. [27]	An overview of resource utilization of steel slag as absorbent material for wastewater treatment.	2020
6	Matinde et al. [16]	Mining and metallurgical wastes: a review of recycling and re-use practices.	2018
7	Chowdhury and Yanful [7]	Application of recycling waste products for ex situ and in situ water treatment methods	2020
8	Phiri et al. [8]	The potential for copper slag waste as a resource for a circular economy	2021
9	Gabasiane et al. [28]	Environmental and socioeconomic impact of copper slag—a review	2021
10	Ping et al. [29]	The removal of phosphate and ammonia nitrogen from wastewater using steel slag	2015
11	Reddy et al. [30]	Critical review of applications of iron and steel slags for carbon sequestration and environmental remediation	2019

2. Various Recycling Smelter Slags

Smelter slags are produced by different industrial activities. Although all types of slag are not effective and nontoxic materials, most ferrous and nonferrous slag materials used in the previous studies were found to be effective engineering materials and reactive adsorbents for different treatment methods. Two types of slag are very common. These are slags from the ferrous metallurgical industry and slags from the nonferrous metallurgical industry [5,24]. As discarded waste products, the world produces approximately 320 to 384 million tons of iron and steel slags, 15 million tons of nickel smelter slags, 15 million tons of copper slags, and 19 million tons of ferromanganese slags each year [6,25,26]. The chemical composition consisting of TiO₂, Fe₃O₃, FeO, Fe₃O₄, BaO, MgO, CaO, Al₂O₃, MnO, and SiO₂, etc. is present on/in the layers of slags produced in various industrial operations [5,8,9,27]. The major composition of the smelter slags was also investigated and confirmed by different surface analysis techniques, such as XRF, XRD, XPS, and Raman analyses [2,25,26]. These minerals or chemical compositions are extremely reactive, and they can be used in a variety of technical applications.

Table 2. Types of slags and their applications.

Types of Slags	Major Compositions, % (wt/wt)	Engineering Applications	Reactivity	References
Blast Furnace (BF) Slags	FeO (35–42%), CaO (35–40%), SiO2 (8–9%); MgO (8–15%)	Used as aggregate, ballast, and lightweight; building materials; glass raw materials, mineral wools, lime fertilizer, and soil stabilization/conditioner, in the construction of reactive barriers, and water and wastewater treatment	Affinity to heavy metals, Phosphorous etc.	Zhao et al. [31]; Zhou et al. [32]; Reuter et al. [1]; Reddy et al. [30];
Steel slags	FeO (2–8%), CaO (40–60%), MgO (3–10%), MnO (1–8%), Al2O3 (15–29%)	Used as construction materials, pavement materials, and engineering materials in the construction of reactive barriers, filter beds, and various purposes of landfilling, etc.	Cr(VI), Pb, Cd and TCE, DCE	Jones [6]; Reddy et al. [30]; Conno et al. [22]; Zhao et al. [31]
Stainless steel slags	CaO (42%), SiO2 (28.3%), CrO, FeO (3%) etc.	Used in in situ Brownfield remediation, metal recovery, road and hydraulic construction, reactive materials in the permeable barriers, wastewater treatment, etc.	Volatile organic contaminants (VOCs), Cr and Ni	Reuter et al. [1]; Capobianco et al. [33]; Zhang and Hong [34]
Ferroalloy slags	SiO2 (>40%), MgO (>8%) and Al2O3 (>10%)	Used as aggregate, cement mixtures, armour stones, landfilling activities, road construction material, etc.	toxic metals (e.g., Cr(VI), Pb, Cd and As)	Reuter et al. [1]
Copper slags	FeO (40%), SiO2 (32%), CaO (3.95%), MgO(2.82%), and Al2O3 (2.5%)	Used in treating metal-contaminated water and wastewater, supplementary Cementitious materials	Cr(VI), Pb, P, and As	Kiyak, et al. [35]; Jones, [6]; Zhao et al. [31]
Lead- and zinc-containing slags	CaO, SiO2, FeO, ZnO	Used in the metal recovery industry, supplementary Cementitious materials	--	Jones [6]; Zhao et al. [31]
Granulated phosphorus slag	CaO (40–50%), FeO (2.5%), SiO2 (30–40%), MgO (5%), and Al2O3 (2.5%)	Used as phosphorous fertilizer, and lightweight building material	Nutrients	Reuter et al. [1]
Waste incineration slag	SiO2 (57%), Al2O3 (10.8%), CaO (12%) and NaO (6.1%)	Used in road construction, concrete aggregate, or cement production	Heavy metals	Reuter et al. [1]; Czop, and Piekarczyk [36]
Silicate Iron Slags	SiO2 and FeO	Used as fertilizer production	Heavy metals	Mehmood et al. [10]
Nickel smelter slags	FeO (58%), SiO2 (40%), Al2O3 (2.5%), CaO (2%), MnO (1.5%)	Investigated for in situ ex situ environmental treatment, water treatment, etc.	Heavy metals	Addai [37]; Chowdhury and Yanful [7]

lists the many types of slag and their uses.

2.1. Slags from Ferrous Metallurgical Industry

Iron and steel slags are obtained during the pig iron and crude steel production processes [5,17,37]. Slags from the ferrous metallurgical industry can be classified into four major types: blast furnace (BF) slag, basic oxygen furnace (BOF) steelmaking slag, secondary metallurgical process (SMP) slags, and electrical arc furnace slag (EAFS) [1]. Figure 1 shows a general process diagram for ferrous slag production. BF slag is produced at higher temperatures (1450 to 1550 °C) during the reduction processes of iron ore, coke, limestone, and other materials. Steel slags are created when scrap metals are melted in an electric arc furnace (EAF) to make liquid steel [5]. Moreover, steel slags are also formed as a byproduct of steelmaking from iron with other chemical and mineral compositions during smelting and combustion processes [1]. In Figure 1, the slag production rate in a basic oxygen furnace (BOF) or an electric arc furnace (EAF) varies depending on the quality of raw materials used, ranging from 150–200 kg per ton of steel in BOF and 130–180 kg per ton of steel in EAF [5,38]. Steelmaking slags are mostly composed of CaO, MgO, SiO_2, and FeO [38,39]. About 600 million tonnes of slag from the steel industry are generated globally, of which BFS accounts for 58.6% and steelworks for 42.4% [40].

About 50% of the slag was utilized in the plant for sintering and iron-making recycling, and the rest was used directly for road projects [12]. In China, more than 300 million tons of deposited steel slag from BOF have accumulated, occupying farmlands and polluting groundwater and soil [12]. Moreover, ferroalloy slag is another sort of byproduct from the manufacturing of FeMn, SiMn, and FeCr. Ferroalloy slag can be generated by a smelting process using submerged electric furnaces and also blast furnaces or converters. Disposal of these slags requires a large landfill area, so it pollutes the atmosphere, underground water, and soil [1]. Moreover, steel slags (containing a significant quantity of chromium and a certain amount of nickel) can release a noticeable amount of Cr as high as 1 ppm [6]. So, improving the use rate of slags and using an alternative waste management option can be an important way for the ferrous metallurgical industry to achieve sustainable development.

Ameri et al. [41] used steel slag in road construction as a substitute for virgin aggregates in cold mix recycling asphalt pavement preparation [as shown in Table 2]. Steel slag ground into fine powder can be used as cement additives, concrete admixtures, and as a potential material for ceramic production [12]. Steel slag with a porous structure and a large surface area is easy to separate from water due to its high density. Therefore, the application of steel slag in industrial wastewater treatment has received intensive attention in recent years. In developed countries such as Germany, the USA, France, and Japan, slag is used to produce siliceous fertilizer, phosphorus fertilizer, and micronutrient fertilizer [10]. In addition, steel slags contain CaO, SiO₂, and MgO, which are considered fertilizer components. In China, the steel slag fertilizer program started in 2011 to manufacture fertilizer components from discarded waste products such as steel smelter slags [12].

2.2. Slags from Nonferrous Metallurgical Industry

Copper slag is a smelter waste product produced by the nonferrous metallurgical industry during matte smelting and conversion during pyrometallurgical processes [1,35]. Globally, approximately 14 million tonnes of copper slag are produced [6]. The major chemical compositions of copper slag are FeO, SiO₂, CaO, MgO, and Al₂O₃, etc. [35]. On the other hand, the lead (Pb) smelting slags are a mixture of four components, e.g., CaO, SiO₂, FeO, and ZnO. Pyrometallurgical extraction of lead–zinc complex ore through a smelting furnace system generates similar to zinc and lead-containing slags [1].

Phosphorus slag is produced as a waste product during the extraction of elemental phosphorus, which consists mainly of CaO and SiO₂ [1]. The amount of slag produced during the extraction process is rather high. There could be approximately 7.5 tons of slag generated per tonne of phosphorus extraction [12]. Slags containing phosphorus can be utilized as phosphorous fertilizer, depending on the amount of CaO, P, and other minerals present in the slag. Slag is also used to produce siliceous fertilizer and phosphorus fertilizer [10,12]. Waste incineration slags are produced from the total oxidation process of the combustible materials present in municipal and industrial wastes. The incinerator residues can be utilized as road construction materials, concrete aggregate, or cement materials [1]. In addition, high-quality stones can be manufactured from municipal solid waste (MSW) incinerating residuals.

Nickel smelter slags, a mixture of different iron oxides, iron silicate, aluminium, magnesium, calcium, and nickel sulfide or oxides can be used as an adsorbent during in situ ex situ environmental remediation [2]. More than 50% of different iron oxides were found in nickel smelter slag, indicating a strong affinity for toxic metals in any aqueous solution [7,11]. Most furnace slags from copper/nickel/cobalt smelters consist of 0.04–1.2% Ni, 0.21–0.7% Co, and 0.6–3.7% Cu, and could leach out the elements into the soil and water environment during treatment or other applications [11,41,42,43]. Figure 2 shows a general process diagram for nonferrous (copper, nickel, lead–zinc, and phosphorous) slag production.

Copper, nickel, and lead–zinc slags are formed during roasting and smelting processes. Additional secondary furnace processing is required for phosphorous slags. The annual production of nickel, lead, and zinc slags in the USA is about one million tonnes, whereas copper and phosphorous slags’ production is estimated to be around four million tonnes [7,8,12,35].

Table 2 shows the types of slag, their compositions, and their applications. It is clear that ferrous and nonferrous smelter slags can be used as aggregate, cement mixtures, supplementary cementitious materials, building materials in the construction industry, landfilling activities, armour stones, fertilizers in the agricultural industry, soil stabilization/conditioning, and reactive materials in water and wastewater treatment, etc. Most smelter slags contain FeO, CaO, MgO, Al₂O_3, and SiO₂ in different amounts based on their sources and types (as shown in Table 2).

3. Different Toxic Contaminants’ Removal

Most heavy metals (Pb, Cu, Zn, Cd, Cr), metalloids (arsenic (As)), excess nutrients, and chlorinated compounds such as trichloroethylene (TCE) and dichloroethylene (DCE) can be removed to different extents by ferrous and nonferrous slag materials.

Table 3. The applications of different slags for water and wastewater remediation.

Contaminants	Reactive Slag		Study	Removal	Major	Removal	References
	Adsorbents		Methods
	Nonferrous Slags	Ferrous Slags		Mechanisms	Controlling Factors	Efficiency (mg/g)
As (III) and As(V)	Nickel smelter Cu smelter slags;	Steel Slags; Blast furnace slag (BFS),	Batch and adsorbing column study;	Electrostatic attraction and oxidation-reduction processes;	The solution pH;	0.8 to 13.7 mg/g	Borrayo et al. [24]; and Chowdhury and Yanful, [7]; Turk et al. [44]; Nguyen et al. [45]
					adsorbate and adsorbent ratio; contact time, reaction rate
Cr(VI) and Cr(III)	Copper smelter	Blast Furnace slag, ferrochrome slag,	Batch and adsorbing column study;	Treatment mechanisms, such as oxidation and precipitation;	Acidic pH, Fe dissolution, slag dose, mixing speed, contact time	0.16–55 mg/g	Kiyak et al. [35]; Park et al. [46]; Mercado-Borrayo et al. [47]
Pb species	-	Steel slag, Granular blast furnace slag (BFS); Electric arc furnace slag (EAFS);	Batch and adsorbing column study	- Removal methods, such as adsorption, co-precipitation, hydroxide precipitation as hydroxide, sulfide, and ion exchange, etc.	The solution pH, the presence of other ligands, contact time, slag particle size, flow velocity, etc.	0.07–8 mg/g	Dimitrova, [48]; Nilforoushan and Otroj [49]; Huy et al. [50]; Mercado-Borrayo et al. [47]
Zn species	Ni smelter slags;	Electric arc furnace slag (EAFS), Blast furnace slag	Batch	- Removal mechanisms, such as precipitation, ion exchange,	Solution pH; adsorbate and adsorbent ratio; contact time etc.	0.5–9.5 mg/g	Nguyen et al. [3]; Addai, [37], Chen et al. [51]
			experiment
Cd species	Ni smelter slags;	Steel slags, Blast furnace slag; Activated modified steel slag	Batch	- Removal methods, such as precipitation, ion exchange,	Solution pH; adsorbate and adsorbent ratio; contact time etc.	0.21–3.47 mg/g	Addai, [37]; Yu et al. [27]; Nguyen et al. [3]; Mercado-Borrayo et al. [47]
			experiment
Cu species	Ni smelter slags;	Steel slags, Blast furnace slag	Batch	- Removal mechanisms, such as precipitation, ion exchange,	Solution pH; adsorbate and adsorbent ratio; contact time etc.	3.8–300 mg/g	Kim et al. [52]; Addai, [37], Nguyen et al. [3]
			experiment
Nitrogen compound		Steel slags	Batch	Adsorption methods such as physicochemical adsorption and precipitation	Particle size and dosage of slag,	10.29 mg/g	Ping et al. [2]
			experiment		solution pH and contact time
Phosphorus compound	Copper	Steel slag, Cooled basic oxygen furnace slag	Batch study	- Removal mechanisms such as chemical adsorption and precipitation	Particle size and dosage of slag,	0.26 to 3.24 mg/g	Park et al. [53]; Letshwenyo and Sima, [54]
	smelter slag;				solution pH and contact time
TCE,		Steel slag; Basic Oxygen Furnace (BOF) slag; Stainless steel slags	Batch	-	The pH and contact time	0.4 mg/g	Smith, [55]; Tsai et al. [56];
DCE			experiment	- Removal methods such as chemical adsorption and precipitation
AMD		Basic Oxygen Furnace (BOF) slag; Stainless steel slags	Batch study	- Treatment mechanisms such as physicochemical adsorption and precipitation	Slag doses, the pH, and contact time	12.5 mg/g of Fe;	McCarthy, [57]; Feng et al. [58]; and Name and Sheridan, [59]
						20 mg/g of sulfate;
						16.21 mg/g of Cu and 32.26 mg/g of Pb

Notes: AMD: Acid Mine drainage; TCE: Trichloroethylene; DCE: Dichloroethylene.

demonstrates the applications of ferrous and nonferrous slags for water and wastewater remediation. It also shows different removal mechanisms, major controlling factors, and removal efficiencies of smelter slag. Major removal mechanisms are adsorption, precipitation, ion exchange, oxidation, and reduction processes, etc.

3.1. Arsenic Removal from Water and Wastewater

Arsenic (As) contaminated wastewater is produced by industrial production processes in metallurgy, agriculture, forestry, electronics, pharmaceuticals, glass, and the ceramics industry. Arsenic can also be released from natural or other anthropogenic sources [7]. Arsenic is mostly consumed by humans through water and food. Long-term and excessive exposure has been linked to skin, lung, kidney, and bladder cancers [60]. Different studies reported As removal from water and wastewater by recycling smelter slags in different environmental conditions. According to Borrayo et al. [24], the maximum removal of As by steel slags was found to be 1.99 mg/g. The slags can be used in well-mixed batch reactors to perform an adsorption method for As removal from contaminated water and wastewater [7,24]. Nickel smelter slags can also be used to remove As from aqueous solutions. The study conducted by Chowdhury and Yanful [7] found that the maximum removal capacity of nickel smelter slags was found to be approximately 1.78 mg As per g of slag. The batch and column studies were conducted to determine As removal capacities from contaminated water. Chowdhury et al. [2] discovered redox-mediated reactions while As(V) was being removed by nickel smelter slags. The study further reported that electrostatic attraction and oxidation-reduction reactions between As species and nickel smelter slags were the main mechanisms. Thus, recycled nickel smelter slag could be an effective, low-cost reactive medium for water and wastewater treatment and can be used in both in situ and ex situ treatment processes. According to Chowdhury [11], the chemical interaction between nickel smelter slag and As species could be as follows (Equation (1)):

Fe-FeO-SiO₄ + As species (As³⁺, As⁵⁺, AsO₄²⁻; AsO₃²⁻)
→ Fe-As (e.g., FeAsO₄ or Fe-As-SiO₄) …

(1)

The maximum As(III) adsorption capacity of blast furnace slag (BFS) was reported to be 1.40 mg As(III)/g of BFS at 1 mg/L of As(III) initial concentration and at 25 °C [42]. The mechanism for As(III) removal involves the oxidation of As(III) to As(V) and its adsorption or precipitation onto BFS. Kanel et al. [61] further reported that BFS materials can be used as a permeable reactive barrier (PRB) material to remove As(III) and As(V) from groundwater.

According to Turk et al. [44], the maximum removal of As(V) and As(III) achieved was 98.76% (0.99 mg/g) and 88.09% (0.83 mg/g), respectively, at a copper smelter slag dose of 10 g/L with an initial As(V) and As(III) concentration of 300 μg/L. Nguyen et al. [45] conducted batch and column experiments using stainless steel slags for As removal from contaminated water. The maximum removal achieved was 13.7 mg/g. They further reported that no significant effect on As removal was observed when contaminated water contained common ions and dissolved organic matter. The study found that slags can be reused several times after the completion of the desorption of adsorbed As from adsorbents. Figure 3 shows the primary mechanisms of As removal by different smelter slags. From previous studies, it was confirmed that ferrous and nonferrous smelter slags (e.g., steel slags, BFS, copper smelter slags, nickel smelter slags, etc.) can be used for As removal from water and wastewater treatment.

In Figure 3, it is evident that the chemical composition of BOF, steel, copper, or nickel smelter slags showed a significant amount of different adsorbents that have a high affinity to As species in any aqueous solution. Oxidation-reduction, electrostatic attraction, or physicochemical sorption would be the removal mechanisms of arsenic from aqueous solutions [7,24,26,44]. These slags’ surfaces had an affinity for As species, and thus various FeO–As compounds (e.g., FeAsO₄ or Fe-As-SiO₄) were formed [2]. These results were confirmed through batch and continuous column experiments [2,24]. According to Mohan et al. [62], and Bibi et al. [63], arsenic removal by these slags’ materials occurs by reaction with iron oxides and other metallic oxides contained in the slag. Adsorption affinity is dependent on the surface area of slag materials, while the existing forms of arsenic species control physicochemical reactions.

3.2. Chromium (Cr) Removal from Water and Wastewater

Industrial processes can produce Cr-contaminated wastewater, which can then migrate into the subsurface or surface waterbodies. Cr-contaminated wastewater can also be generated from wastewater spills or waste deposition. The major sources of Cr (VI) from industrial processes are electroplating, tanning, wood preservation, chromate pigments in dyes, paints, inks, plastics, and the preparation of chromate compounds [11]. Cr(VI) is highly soluble and toxic, causing a wide range of diseases in humans (i.e., a carcinogen and mutagen) [11,46].

Copper smelter slag, an alternate Fe(II) source, in the Cr(VI) reduction process, appears to be attractive because of its low cost [35]. The common removal mechanisms are the reduction of Cr(VI) to Cr(III) and, second, precipitation of Cr(III) [11,35]. The maximum Cr(VI) removal by copper smelter slag was determined to be 1.2 mg/g when the adsorption reaction was carried out at 100 mg/L of initial Cr concentration and 5 g/L of reactive smelter slag [35]. Copper smelter slag can be used in situ (e.g., permeable reactive media) or as an ex situ batch reactor [8,35,46].

Blast furnace slag from the ironmaking industry was used to remove Cr(VI) from wastewater. Park et al. [46] reported that in an acidic environment, a large amount of iron was also released from the slag into the aqueous phase, which was the driving force for Cr(VI) removal from the aqueous solution in the batch reactor. When the adsorption process was kept at a constant pH of two for 24 h and the initial Cr(VI) concentration was 200 mg/L, and the maximum removal efficiency was 55 mg/g. In an acidic solution, the redox reaction between Cr(VI) and Fe(II) occurred easily and spontaneously, as shown in Equation (2) [64]:

HCrO⁻₄ + 3Fe²⁺ + 7H⁺ → Cr³⁺ + 3Fe³⁺ + 4H₂O; E₀ = +0.58 V

(2)

Steel slags can also adsorb Cr(VI) (e.g., 0.16 mg/g) from wastewater in a batch reactor under ideal experimental conditions [49]. According to Han et al. [65], Fe²⁺ released from BOFS (basic oxygen furnace slag) can effectively remove Cr(VI) from wastewater, and the main mechanism of Cr(VI) removal by BOFS appears to primarily consist of several sequential reaction steps, including BOFS dissolution, Cr(VI) reduction, and CaSO₄·2H₂O precipitation, adsorption, and crystallization on the BOFS surface. Cr(VI) removal capacity by BOFS was calculated to be 12 mg/g. Erdem et al. [66] investigated Cr(VI) uptake from industrial wastewater using ferrochrome slag obtained from Etiholding’s Elazg Ferrochrome Plant in Turkey. More than 90% of Chromium was removed from the water when the batch adsorption experiment was conducted at 5 g/L of ferrochrome dosage and an initial Cr(VI) concentration of 10 mg/L with a contact time of 2 h and 25 °C. 1.8 mg/g was reported to be the highest removal capacity [66].

From the previous studies, it was clear that Cr removal can be achieved by blast furnace slag, copper smelter slag, BOFS, or ferrochrome slag. The removal rate of Cr(VI) increased with a decrease in pH or increase in slag dosage and equilibrium contact time (determined to be 2 to 3 h) [47,64,65,66].

The use of smelter slags for Cr removal from wastewater has been illustrated in Figure 4. The smelter slags follow three steps: the formation of Fe ions during mixing processes, the reduction of Cr(VI), and the precipitation of Cr(III). Figure 4 also shows how recycled slags help in the development of green water and wastewater treatment technology. The proper handling and reduction of disposal costs of smelter slags can be achieved by inventing the application of smelter slags in different engineering applications. Figure 4 is one of the efforts to achieve the goal.

3.3. Lead (Pb) Removal from Water and Wastewater

The main sources of toxic lead (Pb) are battery manufacturing, metallurgy, metal finishing, mining, metallurgy, and chemical industries [48]. Pb is a very toxic metal, causing a cumulative effect on the human body. Thus, unplanned discharge into the environment causes a serious hazard to the human body. It is considered a Group 2A probable human carcinogenic chemical compound. According to Huy et al. [50], steel slags can remove Pb ions from water and wastewater. Steel slags successfully removed more than 95% (i.e., 5 mg/g) of Pb ions from contaminated water when steel slags were used as a filter media in any filter column under acidic conditions (e.g., pH 3.5). The removal was dependent on the solution pH, the presence of other ligands, the contact time, etc. Furthermore, under optimal experimental conditions, steel slags can adsorb Cr(VI) from wastewater (e.g., an adsorption capacity of 0.07 mg/g) [47]. The recycled electric induction furnace slag was used to remove Pb ions from aqueous solutions. The experiment was conducted in initial concentrations of 1 mg/L and 0.6 g/L of slag dose [47]. A similar result was obtained in a batch experiment with a contact time of 4 h [48]. According to Dimitrova [48], granular blast furnace slag (BFS)-packed columns can remove Pb ions from aqueous solutions. The results showed the maximum removal efficiency to be 8 mg/g when the solution pH was kept at four [48]. In the same way, the removal efficiency depended on the solution pH, lead concentration, competing metal ions, slag particle size, flow velocity, and contact time.

According to Nilforoushan and Otroj [49], electric arc furnace slag (EAFS) was used in the absorbing bed in the presence of 2 mg/L of Pb²⁺ and at pH 9.0–9.5. The removal efficiency was found to be 100% after 48 h of column operation. The removal capacity was 98%, while the initial concentration was kept at 10 mg/L.

The possible mechanisms to remove lead from furnace slags are adsorption, coprecipitation, and hydroxide precipitation as hydroxide, sulphide, and ion exchange [54]. According to Abdelbasir and Khalek [67], blast furnace slag (BFS) is a low-cost sorbent. At pH 6, after 60 min of contact time, the maximal absorption capacity for Pb²⁺ was 30.2 mg/g. They further reported that ion exchange was the primary process of adsorption between BFS and metal ions.

Figure 5 demonstrates the influencing factors between a toxic metal ion and smelter slags during column and batch studies. Moreover, nickel smelter slags also removed Pb ions from aqueous solutions, and the removal mechanisms depended on chemical sorption and ion exchange [37]. The removal efficiency (e.g., 4 mg/g) of nickel semester slags also depends on pH, lead concentration, competing metal ions, slag particle size, flow velocity, contact time, etc. [37]. The influencing factors in column and batch systems are also the chemical compositions of slags, toxic contaminants, solution pH, and temperature, as well as mixing and reaction methods (as shown in Figure 5).

3.4. Cu Removal from Water and Wastewater

Copper (Cu) is released into the environment in large quantities by metal finishing, paint, chemicals, mining, wood production, fertilizer, and effluent from electrical industries. Exposure to high levels of Cu can be hazardous to humans, causing irritation in the eyes, nose, and mouth as well as tissue damage. According to Kim et al. [52], steel slags remove Cu ions from water and wastewater. The primary removal mechanisms are precipitation and adsorption on the surface of metal oxide. The maximum removal by steel slag was found to be 300 mg/g when a batch experiment was conducted for 4 h at pH 5, 10 g of slags, and 100–5000 mg/L of Cu solution. According to Addai [37], nickel smelter slags can remove Cu ions from aqueous solutions, and their maximum removal efficiency was found to be 3.8 mg/g for Cu while conducting the experiment at 5 mg/L of initial concentration of Cu, 10 g/L of smelter slags, and 10 h of contact time at 25 °C. Nguyen et al. [3] reported that an iron industry blast furnace slag waste removed Cu ions from water, and the maximum removal was achieved at 5.22 mg/g (when the batch experiment was performed at pH 6.5).

3.5. Zn Removal from Zn Contaminated Water

The sources of Zn release into the environment are the plastic, newsprint, galvanizing, and zinc plating industries. Excessive exposure to Zn can damage living tissues and cause skin irritations, nausea, restlessness, and lung disorders [68]. Slags can remove Zn ions from aqueous solutions, thus treating contaminated wastewater. According to Chen et al. [58], electric arc furnace slag (EAFS) removed Zn, and its removal capacity was found to be 0.153 mmol/g. Moreover, nickel smelter slags also removed Zn ions from aqueous solutions. The removal capacity was found to be 33% when a batch experiment was performed at 10 mg/L of Zn concentration [37].

Chen et al. [51] also used steel slags for Zn ion adsorption from water. The removal was achieved at 0.148 mmol/g, and the adsorption mechanism was physico-chemisorption. Nickel slag shows affinity to the metals studied in the order of Pb > Cu > Zn > Cd [37], whereas the adsorption capacity for the metals in the case of blast furnace slag waste is in the order of Pb > Cu > Cd, Zn and Cr [3]. The Zn removal capacity of blast furnace slag was calculated to be 3.46 mg/g while conducting column tests in an optimum environmental condition [3].

3.6. Cadmium (Cd) Removal from Waste Water

Cadmium (Cd), a toxic heavy metal, can be released from different industrial activities such as metallurgy, electroplating, pigment, fertilizers, batteries, mining, and electroplating. Exposure to cadmium over a long period of time can cause kidney failure, the development of dyspnea, hypertension, and prostate cancer. Increased levels of cadmium in the body can also result in death. Blast furnace slag can remove Cd ions from aqueous solutions, and maximum removal was achieved at 3.47 mg/g in the furnace slag column adsorption study [3]. According to Addai [37], the amount of Cd removed by nickel smelter slag was less than 10%. Stainless steel slag obtained from scrap metal recycling, on the other hand, removed more than 90% of Cd from aqueous solutions [50]. Yu et al. [27] reported that activated modified steel slag at high temperatures could adsorb Cd ions from aqueous solutions. The removal rate of Cd from modified steel slag was almost 94% under optimal conditions. Mercado-Borrayo et al. [47] reported that recycled electric induction furnace slag was used to remove Cd ions from aqueous solutions. The removal capacity was achieved at almost 64% (0.21 mg/g) when the initial concentration was kept at 1 mg/L and the slag dose was 0.6 g/L.

3.7. Nitrogen (N), Phosphorus (P), DCE, and TCE Removal from Water and Wastewater

Excess nutrient concentrations need to be treated in order to control or prevent eutrophication problems (i.e., the nutrient load causes abnormal growth of algae and aquatic plants that are decayed by microbial decomposition, reducing the concentration of dissolved oxygen resulting in the death of aquatic species and odour issues). According to Yu et al. [27] and Ping et al. [29], steel slags can remove ammonium nitrogen and phosphorous from wastewater. It could be considered an economical and effective method and is used in wastewater treatment plants. They further reported that ammonium nitrogen and phosphorous removal from wastewater can be used after biological treatment. The maximum removal capacity of phosphorus from wastewater was 1.37 mg/g. The main mechanisms for adsorption were chemical adsorption and precipitation [27]. According to Yu et al. [27], under optimal conditions, the maximum adsorption capacity of phosphorus by smelter steel slags was determined to be 3.24 mg/g and ion exchange and physical adsorption were the main adsorption mechanisms when modified steel slags were used as an adsorbent. In addition, a series of batch experiments were designed using steel slags for ammonium nitrogen removal from aqueous solutions [29]. The maximum removal capacity of ammonia nitrogen was 10.29 mg/g, while the initial concentration was kept at 40–240 mg/L and steel slag doses of 4 g/L. The removal efficiency of PO₄³⁻ and NH₄⁺-N depends on the particle size, dosage of slag, solution pH, and contact time when the batch experimental method is used [29].

In addition, copper smelter slag was also used in the batch study to remove phosphorus from secondary wastewater effluent. The maximum removal efficiency was assessed to be 0.26 mg/g when the batch test was carried out at 4 mg/L of phosphorous, 160 g/L of Cu smelter slag, and 4 h of contact time at initial pH values of 6.4 and 7.5, respectively [61]. Park et al. [53] used rapid-cooled basic oxygen furnace slag for P removal. The removal capacity was found to be used at 3.6 mg/g during a batch experimental study conducted at 10–200 mg/L of phosphorus and at initial pH values of 5.0 and 7.0.

The toxic chlorinated solvents (trichloroethylene (TCE) and dichloroethylene (DCE)) are released from cleaning activities in industries, mishandling, improper storage, and disposal of spent solvents. It could be produced from workshops, warehouses, car repair centres, etc. It could also be found in hazardous waste disposal sites and contaminated subsurface [56]. Chemical oxidation (like the Fenton reaction) can remove TCE effectively. The Fenton reaction is a mechanism for converting hydrogen peroxide to a strong oxidizing radical (OH⁻). The capacity of Fenton oxidation would be increased during the chemical oxidation method for TCE removal when BOF slag was used as the catalyst and as the supplemental source of iron minerals in Fenton oxidation and persulfate oxidation processes [56]. The results showed that 81% of TCE removal was achieved when an adsorption study was performed at an initial TCE concentration of approximately 5 mg/L with the addition of 1000 mg/L of H₂O₂ and 10 g/L of BOF slag. This BOF slag can also be used as reactive media in a permeable reactive barrier system for groundwater remediation, especially in situ TCE treatment. In addition, Smith [55] investigated the use of a permeable reactive barrier constructed with steel slag media in order to remove groundwater contaminants like TCE. Steel slags of various particle sizes were used to construct an in situ permeable reactive barrier [55].

3.8. Acid Mine Drainage (AMD) Treatment Using Smelter Slags

Acid mine drainage (AMD) is produced during the mining of metals or coal when sulfur-containing materials are exposed to water and air, resulting in sulfuric acid. A very hazardous discharge can be produced when acid and heavy metals interact [69]. The primary sources of AMD wastewater are the drainages from active as well as abandoned mines. Pyrite oxidation (with dissolved O₂, Fe³⁺, and other mineral catalysts such as MnO₂, ferrous oxidation, and iron hydrolysis) discharges a dissolved Fe^2+, SO₄²⁻, and H⁺ solution [69,70]. Acid mine wastewater has an impact on the soil, surface water, and groundwater, and thus has a potential effect on the health of people who are exposed to the contaminated water. Moreover, building materials and different concrete infrastructures are also vulnerable to degradation and cracking due to exposure to corrosive and toxic mine waste. Mine wastewater containing high levels of radioactive materials and heavy metals may pose cancer risks [58,59]. Iron slag, steel slag, or basic oxygen furnace slag (BOF) were effective at reducing acid, toxic metals, iron, and sulfate concentrations [66]. Slags are highly alkaline since most slags contain an elevated concentration of hydrated silica, calcium oxide, and magnesium oxide [11,59]. Thus, slag can increase the pH of acid mine water to almost neutral levels and remove heavy metals through physicochemical sorption and precipitation. Furthermore, the dissolution of calcium hydroxide (Ca(OH)₂) from CaO results in alkalinity in slag-mixed water. At different pHs, different forms of iron in slags react with hydroxide (OH⁻) to form different products [58]. The removal of metals from solutions by slag is due to the formation of precipitates. Gypsum (CaSO₄·2H₂O) and other sulfate precipitates can help remove sulfate from slag that has had AMD added to it.

Ca(OH)₂ → Ca²⁺ + OH⁻

(3)

Ca²⁺ + SO₄²⁻ + OH⁻ → CaSO₄·2H₂O

(4)

Feng et al. [58] conducted a study using iron slag for AMD treatment. The results showed a much higher adsorption capacity at a higher pH value of 5.5–8.5. Adsorption, precipitation, and coprecipitation were the toxic metals’ removal mechanisms if iron slags were used as adsorbents. Slags collected from a basic oxygen furnace could remove more than 90% of heavy metals and anions from AMD in a batch experiment with 30 g/L of slag, pH values ranging from 2 to 6.68, and a contact time of 24 h [58].

Name and Sheridan [59] reported that stainless steel slags could remove 63.6% (6.36 mg/g) of iron and 39.8% (20 mg/g) of sulfate from AMD when a batch experiment was carried out at 100 g/L of slag dose, acidic conditions (pH values of 2.5 to 5.9), and 4-h contact time. When basic oxygen furnace slag was used for AMD treatment, 99.7% (10 mg/g) of iron and 75% (12.5 mg/g) of sulfate removal were achieved at an initial sulfate concentration of 5000 mg/L and an iron concentration of 1000 mg/L. Moreover, ironmaking slag and steelmaking slag obtained from Saldanha Steel South Africa in the form of powder with a mean particle size of 24.5 and 24.1 µm were also used to treat Cu and Pb in acid mine wastewater [58]. The isothermal sorption investigation on the steel slag found that the maximum removal of Cu and Pb in acid mine wastewater was achieved at 16.21 mg/g and 32.26 mg/g, respectively, while for iron slag, it was found to be 88.50 mg/g and 95.34 mg/g, respectively. The combined slag sorption and flotation technique used to remediate an acid mine drainage from a South African gold mine yielded a promising result [58].

Steel slag can be used as a passive treatment method for the remediation of acid mine drainage. According to Saha and Sinha [71], iron slag has more affinity for metals than BOF slag and SS slag. The removal capacity heavily depends on the surface area of the slags. They further reported that the metal removal efficiency of iron slag was greater than that of BOF slag since BOF slag has a surface area of 22.333 m²/g whereas the surface area of SS is 31.929 m²/g.

4. In Situ and Ex Situ Treatment

Recycling smelter slags can be used for both in situ (e.g., filter bed, permeable reactive barrier, injection method, constructed wetland, oxidation) and ex situ (e.g., filter bed, suspended batch reactor or continuous reactor system, oxidation and precipitation, carbon sequestration, etc.) treatment [7,31,50,56,72,73]. Reactive slags can be used as barrier media to treat toxic metals and chlorinated solvents [7,56]. Flow rate, residence time, barrier thickness, hydraulic conductivity of barrier media, reaction rate, and hydrogeological condition of the contaminated site influence slag removal efficiency in permeable reactive barriers [7]. Activated reactive slag solution injected into the contaminated source zone would be another possible in situ treatment option for contaminated groundwater treatment [11]. According to Names and Sheridan [59], acid mine drainage (AMD) is treated by creating a natural system similar to a wetland consisting of reactive slag material layers. An alkaline bed can be constructed by using slags that could neutralize the acidic mine waste in the constructed wetland. Batch reactors or continuous filter beds using smelter slags can be used in water and wastewater treatment plants to reduce targeted contaminants [7,31]. Slags can be injected into the source zone during various in situ oxidation processes to destroy the targeted contaminants [56].

Table 4. Different in situ and ex situ treatments using reactive slags.

Treatment	Treatment System	Mechanisms	Types of Water and Wastewater Treated	Possible Slag Media		References
				Ferrous	Nonferrous
Permeable Reactive Barrier	In situ continuous	Physicochemical Adsorption	Contaminated groundwater	BOF Slag, Steel slags,	Nickel smelter slags	Tsai et al. [56], Chowdhury et al. [2];
Injection method	In situ continuous	Slags used as Catalyst for Oxidation reduction	Contaminated source zones	BOF slag	-	Tsai et al. [56]; Chowdhury and Yanful, [7]
Constructed Wetland	In situ System	Adsorption, ion exchange and Precipitation	Mine wastewater, Municipal wastewater	BOF Slag, Steel slags		Mayes et al. [74]; Names and Sheridan, [59]; Ge at al. [75]
Filter bed	Ex situ continuous system	Physicochemical adsorption, precipitation	Surface water, Municipal wastewater, Metal contaminated wastewater	Steel slags	Nickel smelter slags	Huy et al. [50]; Chowdhury and Yanful [5]
Batch reactor	Ex situ batch System	Physicochemical adsorption	Metal contaminated wastewater		Ni smelter slags, Steel slags, Copper smelter slags	Turk et al. [44], Chowdhury and Yanful [5];
Flotation of adsorptive slags (Ex situ treatment)	Ex situ batch system	Direct flotation by Sodium dodecyl sulphate (SDS)	Acid mine water, Metal contaminated wastewater	Iron slag,	Nickel smelter slags	Feng et al. [58]; Chowdhury and Yanful [5]
Continuous flow reactors	Ex situ continuous system	Adsorption, ion exchange and Precipitation	Mine wastewater, Metal contaminated wastewater	steel slags, BOF slag,	Nickel smelter slags	Name and Sheridan [59]; Chowdhury et al. [2]

presents the various in situ and ex situ treatment systems that have been used with reactive slags over the last two decades. It shows that ferrous and nonferrous smelter slags can be used as reactive materials in injection methods for site remediation engineering, the construction of a permeable reactive barrier, a constructed wetland, filter bed or continuous flow reactors for adsorption, ion exchange, and precipitation in water and wastewater treatment plants, etc.

To control environmental pollution, any water or wastewater should be treated properly before being discharged into nature. In the United States, recycled slag has been used for in situ treatment. The first large-scale use of basic oxygen furnace (BOF) slag to treat arsenic from groundwater was at a DuPont facility in East Chicago, Indiana. A 2000 feet long by 35 feet deep permeable reactive barrier (PRB) was constructed in 2002 to remediate a 1–2 mg/L arsenic plume [76]. In the United States, the Massachusetts Alternative Septic System Center also used a permeable reactive barrier containing BOF slag in order to treat wastewater from septic tanks. Atomized slag is used as an alternative to sand and fine gravel in construction materials, water treatment media, filter media, coagulation media, abrasives, and desulfurizing agents [77]. In PRB, the continuous trench system is used in Korea in order to backfill with atomized slag and reactive admixtures of Fe metal, allowing water to pass through the barrier at its natural gradient [77].

With the use of iron slag, a wastewater treatment facility for the New Zealand settlement of Waiuku was able to reduce the phosphate concentration by 80%. Along with the elimination of phosphate, similar operations in Canada and Massachusetts also saw a decrease in BOD and E. coli levels [78]. Roychand et al. [79] claim that the waste product of the steel industry, slag, has the ability to remove phosphorus from water. In Waiuku, New Zealand, a large-scale continuous wastewater treatment facility employing slag demonstrated 77% phosphate removal over a five year period [80,81]. Adsorption is a key mechanism for removing phosphorus from a slag medium. However, over time, its adsorption capacity decreases and it starts to lose its effectiveness, necessitating either complete replacement or rejuvenation of the existing slag filters.

Slags were used as a substrate in CW at the large-scale demonstration wetlands in Xi’an, the largest megacity in northwest China [75]. Municipal and industrial wastewater discharges had a significant negative influence on the Zaohe River (a Weihe River tributary), which flowed into the Yellow River near Xi’an in China. Near the intersection of the Zaohe and Weihe Rivers (34°22′54″ N, 108°51′05″ E), two horizontal subsurface flow (HSSF) CW systems were built in the floodplain. The two wetlands were filled to a total thickness of 0.6 m using locally available slag (CaO slag) and gravel substrates, respectively. In the typical climate of northwest China, two similar large-scale horizontal subsurface flow (HSSF) CWs with gravel or slag substrates (surface area, 340 m²; depth, 0.6 m; hydraulic loading rate (HLR), 0.2 m/day) were selected for remediating contaminated urban river water [75]. Total phosphorus (TP) and total nitrogen (TN) elimination in HSSF CWs were found to be greater than 75%. Large-scale HSSF CWs using either gravel or slag as a substrate could, overall, achieve comparable pollutant removal rates and offer a potential remedy for cleaning up polluted urban rivers in northern China [75].

Steel slags were also used as substrates in vertical flow-constructed wetlands (VFCWs) to treat municipal wastewater in La Motte d’ Aigues City in France [82]. It has a treatment capacity of around 240 m³/day and is composed of two stages of VFCWs. In this CW, two different types of slag, such as basic oxygen furnace (BOF) slag and electric arc furnace (EAF) slag were used to construct their bed. In France, CWs filled with steel slag produced high removal performances (>80%) for chemical oxygen demand (COD), total suspended solids (TSS), and total Kjeldahl nitrogen (TKN) [82].

Metallurgical slags (such as BOF and SS) were used to treat acidic mine drainage around the Witwatersrand Basin area and the Mpumalanga coal fields in South Africa [66]. Slag leach bed (SLB) is a passive technique that has been constructed in South Africa to treat mine effluent and water. The concentration of acid, iron, and sulfate could be reduced using both the BOF and SS slags [59]. Steel slags have a high calcium content, a very high pH (>11%), and provide a high alkaline environment to balance the acidic drainage (pH > 7), according to the NSA (2013) [78]. In the Huff Run watershed in Ohio, USA, steel slags were used as a passive treatment for acid mine drainage to reduce the effects of low-flow pH changes in the water further downstream [78].

The results from the different studies suggested that the use of recycled smelter slag waste products as a reactive medium for site remediation, as well as water and wastewater treatment, would be sustainable and promising [30,31]. The study revealed (as shown in Table 2 and Table 3) that ferrous slags are more effective in removing a wide range of toxic chemicals than nonferrous smelter slags. The technology is low cost and effective (it would be constructed and operated with inexpensive, readily available, and easily reusable treatment materials) and offers attractive viable options in engineering applications. The major associated cost for reactive smelter slags in cases of environmental engineering application would be the crushing of the slag down to the desired particle size. Thus, compared to the waste disposal cost, using recycled slag in the treatment industry is more viable and beneficial from a sustainable economic point of view. In addition, smelter slag material from the steel industry can be utilized for the manufacturing of high-quality products such as cement, building materials, aggregates for road construction, and fertilizers. Unfortunately, despite its use in the production of different engineering materials, small quantities also end up in waste disposal sites or are used for landfilling. Thus, to enhance the reutilization of slag materials, suitable rules and regulations could be redeveloped, which could accelerate resource recovery or conversion activities.

5. The Cost of Smelter Slags

Cost is a key consideration in the implementation of any successful engineering technique. The effect of the materials’ cost, availability, preparation, installation process, handling, and operation controls its widespread application in real-world projects [83]. Recycled slag for environmental remediation promotes sustainable growth. Crushing is the main expense involved in using recycled smelting slag in any treatment project [84,85]. According to Mercado-Borrayo et al. [49], slag material costs 0.025 USD/kg, while the commercial adsorbent, nano zero valent iron (nZVI), costs 122 USD/kg. The unit costs of treatment utilizing slags (a noncommercial adsorbent) are 0.55 USD/m³, while the unit costs of treatment using commercial adsorbent (nZVI) are 0.72 USD/m³. Moreover, the cost of crushing the slags into sizes that can be used in an adsorption process, (according to a communication with John Aarts Crushing Company, London, ON, Canada) would be $8 per tonne of slag materials [2].

Steel slag provides a practical and affordable solution to the problem of acid mine drainage (AMD) remediation. The cost of the steel slag used in the Huff Run Watershed in Ohi, USA, was $12 per tonne. Steel slags for acid mine drainage treatment were predicted to cost between $10 and $15 in Pennsylvania counties [78]. According to Kanel et al. [61], BFS from the steel industry is freely available. Nevertheless, after accounting for transportation costs, chemical costs, etc., the finished products would cost about US$12 per tonne. In comparison, the least expensive commercially available carbon costs around USD 1000 per tonne [61]. BFS is relatively less expensive (USD 12 per tonne) than other industrial wastes, such as ferruginous manganese ore (USD 50–56 per tonne), red mud (USD 25 per tonne), and bassage fly ash (USD 15 per tonne) [61]. Li et al. [9] examined the cost of steel slag from private contractors and discovered that it was five times more expensive than natural sand. Steel slag costs between USD 34 and 107 per tonne, depending on the particle size and mineral compositions, while river sand costs roughly USD 23 per tonne, according to Chinese market pricing in 2020.

The literature review indicates that innovative low-cost reactive slag materials for ex situ in situ treatment are a promising green technology [8,37,85,86]. According to Gisi et al. [13], BF slags cost USD 0.04/kg, yet industrial byproducts such as bagasse fly ash, blast furnace slag, peat, and red mud are some of the most competitive and have associated costs of USD 0.002, 0.04, 0.04, and 0.025, respectively (as shown in

Table 5. Examples of cost evaluation of slags and other adsorbents.

Adsorbents	Treatment Cost USD per kg	Removal Efficiency (mg/g)	References
Red mud	USD 0.025	99% (19.72 mg/g) of Cu(II), 90% (12.5 mg/g) of Zn(II), 80% (10.57 mg/g) of Cd(II), 95% (26.4 mg/g) of Cr(VI)	Gisi et al. [13]
Peat	USD 0.04	95% (23 mg/g) of Cu(II), 99% (29 mg/g) of Cr(VI), >75% (5.8 mg/g) of Cd(II), 99% (40 mg/g) of Pb(II)	Gisi et al. [13]
Bagasse fly ash	USD 0.02	99% (43 mg/g) of Pb(II), >80% (12.5 mg/g) of Zn(II), 90% (13.5 mg/g) of Cd(II)	Grassi et al. [87]
Blast furnace slag	USD 0.04	99.7% (10 mg/g) of iron and 75% (12.5 mg/g) of sulfate removal, 90% (1.4 mg/g) of As (III)	Kanel et al. [61]
Nickel smelter slag	USD 0.001	>90% (3 mg/g) of As (V)	Chowdhury and Yanful [7]
Steel slag	USD 0.0012	>90% (30 mg/g) of Cu(II), >90% (29.35 mg/g) of Ni	NSA [78], Gisi et al. [13]
BFS (blast furnace slag)	USD 0.04	>80% (1.4 mg/g) of As (III), >85% (17.66 mg/g) of Zn (II),	Gisi et al. [13]
Chitosan	USD 0.025	>80% (0.8 mg/g) of As (V), >75% (7.94 mg/g) of Cr(VI),	Gisi et al. [13]
Carbonaceous sorbents from sewage sludge	USD 0.0015	80% (0.8 mg/g) of Cu(II)	Rio et al. [84]

). The development of reactive, affordable, accessible, and effective slag materials for toxic contaminant removal would be a good replacement for currently existing and expensive reactive adsorbents [37]. The prices listed in Table 5 should be adjusted with the current inflation rate in order to evaluate the accurate treatment cost. The cost of removing toxic contaminants with smelter slag in 2022 can be estimated using a 1–3% annual increase. Table 5 shows the cost evaluation of slags compared to other adsorbents during the years 2016–2020.

6. Challenges of Spent Smelter Slags

The adsorption efficiency of smelter slag is affected by many factors, such as lattice aperture size, channel dredging, the cation position and ratio of Si and Al, Fe, the presence of competing ions in the aqueous solution, etc. Their lifetime, regeneration process, and slag bed leaching issues sometimes proved ineffective [5]. Moreover, spent slag adsorbents contain solid wastes loaded with high concentrations of toxic contaminants [5,37]. Two strategies: (i) regeneration, recycling, and reuse; and (ii) immobilization treatment, can be applied to reduce the negative environmental impact of spent slag adsorbents. The regenerability, recyclability, and reusability of spent slag adsorbents not only contribute to the economics of the adsorption process but also mitigate the environmental impact of the slag’s materials. It has been established that immobilizing hazardous wastes with cementitious materials, as well as thermal treatment techniques, effectively lowers the mobility of toxic contaminants in the environment [88,89].

The most popular regeneration techniques for various adsorbents include thermal, chemical, or a mixture of these techniques [90,91]. Thermal regeneration entails heating an adsorbent to a specific temperature to break the bond between the adsorbate and adsorbent without deteriorating the adsorbent [90]. Chemical regeneration is the degradation of a specific species in a solution using a specific solvent and/or chemical species [90,91]. These techniques can be easily used at industrial scales and are relatively inexpensive and straightforward. Thermochemical processes have inherent challenges such as a loss of structure within the reactive adsorbents, reduction of absorption capacity, and loss of reactivity during each treatment cycle [90,91,92].

The desorption behaviour of Pb²⁺ and Cd²⁺ ions from BFS in 0.01 M NaNO₃ electrolyte and 0.1 M HNO₃ was examined by Zhou and Haynes [18]. While 0.1 M HNO₃ was effective in desorbing Pb²⁺ ions from spent BFS adsorbents, there was a loss of adsorbent capacity due to the strong inorganic acid’s chemical attack. Desorption of both Pb²⁺ and Cd²⁺ was reported to be poor in 0.01 M NaNO₃. Due to the high alkalinity of the slag and the strong adsorbate–adsorbent interactions, 0.1 M HNO₃ was ineffective for desorbing Cd²⁺. In addition, adsorption–desorption tests for nickel smelting slag were performed in order to assess the slag’s potential for reuse in the treatment system [37]. In the study, 10 g/L of nickel smelting slags were mixed with 10 mg/L of Pb and 10 g/L of Cu for the batch experiment. After 10 h of adsorption, elution with 50 mL of 0.1 M HNO₃ was conducted for 10 to 12 h in order to conduct desorption tests. After a 24-h desorption test, the findings of this study revealed more than 90% of desorption from a single Pb or Cu-adsorbed nickel smelting slag [37]. Low pH conditions are favourable for cation desorption from spent adsorbents [93,94,95]. The main causes of the desorption of metal cations under low pH conditions are: (1) desorption and/or dissolution of adsorbed cations since both their sorption and precipitation occur at a high pH; (2) competition between metal cations and H⁺ ions leading to the displacement of cations into the solution; (3) acid conditions favouring dissolution of Fe and Al oxide/silicate adsorption surfaces, releasing the adsorbed and/or surface precipitated ions; and (4) alkaline and acid reactions lowering sorption capacity.

7. Conclusions

Although smelter slags can remove contaminants from water and wastewater, their application has some limitations. Thus, future research is indeed required to overcome all the limitations of smelter slag application in contaminated water treatment. These could include smelter slag surface activation, reactive smelter materials’ uses in treatment plants (such as biological treatment), application in the constructed wetland, toxic metal leaching study, regeneration study, field scale study, in situ continuous treatment for contaminated site remediation, etc. Slags from ferrous and nonferrous metallurgical industries can contribute to the circular economy in different ways. Smelter waste products can be considered circular products by applying the following actions:

(1)

recycling at the source;

(2)

recovering the precise compounds before final disposal;

(3)

remanufacturing of different engineering materials;

(4)

producing reactive adsorbents; and

(5)

activating slag materials for both ex situ and in situ treatment.

Recycled slag materials in the treatment industry can contribute to long-term sustainability goals through the 5Rs (i.e., reduce, reuse, recycle, remove, and recover). This application can successfully recover resources from discarded waste products. Ferrous and nonferrous smelter slags have the potential to be used in green technology and the closed-loop circularity of slag materials can be developed through the 5Rs in the technological field. Researchers pay more attention to the application of smelter slag in treatment industries due to its economics, circularity, and scalability. Scalability and a circular economy could be achieved by incorporating the treatment and activation of recycled slags into the overall process optimization system.