A Novel Eco-Friendly Circular Approach to Comprehensive Utilizing Bittern Waste and Oyster Shell

Abstract

Efficient waste management, especially in relation to swaste reuse, has become a pressing societal issue. The waste bittern generated during salt production and discarded oyster shells present formidable environmental challenges and a waste of resources for some coastal regions. Therefore, this work developed a two-stage circular process for the environmentally friendly and efficient utilization of both waste materials. In the first stage, CO₂ gas and an organic extraction phase comprising Tri(octyl-decyl)amine (R₃N) and isoamyl alcohol were introduced into the waste bittern to obtain MgCO₃·3H₂O (s). The second stage involved reacting the reacted organic extraction phase with oyster shell powders to produce CaCl₂·2H₂O (s) and CO₂ (g) and regenerate R₃N. This work focused on investigating the yield of MgCO₃·3H₂O and the regeneration ratio of R₃N, which are crucial indicators for the two stages involved in the process. The results indicate that, under optimal operating conditions, a maximum yield of 87% for MgCO₃·3H₂O was achieved, and the regeneration ratio of R₃N reached 97%. Furthermore, the reaction mechanism and thermodynamic functions of the R₃N regeneration process were elucidated as a crucial element in achieving a circular process. The findings of this work offer a sustainable solution to environmental pollution from waste bittern and oyster shells, and provide a promising avenue for green chemical production.

1. Introduction

Effective waste treatment and utilization, particularly of biomass wastes [1,2] and difficult-to-handle garbage [3], can promote sustainability and cost-effectiveness within green chemistry and engineering [4]. Waste bittern is a by-product of the salt-making industry and desalination processes, consisting mainly of salar bittern and sea bittern. Its release into the environment can have detrimental effects on living organisms [5,6]. Usually, the concentrations of magnesium ions and chloride ions in waste bittern are higher than those of other ions. Thus, the recovery of both salts can alleviate or eliminate environmental pollution associated with hem.

There are several techniques for salt recovery. For instance, chloride ions are recycled by concentration, crystallization, and electrolysis [7,8,9], whereas magnesium ions are recycled by trioctylamine and N235 extraction [10,11], membrane electrolysis [9], and via the preparation of Mg(OH)₂, MgCO₃, and struvite [12,13,14]. However, these techniques for removing ions from waste bittern suffer from drawbacks such as secondary pollution, high costs, and low efficiency. Therefore, there is a need for an environmentally friendly and efficient process to treat waste bittern before disposal into the environment.

Oyster breeding is a thriving global industry that generates a significant by-product: oyster shells, consisting mainly of calcium carbonate (approximately 95%). However, these shells are frequently disposed of in coastal waters or landfills, resulting in the emission of harmful gases, including NH₃, H₂S, and amines, as well as water pollution and the proliferation of insect breeding sites exacerbating environmental pollution [15]. Many research studies suggest that oyster shells can be utilized in various ways, including as fillers in the material industry. Many research studies suggest diverse uses for oyster shells, including as fillers in the material industry [16,17], building material [18,19], composite material [20,21,22], and in chemical engineering for wastewater treatment [23,24], carbon dioxide absorption [25,26], catalysis [27,28], and as a source of calcium.

However, these alternative uses often involve processing single, cheap oyster shells with large amounts of chemical raw materials or energy, resulting in high costs and poor returns. To address these challenges, a new process should be developed that allows oyster shells and other waste materials to interact with each other. This reduces the need for additional chemicals or energy, ultimately leading to reduced costs and promoting the simultaneous reuse of multiple waste streams. This process represents a critical step toward the efficient and sustainable utilization of oyster shells and other waste materials.

Therefore, the work explores a novel environmentally friendly process that utilizes oyster shells and waste bittern, both of which are sourced from the sea, to produce magnesium carbonate and other by-products. Two stages are planned in this process. In the first stage, an organic extraction phase comprising R₃N, isoamyl alcohol, and CO₂ from oyster shells are introduced into a waste bittern to yield MgCO₃·3H₂O. In the second stage, the used organic extraction phase is reacted with oyster shell powders to form CaCl₂·2H₂O, CO₂, and R₃N. Both R₃N and CO₂ are considered as recyclable materials and are reintroduced into the first stage. The process is optimized by systematically investigating the effects of reactant dosages, reaction time, stirring speed, temperature, and so on. Additionally, the reaction mechanism of the R₃N regeneration process, which is crucial to this process, was studied. Compared to previous relevant research, this process represents a significant advancement in the simultaneous and environmentally friendly utilization of two primary sea wastes to produce chemical products without generating any new waste. All by-products are recycled within the system, resulting in an efficient and sustainable process that effectively utilizes waste materials. Therefore, the results of this study provide a new perspective on the utilization of oyster shells and bittern.

2. Materials and Method

2.1. Materials

Waste bittern was obtained from Quanzhou Seawater Salt Factory in Quanzhou, China, and the oyster shells were sourced from a seafood market in Xiamen, China. They were characterized by an X-ray fluorescence spectrometer (ARL Perform‘X 4200, Thermo Fisher Scientific, Waltham, MA, USA), and the results are shown in

Table 1. Chemical composition of waste bittern.

Element	Cl	Mg	Na	S	K	Al	Br
Mass fraction/%	18.24	4.76	4.15	2.71	1.35	0.54	0.25

and

Table 2. Chemical composition of oyster shell.

Element	Ca	Na	S	Cl	Mg	Si	Al
Mass fraction/%	21.29	0.64	0.20	0.42	0.21	0.09	0.05

, respectively. Ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA, ≥99.5%, CAS: 60-00-4), zinc oxide (≥99.0%, CAS: 1314-13-2), ammonia solution (25~28%, CAS: 1336-21-6), ammonium chloride (≥99.5%, CAS: 12125-02-9), chrome black T (IND, CAS:1787-61-7), phenolphthalein (IND, CAS: 77-09-8), sodium hydroxide (≥96.0%, CAS: 1310-73-2), calcium chloride (≥96.0%, CAS: 10043-52-4), and isoamyl alcohol (≥98.5%, CAS: 123-51-3) were all obtained as analytical reagents from Xilong Science Co., Ltd. (Guangdong, China). Hydrochloric acid (36.0~38.0%, CAS: 7647-01-0) and calcium-carboxylic acid indicator (IND, CAS: 3737-95-9) were supplied as analytical reagent by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Tri (octyl-decyl) amine (R₃N, ≥95%, 68814-95-9) was sourced as analytical reagent from Wuhan yuancheng co-create technology Co. Ltd. (Hubei, China). Carbon dioxide (≥99.99%, CAS: 124-38-9) gas was from Quanzhou tianxing gas co. Ltd. (Fujian, China).

2.2. Method

The experimental process was divided into two parts, including waste bittern utilization process (Section 2.2.1) and oyster shell utilization process (Section 2.2.2). The process flow diagram is shown in Figure 1. All experiments were conducted thrice, and the maximum relative error observed was below 5%.

2.2.1. Waste Bittern Utilization

The experimental process involved the mixing of waste bittern with CaCl₂, resulting in the formation of CaSO₄·2H₂O. The resultant mixture was subjected to a filtration process, which separated the solid CaSO₄·2H₂O residue from the waste bittern filtrate, which was free of SO₄²⁻ ions. The equation of the reaction is as follows:

CaCl₂ + MgSO₄ + 2H₂O → MgCl₂ + CaSO₄·2H₂O↓

(1)

The next step involved subjecting the waste bittern filtrate without SO₄²⁻ ions to a process that included mixing it with carbon dioxide, R₃N, and isoamyl alcohol using a half-moon impeller in a 1000 mL three-necked-flask at 313.15 K. Filtration of the resulting mixture yielded a solid residue of MgCO₃·3H₂O and a filtrate consisting of residual waste bittern and an organic solution [29,30,31,32]. The organic solution was subsequently employed in the oyster shell utilization process. The composition of the solid residue was analyzed using X-ray fluorescence spectroscopy (XRF), and the result is tabulated in

Table 3. Chemical composition (%) of MgCO₃·3H₂O as analyzed by XRF.

Compound	Mg	Ca	Si	S	Al
Wt%	50.13	0.996	0.059	0.035	0.042

. Based on these results, the maximum purity that can be achieved is 98.75% in this work.

The equation of the reaction in this step is as follows:

MgCl₂ + 2R₃N + CO₂ + 4H₂O → MgCO₃·3H₂O↓ + 2R₃N·HCl

(2)

The effects of different operating parameters, including the dosages of waste bittern, R₃N, and isoamyl alcohol, reaction time, and stirring speed, on the yield of MgCO₃·3H₂O was systematically investigated. Further details on the characterization of the relevant substances in the waste bittern utilization process can be found in Appendix A. The yield of MgCO₃·3H₂O was calculated as follows:

$${\lambda }_1=\frac{n_1}{n}\times 100\%$$

(3)

where λ₁ denotes the yield of MgCO₃·3H₂O, n₁ represents the moles of MgCO₃·3H₂O, and n represents the moles of magnesium in the waste bittern. The number of moles was determined by measuring the concentration of magnesium ion through titration method and calculating the volume used.

The titration method involved two steps. In the first step, the sample was diluted 100 times and mixed with triethanolamine solution, potassium hydroxide solution, and calcium–carboxylic acid indicator to determine the concentration of calcium ion. The resulting solution was titrated with a known concentration of EDTA solution (0.0213 M) until the color changed from purple to blue. In the second step, another sample from same solution was also diluted 100 times and mixed with triethanolamine solution, potassium hydroxide solution, ammonia–ammonium chloride solution, and chrome black T solution to determine the concentration of magnesium ion. This mixture was titrated with the known concentration of EDTA solution (0.0213 M) until the color changed from purple to blue. The concentration of magnesium ions was calculated based on the volume of twice-EDTA-used solution. Each titration experiment was repeated three times, and the relevant titration data can be found in Appendix B.

2.2.2. Oyster Shell Utilization

Initially, the oyster shells were thoroughly washed and ground into a fine powder. Next, the purified oil phase obtained from the first stage process, which contained R₃N·HCl, was blended with the oyster shell powder and distilled water to produce R₃N, CaCl₂, and CO₂. Calcium chloride dihydrate was then acquired by crystallizing a calcium chloride solution. The composition of CaCl₂·2H₂O was evaluated using XRF, and the corresponding results are provided in

Table 4. Chemical composition (%) of CaCl₂·2H₂O as analyzed by XRF.

Compound	Cl	Ca	Mg	Na	S
Wt%	52.79	31.38	0.694	0.622	0.217

. Based on these results, the maximum that can be achieved is 96.63%.

The chemical reaction is as follows:

CaCO₃ + 2R₃N·HCl → CaCl₂ + 2R₃N + H₂O + CO₂↑

(4)

The characterizations of the relevant substances involved in the oyster shell utilization process can be found in Appendix A.

Although obtaining CaCl₂·2H₂O is an essential objective in the oyster shell utilization process, the regeneration of R₃N is even more crucial because it plays a pivotal role in achieving a continued cycle in the two-stage process. Specifically, the occurrence of the R₃N regeneration reaction directly impacts the feasibility of the circle process, while also positively affecting the conversion rate of calcium chloride. Therefore, the regeneration ratio of R₃N is a crucial assessment index for this process, and it was determined using the following calculation:

$${\lambda }_2=\frac{{n_0}_{({\mathrm{R}}_3\mathrm{N}·\mathrm{H}\mathrm{C}\mathrm{l})}{-n}_{r({\mathrm{R}}_3\mathrm{N}·\mathrm{H}\mathrm{C}\mathrm{l})}}{{n_0}_{({\mathrm{R}}_3\mathrm{N}·\mathrm{H}\mathrm{C}\mathrm{l})}}\times 100\%$$

(5)

where λ₂ represents the regeneration ratio of the R₃N, and n_r (R₃N·HCl) and n₀ (R₃N·HCl) denote the final and initial moles of R₃N·HCl in process, respectively. The molar concentration of R₃N·HCl was determined through titration using a calibrated NaOH solution. The effects of operating parameters, including phase ratio between aqueous and oil phases and reaction temperature, on the regeneration ratio were investigated. Additionally, the thermodynamic function and reaction mechanism of the regeneration process were established in this work.

3. Results and Discussion

3.1. Effects of the Operating Conditions

3.1.1. Effect of Phase Ratio of Reactants on the Yield of MgCO₃·3H₂O

Figure 2 presents the effects of the phase ratio of waste bittern, R3N, and isoamyl alcohol on the yield of MgCO₃·3H₂O. Figure 2a shows an increase in the phase ratio between R3N and waste bittern, and an initial improvement in the yield of MgCO₃·3H₂O followed by a subsequent decrease. The results demonstrate that R₃N, as an extractant, directly impacts the yield of the process. However, due to the high viscosity of R₃N, the mixing efficiency between reactants is reduced when the phase ratio between R₃N and waste bittern is larger than 4.8, ultimately resulting in a decrease in the yield. Isoamyl alcohol, as a diluent, has the capability of reducing the viscosity of the organic solution. Therefore, Figure 2b demonstrates that the addition of isoamyl alcohol improves the yield of MgCO₃·3H₂O by increasing the probability of contact between reactants through the viscosity reduction. However, when the dosage of isoamyl alcohol is higher, it can surround R₃N molecules, thereby hindering the contact between reactants and R₃H and ultimately resulting in a decrease in the yield. The results show that the best improvement in the yield of MgCO₃·3H₂O is achieved when the phase ratio of isoamyl alcohol is the same as that of R₃H. The optimal phase ratio of waste bittern, R3N, and isoamyl alcohol in this work for achieving the highest yield is 1:4.8:4.8, respectively, which results in a yield of 87%.

3.1.2. Effect of Reaction Time and Stirring Speed on the Yield of MgCO₃·3H₂O

The effects of the reaction time and stirring speed on the yield of MgCO₃·3H₂O are shown in Figure 3. As seen in Figure 3a, the yield of MgCO₃·3H₂O increased with an increasing reaction time and balanced at the 7th hour, after which there was no significant change in the yield, indicating that the reaction had reached its optimal point. Longer reaction times resulted in more extraction reactions between the reactant molecules and a higher yield of MgCO₃·3H₂O; further increases in time had a limited effect on the yield. Therefore, an optimal reaction time of 7 h can be used to obtain a high yield of MgCO₃·3H₂O in this work.

It is evident from Figure 3b that the yield of MgCO₃·3H₂O gradually increased with an increase in stirring speed from 300 to 600 rpm and then remained generally constant. This is due to the fact that, at low stirring speeds, the oil phase and water phase were not mutually soluble, making it difficult for the reactants to come into adequate contact. A higher stirring speed, on the other hand, increased the contact between the reactants, resulting in an increased reaction rate and a higher yield of MgCO₃·3H₂O. However, there is a limit to the mixing capacity with a further increase in stirring, and higher stirring speeds also mean a higher energy consumption. Therefore, the optimal stirring speed can be established as 600 rpm in this work.

3.1.3. Effect of Phase Ratio of the Aqueous and Oil Phase on R₃N Regeneration Rate

The regeneration rate of R₃N increased gradually up to a maximum of 97% as the phase ratio of aqueous and oil phases rose from 0.0 to 0.67 and then remained relatively constant, as shown in Figure 4. Hydrochloric acid was produced during the regeneration process of R₃H chemically with oyster shell powder, thereby improving the regeneration rate. Without the aqueous phase, it is difficult for the hydrochloric acid in oil phase to directly react with oyster shell powder, and the regeneration rate will be greatly reduced. However, the presence of an aqueous phase can dissolve hydrochloric acid from the oil phase and a small amount of oyster shell powder and quickly initiate a chemical reaction between the two substances, thereby promoting an increase in the regeneration rate of R₃N. Therefore, increasing the phase ratio between the aqueous and oil solutions up to 0.67 is beneficial for improving the regeneration rate. However, when the phase ratio is further increased, the molecular exchange between the oil and aqueous phases may be nearing saturation, leading to the regeneration rate of R₃N·HCl no longer growing, or even decreasing, due to a reduction in the concentration of hydrochloric acid in the aqueous solution that reduces the mass transfer between the two phases. According to Figure 4, the optimal phase ratio of the aqueous and oil phase can be set as 0.67 in this work.

3.1.4. Effect of Reaction Temperature on R₃N Regeneration Rate

According to the data presented in Figure 5, the regeneration rate of R₃N increased with an increase in the reaction temperature up to 318.15 K, achieving a maximum of 97%. Subsequently, the regeneration rate remained almost constant with a further rise in temperature. The R₃N regeneration process is known to be endothermic, and detailed information can be found in Section 3.2. The higher reaction temperature facilitated the decomposition rate of R₃N·HCl, resulting in an increased yield of R₃N. However, temperatures beyond 358.15 K could cause the solvent in the oil phase to evaporate rapidly, leading to an increase in concentration and a slight reduction in the regeneration rate of R₃N.

3.2. Thermodynamic Function Calculation and Reaction Mechanism of the R₃N Regeneration Process

The reaction mechanism of the R₃N regeneration process was investigated in this work. The organic regeneration solution of R₃N, an aqueous solution of CaCl₂, and CO₂ gas were obtained by heating a mixture of the oil phase (R₃N·HCl), distilled water, and oyster shell powder. The reaction equation is expressed as Equation (4). According to the equation, the equilibrium constant calculation expression is as follows:

$$K=\frac{\left[{CaCl}_{2\left(w\right)}\right]{\left[{R_{3}N}_{\left(o\right)}\right]}^2}{{\left[{R_{3}N·HCl}_{\left(o\right)}\right]}^2}$$

(6)

The equilibrium constants at different temperatures were calculated, and the results are listed in

Table 5. Equilibrium concentrations and equilibrium constants at different temperatures.

T/(K)	c(R3N·H+)(o)/(mol/L)	c(CaCl2)(w)/(mol /L)	c(R3N)(o)/(mol/L)	ln K
353.15	0.032	0.603	2.065	7.798
343.15	0.046	0.594	2.052	7.081
333.15	0.066	0.579	2.034	6.313
323.15	0.086	0.564	2.012	5.736
318.15	0.106	0.549	1.992	5.270
302.15	0.139	0.524	1.959	4.642

.

According to van’t Hoff equation,

$$\text{ln} K=-\frac{\mathsf{\Delta }H}{RT}+C$$

(7)

where K is the equilibrium constant, ΔH denotes the enthalpy change (J·mol⁻¹), R is the molar gas constant (R = 8.314 J·mol⁻¹·K⁻¹), T is the absolute temperature (K), and C is a constant. Figure 6 displays the linear relationship between ln K and 1/T. The reaction was found to be endothermic, as evidenced by the negative slope of the line. The value of the slope was used to calculate ΔH, which was determined to be 56.0 kJ·mol⁻¹.

The Gibbs function change (ΔG) and the entropy change (ΔS) of reaction (4) were calculated using the equilibrium constant and Gibbs function as follows:

ΔG = ΔH − TΔS

(8)

ΔG = −RTlnK

(9)

The values obtained at 298.15 K were −10.1 kJ·mol⁻¹ and 221.8 J·mol⁻¹·K, respectively, indicating that the reaction can occur spontaneously at room temperature despite being endothermic. However, a higher temperature favors the yield of CaCl₂, the overall reaction progress, and the regeneration rate of R₃N.

Reaction (4) can be divided into two steps, expressed as Equation (10) and Equation (11), respectively:

R₃N·HCl → R₃N + HCl

(10)

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑

(11)

The standard thermodynamic data of Reaction (11) is shown in

Table 6. Thermodynamic functions data of reaction (11).

Thermodynamic Functions (298.15 K)	CaCO3	HCl	CaCl2	H2O	CO2
ΔfHm (KJ/mol)	−1206.92	−92.307	−795.4	−285.83	−393.509
ΔfGm (KJ/mol)	−1128.79	−95.299	−748.8	−237.129	−394.359
Sm (J/mol/K)	92.9	186.908	108.4	69.91	213.74

, and the thermodynamic functions of reaction (11) are calculated as ΔH = −83.2 kJ·mol⁻¹, ΔG = −60.9 kJ·mol⁻¹ and ΔS = −76.7 J·mol⁻¹·K⁻¹ at 298.15 K. The results indicate that reaction (11) is exothermic and can occur spontaneously under room temperature and atmospheric pressure, without the need for additional energy.

According to the calculation rules governing thermodynamic functions of related reactions, reaction (10) has thermodynamic values of ΔH = 69.6 kJ·mol⁻¹, ΔG = 25.4 kJ·mol⁻¹, and ΔS = 149.2 J·mol⁻¹·K⁻¹ at 298.1 K. These values indicate that the decomposition process of R₃N·HCl is endothermic and cannot occur spontaneously at room temperature.

The combination of reactions (10) and (11), achieved by adding oyster shell powder to the R₃N·HCl solution, reduces the activation energy of the entire reaction, facilitating the decomposition of R₃N·HCl at low temperatures and enabling a high conversion with minimal heat input. This approach is environmentally friendly and practical, not requiring harsh reaction conditions. The results also demonstrate the superiority and feasibility of the joint utilization of oyster shells and waste bittern.

4. Conclusions

This work developed an innovative, environmentally friendly two-stage process that utilizes oyster shells and waste bittern to produce magnesium carbonate and calcium chloride. In the first stage, a mixture of CO₂, R₃N, and isoamyl alcohol is injected into a waste bittern to obtain MgCO₃·3H₂O. The organic extraction phase from the first stage then reacts with the oyster shell to produce CaCl₂·2H₂O, CO₂, and R₃N in the second stage. The optimal conditions for the process were established as follows: in the first stage, a maximum yield of 87% for MgCO₃·3H₂O was obtained, with a phase ratio of waste bittern, R₃N, and isoamyl alcohol of 1:4.8:4.8, a reaction time of 7 h, and a stirring speed of 600 rpm. In the second stage, the regeneration rate of R₃N reached a maximum of 97% at a reaction temperature of 358.15 K and a phase ratio of 0.67 between the aqueous and oil phases. The process generates nearly zero waste as all by-products are recycled in the system, solving the problem of environmental pollution caused by waste bittern and oyster shells. This technology provides a new perspective for green chemical production. For future industrial applications, two main challenges in this system should be addressed: (1) the development of a more effective design for multiphase reactors to improve the efficiency of mixing and finally enhance the reaction and regeneration ratios, and (2) the optimization of the circulation efficiency of R₃N, which is a crucial component that connects two stages, to reduce its loss and ensure its efficacy for future applications.