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Treatment Technology and Research Progress of Residual Xanthate in Mineral Processing Wastewater

Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China
Yunnan Key Laboratory of Green Separation and Enrichment of Strategic Mineral Resources, Kunming 650093, China
Author to whom correspondence should be addressed.
Minerals 2023, 13(3), 435;
Submission received: 15 February 2023 / Revised: 15 March 2023 / Accepted: 16 March 2023 / Published: 18 March 2023
(This article belongs to the Special Issue Clean Utilization of Nonferrous Metal Resources)


Xanthate is the most widely used and effective collector in the flotation of sulfide minerals. However, the residual xanthate in flotation wastewater may cause serious environmental pollution and even human health hazards. At present, a variety of treatment technologies have been developed to degrade xanthate pollutants in wastewater, with the aim of meeting safe discharge standards. This work reviews the research status of xanthate wastewater treatment technologies in recent years. Treatment technologies are evaluated, including coagulation flocculation, adsorption, microbiological, Fenton, ozone oxidation, and photocatalytic methods. The reaction mechanisms and advantages, as well as disadvantages, of the various treatment technologies are summarized. Future research on the treatment of xanthate wastewater should focus on combined methods, which will be conducive to achieving a high efficiency and low cost, with no secondary pollution, and with the aim of generating further original and innovative technologies.

1. Introduction

Recently, environmental issues related to the mineral processing industry have received increased attention from researchers. The efficient treatment of mineral processing wastewater is the focus of research in this field. It is well accepted that froth flotation, on the basis of the different surface properties of minerals, has become the most common and effective beneficiation method for separating metallic sulfide minerals from the gangue [1]. It is statistically estimated that approximately 2 billion tons of minerals are processed by flotation methods every year [2], which mainly involves copper, lead, zinc, nickel, etc. Xanthate is the most widely used agent in the flotation of non-ferrous metal sulfide minerals, and its consumption is expected to reach about 371,826,000 tons by 2025 [3]. However, only half of the xanthate in the flotation process is consumed, while the remainder is discharged to the tailings pond [4,5]. The residual xanthate wastewater can leak out and cause odor problems and toxicity to the biota, resulting in the deterioration of the surrounding water quality [6]. When recycling the beneficiation wastewater, this remaining agent will affect the flotation index and cause adverse effects on the flotation selectivity of the minerals [7]. Thus, effective treatment of the residual xanthate in flotation wastewater is of great significance to the sustainable development of the mineral processing industry and for environmental protection.
Various methods have been reported for the treatment of xanthate wastewater [8], primarily including acid decomposition and conventional chemical oxidation. The conventional chemical oxidizing agents mainly consist of hydrogen peroxide (H2O2) and hypochlorite (NaClO, Ca(ClO)2, etc.). However, there are still many issues with these traditional processes. For instance, a continuous supply of chemicals and energy are required when acid decomposition methods are employed. Therefore, most of them are both unaffordable and unsustainable [9]. The conventional chemical oxidation methods have an extremely low treatment efficiency, and the treated xanthate wastewater cannot meet emission standards for release into the environment, which can produce secondary pollution [10]. Therefore, it is vitally important to develop effective methods with higher oxidation capacity, with the aim of better removal of xanthate.
In order to overcome the shortcomings of these traditional treatment methods, researchers have developed techniques such as physical adsorption [7,11], coagulation flocculation [12], and microbiological methods [4,13]. They exhibit superior performance for the treatment of xanthate wastewater, due to the advantages of simple operation and relatively mature technology. During the last few decades, efforts for the treatment of mineral processing wastewater via advanced oxidation processes (AOPs) have been ongoing. Photocatalytic [14,15], ozone oxidation [16], and Fenton [17,18] are frequently cited as the prime examples of such processes. They become effective alternative methods for degrading residual xanthates in flotation wastewater, because of a high xanthate removal rate, short treatment duration, as well as no secondary pollution. With the ever-increasing raising of environmental protection requirements, the development of efficient treatments for xanthate wastewater has become a hot topic, and our goal is to summarize the recent advances in the treatment of xanthate wastewater, and to guide the future utilization of wastewater resources and environmental protection.
In view of this, the aim of this review is to critically analyze the broad-spectrum treatment methods that have been engaged in the published literature on the degradation of xanthate wastewater. The reaction mechanisms and related influencing factors of the various treatment techniques for the removal of xanthate are explored, while the characteristics and hazards of xanthate are briefly discussed. Finally, updated applications for the removal of residual xanthate from flotation wastewater are emphasized. A review such as this will help researchers understand the progress in research, identify the strengths and weaknesses of each treatment technology, and document knowledge gaps that could help shape the direction of future studies in this area.

2. The Characteristics and Hazards of Xanthate

Xanthate is chemically formulated as ROCSSMe (Me is Na and K). It is easily decomposed and has good water solubility [19]. It can be partially degraded naturally under light conditions, and is unstable under strong acid, strong alkali, as well as neutral conditions [20,21]. In the water system, a series of chemical reactions, as shown in Equations (1)–(4), occur, leading to the decomposition of xanthate [20].
ROCS2 + H+ → ROH + CS2
In acidic solutions, xanthate is readily decomposed into alcohols and carbon disulfide (CS2):
In alkaline solutions, xanthate is readily oxidized to dixanthate and further converted to alcohols and carbon disulfide (CS2) in contact with air:
2ROCSS + 1/2O2 + H2O → (ROCSS)2 + 2OH
ROCSS + CO2 + H2O → ROH + CS2 + HCO3
Xanthate and its derivatives are highly toxic and difficult to degrade. Once the xanthate wastewater enters a water body, even in very small amounts, it can cause water quality deterioration and a bad smell, as well as affect the growth and reproduction of algae, fish, and microorganisms in the water [22,23]. It has been reported that 30 μg/L of xanthate had a significant disruptive effect on the embryonic development of salmonids [24]; 19.25 mg/L of xanthate had a dramatic inhibitory effect on chlorella pyrenoidosa [25]. In addition, xanthate can have a negative impact on the nervous system and liver of humans and animals [26,27]. In addition, xanthate easily forms complex pollutants once it interacts with heavy metals, accelerating the migration, transformation, and residence time of heavy metal elements in the environment [25,28]. This activity can cause inhibition of enzymatic activities, genetic mutations, and chromosomal mutations in organisms [29], making the toxicity of xanthate wastewater more serious.

3. Xanthate Wastewater Treatment Technology

3.1. Coagulation–Flocculation Method

Coagulation–flocculation is an effective method that is widely used to remove pollutants from wastewater. The mechanism is designed to allow polymerization sedimentation of organic pollutants, by adding coagulants and flocculants to the wastewater [30]. The commonly used coagulants are iron salts, ferrous salts, aluminum salts, and their polymers [31]. Polyacrylamides and their derivatives are frequently used as flocculants.
The xanthate anion can combine with many metal ions and readily forms salt species with a smaller solubility. Metal ions such as Fe2+, Cu2+, Pb2+, and Cd2+ [32] are usually added to the xanthate wastewater to form metal xanthate precipitates, and the removal of xanthate and soluble contaminants can be achieved by filtration. FeSO4, polymerized ferric sulfate (PFS), and polyacrylamide (PAM) were added to the wastewater containing high concentrations of xanthate by Sun et al. [33]. The removal of xanthate could reach more than 99% under neutral conditions. Meng et al. [31] indicated that the coagulation–flocculation process using polymerized ferric sulfate as a coagulant and polyacrylamide as a flocculant showed an insignificant removal effect in reducing COD (Chemical Oxygen Demand) in tailings wastewater. Yang et al. [30] reported that a coagulation–flocculation process was employed to treat flotation wastewater with a high concentration of suspended solids (SS) and organic pollutants, high alkalinity, and strong coloration, through the use of ferric chloride (FC) and polymerized aluminum chloride (PAC) as coagulants and polyacrylamide (PAM) as a flocculant. The results indicated that the FC/PAC compound coagulant coupled with PAM flocculant showed a high potential and economic feasibility for treating flotation wastewater with high turbidity and high alkalinity.
The coagulation–flocculation method has the advantages of simple operation and easy control. However, this treatment process requires a large amount of coagulants, making the cost high. In addition, the formation of a multitude sedimentation wastes can cause secondary pollution. In addition, this process is highly dependent on pH, which requires a large input of acid and alkali to adjust the solution pH. These drawbacks limit its application in the treatment of practical flotation wastewater.

3.2. Adsorption Methods

Adsorption is a simple, efficient, and economical method for removing residual xanthate from flotation tailings. The main technique is using adsorbents to separate the xanthates from the wastewater. The commonly used adsorbents include activated carbon, fly ash, zeolite, clay minerals, etc.
Activated carbon is effective as an adsorbent for the treatment of low concentration xanthate wastewater. Salarirad et al. [34] investigated the adsorption performance of ethyl xanthate (EX) in flotation wastewater using activated carbon. The results revealed that the removal rate of EX was more than 99% when the initial concentration of EX was 268 mg/L. Additionally, clay minerals and their modified forms have been widely applied as adsorbents for the treatment of xanthate, due to their stable physicochemical properties, large specific surface area, rich pore structure, and surface properties. Huang et al. [35] used cationic gemini surfactant-modified montmorillonite for the removal of ethyl, isobutyl, and isoamyl xanthates, and the three xanthates were completely removed within 60 min. This result indicated that modified montmorillonite was a promising adsorbent for the removal of xanthate from solution, but its regeneration performance needs to be confirmed. In addition, it was shown by Oliveira et al. that zeolites pretreated with sodium ions and then modified with copper ions exhibited excellent adsorption properties for isopropyl xanthate [36].
It was found that bentonite showed good adsorption properties for the removal of xanthate. Amrollahi et al. [3] reported the adsorption characteristics of bentonite modified with copper-manganese ferrite nanoparticles (Be-CuFe2O4, Be-MnFe2O4) on synthetic and actual mine wastewater with xanthate, in which the removal rate of residual xanthate reached more than 94% under optimal adsorption conditions. Rezaei et al. [11] applied acid-activated (H-Be) and aluminum-pillared (Al-Be) bentonite and obtained two modified adsorbent materials, which were successfully used for the removal of residual potassium amyl xanthate from solution. The adsorption performance of Al-Be was superior to H-Be, and the removal rate of residual xanthate reached over 99%. Li et al. [37] investigated the adsorption of xanthate from aqueous solution using multilayer graphene oxide and obtained a desirable removal rate of xanthate. The superior adsorption performance was attributed to the abundant oxygen-containing functional groups and large specific surface area of the multilayer graphene oxide.
Although adsorbents are inexpensive and widely available, their adsorption capacity is limited. They are usually applied to treat low concentration xanthate wastewater. Adsorbent clogging is a prominent issue when treating high concentration xanthate wastewater. In addition, the issue of regeneration and reuse of adsorbents needs to be further explored.

3.3. Biological Method

Microbiological techniques have been widely used to remove organic pollutants from various types of wastewater. Biological treatment of xanthate wastewater is accomplished through the decomposition of xanthate into CO2, CS2, and dixanthate with the action of microorganisms [38]. The degradation mechanisms are as follows [39]:
8ROCSS + 4H2O + O2 → 8ROH + 2CO32− + 2CS2 + 4CS32−
2ROH → 2RHO → CO2 + 2H2O
CS32− → CS2 + S2−
Chen et al. [38,40] found that domesticated microorganisms were beneficial for the biodegradation of xanthate. The results showed that the removal of xanthate reached 81.8% after 8 days of biodegradation, and the treatment process was consistent with the first order reaction kinetics. Furthermore, the biodegradation mechanism of xanthate revealed that CS2, ROCSSH, and monothiolcarbonate were the main degradation products, accompanied by the generation of a small amount of dixanthate oil droplets. Natarajan et al. [4] demonstrated that paenibacillus polymyxa and pseudomonas putida had the ability to degrade isopropyl xanthate, but the xanthate exerted toxic effects on bacterial growth when the concentration of xanthate exceeded 50 mg/L. To overcome the above limitations, bacterial strains resistant to high concentrations of xanthate were cultivated by continuous subculture. The significant effect of this strain on the degradation efficiency of the xanthate was mainly attributed to the acidic products produced by the bacterial metabolism.
Lin et al. [41] reported that an artificial microbial community SDMC (simultaneously degrade butyl xanthate and biomineralize cadmium) composed of hypomicrobium and sporosarcina after screening and domesticating in a short period of time achieved 100% butyl xanthate (BX) removal and 99% Cd biosorption, respectively. The main mechanisms of SDMC degradation and biomineralization of complex contaminants were attributed to decomposition, degradation, biomineralization, C-O bond breaking, and microbially induced carbonate precipitation (MICP) (as shown in Figure 1). In addition, CS2, butyl perxanthate (BPX), and (Ca0.67, Cd0.33) CO3 were the main products produced during the degradation of the composite pollution. Chockalingam et al. [42] reported the biodegradation of potassium isopropylxanthate in aqueous solution by bacillus polymyxa. Xanthate degradation product was observed at 1046 cm−1 via FTIR spectrum analysis, indicating the biodegradation mechanism. Cheng et al. [1] also designed an anaerobic–aerobic biological filter for the degradation of xanthate in synthetic flotation wastewater. As a result, an average COD removal rate of 88.7% for potassium ethyl xanthate was obtained under optimal reaction conditions, and a biological filter with volcanic rock as filler showed a favorable biodegradation performance for xanthate.
In view of their simplicity of operation, economy, and environmentally friendliness, researchers argue that biological methods are suitable for the treatment of xanthate wastewater [41]. However, this method also has defects, such as a low efficiency and long duration. In addition, the CS2 and alcohols produced by the decomposition of the xanthate are toxic to bacterial growth and activity, which not only leads to an increase in the permeability of lipid membranes and the inability to carry out normal metabolism, but also triggers the secondary contamination of CS2.

3.4. Oxidation Methods

Advanced oxidation processes (AOPs) are considered an effective and promising method for degrading refractory organic pollutants in wastewater. Many AOPs, such as ozone oxidation, Fenton oxidation, and photocatalytic oxidation have been widely used in the degradation of xanthates in mineral processing wastewater, due to their powerful oxidation capacities [43]. The reactive oxygen species (ROS) produced by the oxidation method, such as hydroxyl radicals (•OH), sulfate radicals (SO4), superoxide radicals (O2•), and hydroperoxyl radicals (HO2•) can oxidize organic pollutants into harmless small molecule compounds, in the form of CO2 and H2O [44,45]. Therefore, AOPs have great potential for the degradation of xanthate pollutants. The advantages and limitations of various AOPs are summarized in Table 1.

3.4.1. Fenton Methods

In recent years, Fenton oxidation technology has been widely investigated in the wastewater treatment field. The Fenton method includes homogeneous Fenton and non-homogeneous Fenton, according to the catalyst used. In the conventional Fenton oxidation process, Fe2+ catalyzes the reaction of H2O2 under acidic conditions, to form hydroxyl radicals (•OH) [46,47]. The •OH has a high redox potential of 2.8 eV, which can effectively degrade most organic pollutants (e.g., xanthate) in wastewater [46,48]. The amount of •OH produced mainly depends on the concentration of Fe2+ and H2O2 in the solution system. An appropriate concentration of Fe2+ and H2O2 is beneficial for the generation of •OH, but excessive Fe2+ and H2O2 will consume the produced •OH, which degrades the activity of the Fenton reagent [49]. Thus, it is essential to determine the optimal ratio of Fe2+ and H2O2 for xanthate wastewater treatment.
Fenton oxidation was used to remove residual xanthate from sulfide ore beneficiation wastewater by Ai et al. [50]. Under optimal conditions, the removal of xanthate from both synthetic and actual beneficiation wastewater was extremely high and met the required discharge standards, where the maximum allowable concentration of xanthate is 0.005 mg/L according to the Comprehensive Sewage Discharge Standard (GB8978−1996) [51]. In the case of excess Fe2+, H2O2 and Fe2+ were consumed rapidly and Fe3+ ions were formed, resulting in a change of the color of the treated water. Meanwhile, the efficiency of wastewater treatment decreased with a high initial pH. Based on this, another study by Ai et al. [19] reported the combination of ultrasound with a Fenton reagent. This combined process exhibited higher removal rates of xanthate wastewater. However, numerous inherent disadvantages exist of the conventional homogeneous Fenton reactions, such as strong acidic conditions and the production of iron sludge, which limit the wide application of the homogeneous Fenton method in wastewater treatment [52]. To further expand the applicability of the homogeneous Fenton method in the treatment of xanthate wastewater, shorten the reaction time, and improve the reaction efficiency, a non-homogeneous phase Fenton-like method has been developed by researchers. The non-homogeneous Fenton-like reaction is a general term for a class of reactions in which Fe3+, Fe-containing minerals, and other transition metals such as Co, Cd, Cu, Ag, Mn, Al, etc. accelerate or replace Fe2+ and catalyze H2O2 to produce active groups [53,54].
García-Leiva et al. [6] compared the effect of the Fenton process and a photo-Fenton process on the oxidative degradation of ethyl xanthate (EX) in aqueous solution. The results indicated that the photo-Fenton method could promote the complete mineralization of organic compounds and sulfur in xanthate, and its performance for EX removal was better than the conventional Fenton method. The potential application of acidified/calcined red mud (ACRM) as a catalyst of Fenton in the degradation of butyl xanthate was reported in another work [55]. The authors further explored the possible mechanisms of hydroxyl radical generation and butyl xanthate degradation in an ACRM-catalyzed Fenton-like process (as shown in Figure 2). The xanthate was first oxidized by •OH radicals to produce peroxynitrite, and then continued to be oxidized to H2O and CO2, and finally the xanthate was effectively removed. Similarly, Chen et al. [17] discussed the removal of n-butylxanthate from an aqueous solution through a non-homogeneous Fenton-like method that utilized fly ash as a catalyst. UV-Vis spectroscopic analysis (Shimadzu, Japan) showed that CS2 was the oxidation intermediate in the oxidation of n-butylxanthate. Under optimal test conditions, the xanthate and COD removal reached more than 96.90% and 96.66%, respectively. This result indicated the high oxidative activity of the process for the degradation of xanthate in aqueous solution.

3.4.2. Ozone Oxidation

The oxidation potential of ozone is 2.07 eV. Therefore, ozone oxidation may also be very effective for the treatment of xanthate wastewater. The oxidation reaction of ozone is divided into two pathways: one is the direct oxidation of molecular ozone, the other is the indirect oxidation through its own decomposition to produce hydroxyl radicals (•OH) and other free radicals [56,57]. In this process, pH is the main factor that affects the effectiveness of the xanthate wastewater treatment. Under acidic conditions, the organic components in xanthate wastewater are directly oxidized by O3; while in alkaline systems, xanthate is indirectly attacked by the highly oxidizing •OH produced by the rapid reaction of O3 with OH. However, ozone oxidation has the disadvantages of a limited mass transfer of ozone, short lifetime of the generated ozone and reactive oxygen species (ROS), and relatively high energy consumption of the ozone generated. Thus, its application in practical flotation wastewater treatment is limited [58]. In addition, it is difficult to completely oxidize and mineralize the toxic intermediates during the oxidation process through single O3 oxidation [59]. To overcome these drawbacks, using ozone in combination with other processes to improve the O3 oxidation efficiency has attracted a lot of attention from researchers. Current research on the treatment of xanthate using ozone has focused on comparing the degradation performance of ozone and the synergistic effect of ozone with other advanced oxidation processes, the purpose of which is to develop a more effective treatment method for xanthate wastewater.
It has been shown that the COD removal and ozone utilization are quite limited when using a single ozone oxidation process in the degradation of butyl xanthate [60]. The combination of ozone oxidation and UV radiation (O3/UV) is an effective combined process for the removal of refractory pollutants from wastewater [61]. Fu et al. [62] compared the performance of O3 and UV-O3 processes for the removal of four alkyl (sodium ethyl xanthate (SEX), sodium isopropylxanthate (SIPX), sodium n-butyl xanthate (SBX) and sodium namyl xanthate (SAX)) xanthates in a semi-batch bubble column reactor. Although UV spectral analysis showed that the ozone oxidation process had a high removal rate of alkyl xanthate in a short period of time, its mineralization effect was still unsatisfactory. On the contrary, the improvements in xanthate mineralization rate and ozone utilization in the UV/O3 process were mainly attributed to the easy decomposition of O3 under UV radiation, to form reactive radicals. In addition, a decomposition pathway of n-butylxanthate in the vacuum UV-ozone (VUV/O3) process has been proposed (as shown in Figure 3) [16]. UV promoted the decomposition of refractory xanthates by generating more •OH radicals. The C-O bond in the xanthate molecule was attacked by •OH in aqueous solution and the C=S bond in the –CSS functional group was broken by nucleophilic reactions, to form by-products such as n-butanol and CS2 [26]. These by-products were further attacked by •OH, to form CO2, H2O, and SO42−, with a smaller molecular mass. Fu et al. [63] compared the degradation performance of an ethyl xanthate (EX) collector using O3, UV254nm, UV185+254nm, O3/UV254nm, and O3/UV185+254nm processes. The O3/UV185+254nm process showed the highest degree of removal and mineralization of xanthate. However, a solid-phase extraction/GC-MS (Shimadzu, GCMS–QP2010 SE, Japan) analysis showed that the EX collector produced 13 organic by-products, and most of them could be further decomposed by O3/UV185+254 nm, but there were still a few by-products that were difficult to completely mineralize. Yan et al. [64] analyzed in detail the decomposition pathway and final oxidation products of potassium n-butyl xanthate (PBX) removal through an ozone oxidation process. In the presence of ozone, the PBX was significantly degraded to SO42− and n-butanol, accompanied by small amounts of carbon dioxide and gaseous sulfur. Direct and indirect ozone oxidation reactions occurred in the PBX solution. In the indirect ozone oxidation reaction of PBX, the continuous reactions between peroxide radicals (•HO2 and •O2) and PBX caused the decomposition of PBX into small organic and inorganic substances.

3.4.3. Photocatalytic Oxidations

TiO2 Photocatalysts

Titanium dioxide (TiO2) is the one of the most important semiconductor materials in the field of photocatalysis, because of its chemical stability, high catalytic activity, and non-toxicity. TiO2 mainly includes three crystalline types: anatase phase, rutile phase, and brookite phase [65]. The photocatalytic performance of anatase phase TiO2 was more active than the other two crystalline TiO2 types [66]. TiO2 is an n-type semiconductor compound, and its photocatalytic mechanism is shown in Figure 4 [67]. When TiO2 is irradiated in UV light with an energy (hv) greater than or equal to the band gap energy, the photogenerated electron (e) on its step band (vb) is transferred to the conduction band (cb). At the same time, photogenerated holes (h+) are generated in the valence band, and the electron-hole pairs migrate to the semiconductor surface, under the action of the electric field [68]. The holes can directly oxidize the organic compounds that are adsorbed on the surface of the TiO2. It can also react with H2O molecules to produce hydroxyl radicals (•OH) with an extremely high oxidizing power. Photogenerated electrons can reduce O2 molecules, to form superoxide radical anions (•O2) or hydrogen peroxide radicals (HO2•), as shown in Equations (14)–(20) [69,70]. Organic pollutants are oxidized by these reactive oxygen species (ROS) to generate small molecule compounds, such as CO2 and H2O, thereby achieving a clean treatment of organic pollutants.
TiO2 + hv→ hvB+ + ecB
hvB+ + ecB → Energy
hvB+ + H2O → •OH + H+
ecB + O2 → •O2
•O2 + H+ → HO2
2HO2• + ecB + H+ → H2O2 + O2
H2O2 + ecB → HO• + HO
However, pure TiO2 photocatalysts usually exhibit a low photocatalytic efficiency because of their drawbacks, such as a wide band gap, partial absorption of sunlight in the UV region, and rapid recombination of the photogenerated electron-hole pair, which limit the practicality of pure TiO2-based materials [71,72]. In addition, the low adsorption capacity and easy agglomeration of TiO2 particles also limit their further application in the field of photocatalysis [73]. To enhance the photocatalytic activity, modification methods such as doping modification, photosensitization, and semiconductor coupling to form heterostructures have been extensively studied by researchers [74,75,76]. The degradation efficiency of different modified catalysts for xanthate was compared, as shown in Table 2.
Doping modification mainly involves the introduction of new elements into the TiO2 crystal lattice, which leads to the generation of lattice defects in TiO2 and then causes a change in the energy band structure. This can effectively inhibit the complexation of photogenerated electron-hole pairs and narrow the band gap energy. It also causes the generation of oxygen vacancies in TiO2, resulting in the extension of the light absorption range, from the UV region to the visible region, and enhances the TiO2 photocatalytic activity [92,93]. Doping modification mainly consists of metal ion doping, non-metal ion doping, and noble metal doping. Metal ion doping mainly utilizes Cu2+, Mn2+, and Fe3+ [94,95,96]. Non-metal ion doping primarily includes C, N, and S [97]. The doping of noble metals mostly involves Au and Ag [98].
Bian et al. [77] demonstrated that nitrogen-cerium co-doped TiO2 showed an effective degradation of xanthate under visible light irradiation. This work synthesized new Ce/N-TiO2@AC photocatalysts using the sol-gel method, which achieved 96.3% SIBX removal under visible light irradiation. This degradation process can be briefly described as follows: the SIBX molecules were first adsorbed on the surface of the Ce/N-TiO2@AC photocatalyst. Subsequently, the band gap of TiO2 was significantly reduced. This suggested that the valence band (vB) of TiO2 could be strongly activated to form holes (h+), while photoinduced electrons (e) could be further activated into the conduction band (cB). In addition, the electrons captured in the Ce4+/Ce3+ sites could be transferred to the catalyst surface, leading to the generation of numerous strong oxidation free radicals (e.g., •O2 and •OH). Finally, the SIBX molecules attacked by these radicals could be completely decomposed into inorganic sulfates (SO42−). Li et al. [14] evaluated the effects of Ag-TiO2-FAMB photocatalysts on the photocatalytic degradation performance of butyl sodium xanthate. This work summarized the potential mechanisms of xanthate degradation, as shown in Figure 5. The results of free radical quenching tests showed that superoxide radicals (•O2), hydroxyl radicals (•OH), and photogenerated holes (h+) play a joint role in the photocatalytic degradation of sodium butyl xanthate. Among them, superoxide radicals (•O2) play a major role in the degradation of xanthate. Furthermore, the pathway of xanthate degradation showed that peroxanthate (C4H9OCSSO) was the intermediate product of the xanthate degradation process, and xanthate was finally degraded to CO2, SO42−, and H2O.
Recent studies have shown that Ag/semiconductor/clay mineral terpolymer composites present excellent adsorption and photocatalytic synergistic effects for pollutant degradation [99]. Zeolite has a promising application as a TiO2 carrier, due to its abundant porous structure and stable chemical properties [100]. Shen et al. [78] indicated that TiO2/clinoptilolite showed an excellent performance for photodegradation of SIPX compared with that of pure TiO2. The fast degradation of SIPX under UV irradiation was attributed to the synergistic effect of the clinoptilolite adsorbent and TiO2 photocatalyst in the composite nanoparticles. The SIPX was eventually degraded to small molecule inorganic pollutants such as CO2 and SO42−. Zhou et al. [101] investigated the photocatalytic degradation mechanism on xanthate pollutants with Ag/TiO2/clinoptilolite (ATC) nanocomposite as a composite catalyst. The results showed that the photogenerated electrons reacted with the absorbed O2 to produce •O2, and the generation of holes on the TiO2 valence band could react with OH and H2O to produce •OH. Then, the •O2 and •OH could oxidize xanthate directly to CO2 and H2O. Tan and co-workers reported that the new PANI/TiO2/metakaolin showed desirable performance in the degradation of butyl xanthate with visible light irradiation compared to pure TiO2 [79]. The excellent adsorption of butylxanthate of the composites facilitated the removal of xanthate within 4 h under visible light irradiation, and the degradation rate reached 94.8%.
Recently, two-dimensional nanomaterials have been used as co-catalysts, such as graphene oxide (GO), graphitic phase carbon nitride (g-C3N4), and molybdenum disulfide (MoS2) to promote the photocatalytic activity of single TiO2. These 2D nanomaterials have narrow band gaps and can be coupled with TiO2 to form heterogeneous structures. Zhou et al. [80] prepared a new MoS2/TiO2/clinoptilolite (MTC) nanocomposite with ternary non-homogeneous, which showed an excellent photocatalytic activity under visible light irradiation. The excellent adsorption capacity of clinoptilolite and the MoS2/TiO2 heterostructures in the nanocomposites provided favorable conditions for the efficient degradation of xanthate. Tang et al. [81] prepared b-TiO2@MoS2 heterostructures using electrochemical exfoliation combined with a sol-gel technique. This heterostructure showed stronger visible light absorption and broader active sites than the pure b-TiO2 material, and also facilitated the separation and transfer of photogenerated charges. Under visible light irradiation, the maximum efficiency of xanthate degradation could reach 94.80% at a catalyst loading of 0.03 g. New TiO2/graphene (TiO2/GO) composite photocatalytic materials were synthesized and used as photocatalysts to treat xanthate wastewater [15]. The degradation xanthate mechanism is shown in Figure 6. The electrons were transferred from the valence band to the conduction band to generate electron-hole pairs, promoting the formation of strong oxidation free radicals (e.g., •O2 and •OH), and degraded the xanthate. Therefore, TiO2/GO has great potential to treat xanthate wastewater, due to its huge specific surface area, smaller pore size, and lower electron–hole pair complexation rate.

Bi-Based Oxide Photocatalysts

Bismuth (Bi)-based oxide photocatalysts mainly include Bi2O3, bismuth oxide halide BiOX (X = Cl, Br, I), Bi2MO (M = Cr, Mo, W), which have captured the attention of researches for application in flotation wastewater treatment, due to a high quantum yield, strong visible light response, and controllable structure [102,103]. The electronic structure of these bismuth-based materials is unique and exhibits a great photocatalytic degradation capacity under UV-vis irradiation [104]. However, the relatively small specific surface area and faster recombination of photogenerated carriers of single bismuth-based oxides lead to a less desirable photocatalytic activity for organic compounds [105]. Therefore, various strategies have been systematically investigated by researchers, such as heterojunctions, metal and non-metal doping, surface modification, and internal coupling between different bismuth metal oxides, to improve the photolytic ability of photocatalysts [106,107].
BiFeO3 and Bi2Fe4O9 as narrow band gap semiconductors have good magnetic properties, strong acid and alkali resistance, and excellent chemical stability, and are promising visible light drive materials [108,109]. Recently, Yang et al. prepared a bentonite-based bismuth ferrite composite containing BiFeO3 and Bi2Fe4O9 using a mechanical activation technique [84,85]. The activated bentonite-based bismuth ferrate (A-BiFe/Bent) was used as a non-homogeneous Fenton catalyst for the degradation of ethyl xanthate (EX) from flotation wastewater. Excellent activity under visible light was observed, and the degradation pathway as well as a possible reaction mechanism for the catalytic degradation of EX were proposed, as shown in Figure 7 [84,85]. Similarly, as reported by Zhu et al. [83], A-BiFe/Bent as a catalyst combined with a persulfate (PS) activation system under visible light irradiation could effectively degrade ethylxanthate (EX), providing a potential pathway for the treatment of flotation wastewater.
In recent years, carbon-based materials such as graphite and 2D g-C3N4 have been widely used in the field of wastewater treatment, because of their large surface area and good adsorption properties [110]. Mic-g/BiOI nanocomposites showed superior photocatalytic activity in the removal of xanthate residues from sulfide ore flotation wastewater, with a 4.3-times higher removal rate than that of pure BiOI [86]. The reason for this was mainly attributed to the synergistic effect of the desirable contact interface between BiOI and graphite and the good electrical conductivity. Similarly, Yuan and co-workers [87] found that a BiOBr/g-C3N4−10% photocatalyst exhibited superior photodegradation activity of ethylxanthate in mineral flotation wastewater under visible light and had an excellent reusability compared with single BiOBr and g-C3N4. The increase in visible light-driven photodegradation activity was mainly attributed to the construction of S-scheme BiOBr/g-C3N4 heterojunctions and the effective separation of photogenerated carriers.
The application of alkaline bismuth subcarbonate ((BiO)2CO3) as a novel photocatalyst has recently attracted considerable attention [88,111]. Cui et al. [88] obtained (BiO)2CO3 nanowire photocatalysts through a simple hydrothermal method and applied them to degrade sodium isopropylxanthate. It was shown that (BiO)2CO3 nanowires exhibited excellent photocatalytic degradation of xanthates, due to a narrow band gap under UV-vis irradiation. It was also found that (BiO)2CO3@Bi2S3 core-shell nanowires were produced during the photocatalytic decomposition of xanthate solution and showed an enhanced photocatalytic activity after repeated cycles of testing. However, the effect of the S content on the performance of core-shell (BiO)2CO3@Bi2S3 was still unclear. The luminescence (PL) properties of (BiO)2CO3@Bi2S3 core-shell nanowires with different S contents were further studied [111]. The results indicated that (BiO)2CO3 could be easily converted to Bi2S3 in the presence of S and rapidly formed (BiO)2CO3@Bi2S3 heterojunctions, which was beneficial for the efficient separation of electron-hole pairs. In addition, sodium isopropylxanthate was eventually oxidized to SO42− and CO2 by the (BiO)2CO3 nanowires doped with Fe3+ under UV light irradiation. The Fe3+-doped (BiO)2CO3 nanowires exhibited excellent performance in the removal of sodium isopropylxanthate compared to pure (BiO)2CO3 nanowires, due to a narrow band gap and wider specific surface area [89].

ZnO-Based Photocatalysts

ZnO is an n-type semiconductor compound with high photosensitivity and thermal stability, which has the advantages of eco-friendliness, low band gap energy, and low cost. However, ZnO has a wide band gap of about 3.37 eV, resulting in easy recombination of the photogenerated carriers, and it only absorbs UV light <387 nm, which greatly limits its application in wastewater treatment [112,113]. To overcome these drawbacks, various modifications, such as elemental doping, and surface modification of noble metals and heterojunctions [114], have been applied to the ZnO structure, to extend its functionality under solar irradiation. In particular, element-doped nanocomposite photocatalysts have proven to be promising in the field of mine wastewater treatment.
ZnO doped with Mn and Fe was reported as an active photocatalyst for the production of oxygen from water under visible light irradiation. Xiao et al. [115,116] performed photocatalytic degradation of potassium ethylxanthate (KEX) under visible light irradiation with Zn1-xMnxO and Zn1-xFexO photocatalysts. The results indicated that Zn0.95Mn0.05O and Zn0.96Fe0.04O presented excellent photocatalytic degradation efficiency for xanthate, due to the low complexation of photogenerated electron-hole pairs. Additionally, Xin and co-workers [28] synthesized new PW9@ZnO/Ag(PZA) composites that were doped with Ag nanoparticles using a hydrothermal method. The application of PZA to degrade BX in simulated wastewater under UV light and xenon lamp showed good photocatalytic degradation. Meanwhile, the mechanism of PZA photocatalysis was analyzed, including quantum photon absorption, electron-hole pair generation, and separation and redox reaction processes, as shown in Figure 8.

4. Conclusions and Future Research Requirements for the Treatment of Xanthate Wastewater

The traditional coagulation–flocculation and adsorption methods usually have difficulty in making xanthate wastewater meet discharge standards, due to their relatively low removal rate. Biological methods fail to realize the clean degradation of xanthate fully, and the formation of CS2 is toxic and still harmful to the environment. Thus, in the actual wastewater purification processes, a treatment method that can completely decompose the xanthate into small molecule compounds, such as CO2, H2O, and SO42− would be the best candidate.
With the wide application of advanced oxidation processes (AOPs) in wastewater treatment, the Fenton and ozone oxidation have been extensively discussed in the field of xanthate wastewater treatment. This process utilizes the highly reactive •OH that is generated by AOPs to oxidize and decompose the residual xanthate in wastewater. Due to the strong oxidizing ability of •OH, this can make the organic substances of xanthate decompose rapidly and eventually mineralize into CO2, H2O, and SO42−. Although the research on catalytic oxidation technology has made great progress, there are still many problems that need to be solved. For example, the oxidative decomposition pathway and intermediate product analysis of xanthate in wastewater are still controversial. Furthermore, the complex and variable composition and nature of actual xanthate wastewaters render catalytic oxidation technology difficult to scale up for industrial applications.
Photocatalytic oxidation shows outstanding performance in the treatment of xanthate wastewater. The technique involves the generation of electron-hole pairs through electron transitions within the semiconductor under visible or ultraviolet light. This promotes the generation of highly reactive oxygen radicals, which further break down the xanthate molecule. However, various semiconductor photocatalysts have the disadvantages of a fast recombination of photogenerated electron-hole pairs and narrow light absorption range during the degradation of xanthate, so a great deal of research is still needed regarding improving the photocatalytic activity.
Briefly, the treatment of xanthate wastewater is of great significance for the sustainable development of the mineral processing industry and environmental protection. The available various treatment methods have both advantages and disadvantages. The main reasons being that the existing technology is quite specialized, focusing on a specific aspect. In other words, researchers have mainly studied the treatment of xanthate wastewater based on the perspective of single discipline, and this research paradigm is not conducive to technological innovation. Currently, the treatment effect, cost, and efficiency are usually not comprehensively assessed. Furthermore, most research is now focused on simulated xanthate wastewater at laboratory scale, while actual mine plant xanthate wastewater is less tested. In fact, actual mine plant xanthate wastewater often has more intricate physico-chemical properties. Thus, the adaptability and repeatability of each method needs to be highly valued, although desirable results can be obtained with simulated xanthate wastewater at laboratory scale. In addition, combined methods for the treatment of xanthate wastewater will be conducive to achieving high efficiency, low costs, and no secondary pollution. We believe that interdisciplinary research of a more organized manner may generate more original and innovative technologies for the treatment of xanthate wastewater in the future.

Author Contributions

Conceptualization, Writing-review and editing, J.Y.; Data curation, S.L.; Investigation, Data curation, Z.D.; Conceptualization, Methodology, J.L.; Visualization, Investigation, A.Y.; Conceptualization, Writing review and editing, S.B.; Supervision, S.W. All authors have read and agreed to the published version of the manuscript.


This research was funded by the National Natural Science Foundation of China, grant number No. 52164021.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. Degradation/biomineralization pathway sketch map (Reproduced from J. Environ. Manage. 2022, 316, 115304 with permissions from Elsevier, Netherlands) [41].
Figure 1. Degradation/biomineralization pathway sketch map (Reproduced from J. Environ. Manage. 2022, 316, 115304 with permissions from Elsevier, Netherlands) [41].
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Figure 2. Production of hydroxyl radicals and degradation pathway of butyl xanthate (reproduced from Environ. Sci. Pollut. Res. Int. 2016, 23, 15202−15207 with permission from Elsevier) [55].
Figure 2. Production of hydroxyl radicals and degradation pathway of butyl xanthate (reproduced from Environ. Sci. Pollut. Res. Int. 2016, 23, 15202−15207 with permission from Elsevier) [55].
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Figure 3. Decomposition pathway of n-butylxanthate in the VUV/O3 process (reproduced from Process Saf. Environ. Prot. 2016, 102, 64−70 with permission from Elsevier) [16].
Figure 3. Decomposition pathway of n-butylxanthate in the VUV/O3 process (reproduced from Process Saf. Environ. Prot. 2016, 102, 64−70 with permission from Elsevier) [16].
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Figure 4. TiO2 photocatalytic mechanism diagram [67].
Figure 4. TiO2 photocatalytic mechanism diagram [67].
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Figure 5. Xanthate degradation mechanism diagram (Reproduced from Colloids Surf. Physicochem. Eng. Aspects 2021, 624 with permission from Elsevier) [14].
Figure 5. Xanthate degradation mechanism diagram (Reproduced from Colloids Surf. Physicochem. Eng. Aspects 2021, 624 with permission from Elsevier) [14].
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Figure 6. Mechanism of the degradation of xanthate (Reproduced from Chem. Eng. J. 2022, 431 with permission from Elsevier) [15].
Figure 6. Mechanism of the degradation of xanthate (Reproduced from Chem. Eng. J. 2022, 431 with permission from Elsevier) [15].
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Figure 7. Mechanism of A-BiFe/Bent-catalyzed degradation of EX under visible light (reproduced from Opt. Mater. 2022, 132 with permission from Elsevier) [85].
Figure 7. Mechanism of A-BiFe/Bent-catalyzed degradation of EX under visible light (reproduced from Opt. Mater. 2022, 132 with permission from Elsevier) [85].
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Figure 8. Diagram of the photocatalytic reaction mechanism (reproduced from Environ. Res. 2022, 214, 113776 with permission from Elsevier) [28].
Figure 8. Diagram of the photocatalytic reaction mechanism (reproduced from Environ. Res. 2022, 214, 113776 with permission from Elsevier) [28].
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Table 1. The advantages and limitations of various AOPs.
Table 1. The advantages and limitations of various AOPs.
Fenton• Rapid degradation rate
• Mild reaction conditions
• Convenient operation
• High dosage of reagents
• Limited pH range
• Catalysts are not easily recovered
•Excess Fe2+ increases the color and COD of the treated water
• Secondary pollution
Ozonation• Short reaction time
• High removal rate of xanthate
• Limited mass transfer efficiency and utilization productivity
• High treatment costs
•Low mineralization rate of the intermediates
• Intermediate products increase the COD content of the effluent
PhotocatalyticsTiO2 photocatalysts• Low cost
• Innocuous and harmless
• Strong redox capability
• Excellent chemical stability
• Wide band gap
• Electron-hole pairs are unstable
• Narrow light absorption range
• Low adsorption capacity
•High concentration nano-TiO2 suspensions are easy to agglomerate
Bi-based oxide photocatalysts• Innocuous and harmless
• Excellent chemical stability
• Unique layered structure
• Electron-hole pairs are unstable
• Visible light absorption is limited to a specific wavelength range
ZnO-based photocatalysts• Excellent photophysical properties
• Low cost
• High light sensitivity and thermal stability
• Wide band gap
• Photoinduced carriers are unstable
• Low sunlight usage rate
Table 2. The degradation efficiency of different modified catalysts for xanthate.
Table 2. The degradation efficiency of different modified catalysts for xanthate.
PhotocatalystLight SourcePollutantSynthesis MethodsDegradation (%)Refs.
Ce/NeTiO2@ACVisible light irradiationSodium isobutyl xanthateSol-gel method95.80[77]
Ag-TiO2-FAMBVisible light irradiationSodium butyl xanthateSol-gel method98.50[14]
TiO2/clinoptiloliteUV irradiationSodium isopropyl xanthateHydrothermal method>90[78]
PANI/TiO2/metakaolinVisible light irradiationButyl xanthateSol–gel and in-situ polymerization94.8[79]
MoS2/TiO2/clinoptiloliteVisible light irradiationSodium isopropyl xanthateModerate hydrothermal route>90[80]
b-TiO2 @MoS2Visible light irradiationButyl xanthateSol-gel method94.80[81]
TiO2/ graphene nanocompositesVisible light irradiationPotassium butyl xanthateHydrothermal method97.03[15]
TiO2/g-C3N4Visible light irradiationPotassium butyl xanthateHydrothermal method97.10[82]
C, N, S-tridoped TiO2 nanotubesVisible light irradiationPotassium ethyl xanthateHydrothermal method-[83]
A-BiFe/BentVisible light irradiationEthyl xanthateMechanically activated98.60[84]
A-BiFe/BentVisible light irradiationEthyl xanthateMechanically activated97.85[85]
A-BiFe/BentVisible light irradiationEthyl xanthateMechanically activated98.43[86]
micro graphite/BiOIVisible light irradiationXanthateHydrothermal94.36[51]
S-scheme BiOBr/g-C3N4Visible light irradiationEthyl xanthateHydrothermal96.10[87]
(BiO)2CO3 nanowiresUV-visible light irradiationSodium isopropyl xanthateHydrothermal96.00[88]
Fe3+-doped (BiO)2CO3UV-visible light irradiationSodium isopropyl xanthateHydrothermal95.71[89]
BiOBr@TiO2/PU-SFVisible light irradiationEthyl xanthateA facile blending-phase separation-impregnation-precipitation97.85[90]
BiOCl/TiO2/clinoptiloliteVisible light irradiationSodium isopropyl xanthateA facile hydrothermal route combining with a water bath precipitation procedure>90[91]
PW9@ZnO/AgUV-visible light irradiationButyl xanthateHydrothermal99.83[28]
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Yuan, J.; Li, S.; Ding, Z.; Li, J.; Yu, A.; Wen, S.; Bai, S. Treatment Technology and Research Progress of Residual Xanthate in Mineral Processing Wastewater. Minerals 2023, 13, 435.

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Yuan J, Li S, Ding Z, Li J, Yu A, Wen S, Bai S. Treatment Technology and Research Progress of Residual Xanthate in Mineral Processing Wastewater. Minerals. 2023; 13(3):435.

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Yuan, Jiaqiao, Suqi Li, Zhan Ding, Jie Li, Anmei Yu, Shuming Wen, and Shaojun Bai. 2023. "Treatment Technology and Research Progress of Residual Xanthate in Mineral Processing Wastewater" Minerals 13, no. 3: 435.

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