The Impact of Cobalt Species on the Hazardous Characteristics of Cobalt-Leaching Residue: A Case Study from Guangdong Province, China

Abstract

Cobalt (Co) is a hazardous element of significant environmental concern, primarily due to its potential leaching toxicity. However, the current assessments of leached Co residues have focused solely on the total cobalt concentration, often overlooking the distinct Co species that contribute to its hazardous nature. This study attempts to determine the impact of cobalt speciation on the toxicity of cobalt and the related hazardous characteristics. The objective of this study is to enhance the understanding of how different Co species influence the environmental toxicity of leached residues. Cobalt speciation is studied by a multivariate analysis including ignitability, reactivity, corrosiveness, acute toxicity, leaching toxicity, and toxic substance concentration. The tested concentrations are compared with the identification standards and technical specifications in China. Co species, particularly cobalt oxide, are identified as the main contributors to the toxicity of the leached Co residue. It is also noted that Co occurrence significantly affects the calculation results of cumulative toxicity, thus impacting the hazard characteristics of leached cobalt residue. The findings benefit risk evaluators and decision makers by offering a new approach for managing leached Co residues and providing a scientific foundation for the development of relevant laws and regulations in China.

1. Introduction

Cobalt (Co) is the 33rd most abundant elements in the Earth’s crust [1]. It is essential in trace amounts for humans and other mammals as it is an integral component of the vitamin B12 complex. However, high concentrations of Co are harmful [2]. Most Co compounds released into the environment are adsorbed by sediments or soil particles, becoming immobile. However, acidic environments, such as polluted/contaminated soils near mining sites or melting facilities, lead to the remobilization of adsorbed metals, which are thus rinsed out to the surface and groundwater. Under this condition, the uptake of Co by plants and animals may occur and are accumulated therein [3]. Particularly, an excess exposure of Co is harmful to humans and causes various diseases such as contact dermatitis, pneumonia, allergic asthma, and lung cancer [4]. Large amounts of leached Co residue pose potential threats to the environment due to its toxicity [5]. In China, the waste residues produced during Co extraction in the lead–zinc smelting process are classified as toxic hazardous waste [6]. The toxicity of Co is highly influenced by its oxidation state and solubility, as well as its many other intrinsic and extrinsic factors [7]. It is difficult to accurately characterize the hazardous characteristics of Co based solely on its total concentration. For example, the free Co concentration instead of the total Co concentration was proposed as the key parameter correlated with Co toxicity in anaerobic granular sludge [8]. Mantoura et al. [9] modeled the equilibrium states of divalent Co species in fresh water, and, based on the modeled equilibrium concentrations, the toxicities of the substances were ranked in the following order: Co²⁺ > CoCo₃ > CoHCO₃⁺ >> CoSO₄ > Co-humic acid. Co(II) ions are deemed as the toxicologically relevant Co species, since the size, stability, charge, and binding characteristics of Co(II) allow them (but not Co(III) or Co(0)) to participate in specific receptor activation, ion channel transportation, and other interactions, which then result in the adverse effects of excessive Co exposure [10]. Excessive cobalt exposure leads to a complex clinical syndrome, marked by various neurological, cardiovascular, and endocrine deficits, directly linked to the absorption of cobalt ions into tissues and blood circulation. However, the broad range of systemic cobalt levels observed in symptomatic patients suggests that additional factors may contribute to the clinical presentation [11]. Moreover, compared to other forms, the water-soluble Co forms such as cobalt sulfate (CoSO₄) and cobalt chloride (CoCl₂) cause relatively higher toxicities. In the current standards for the hazardous waste identification of Co in China, the integrated pollution index is often used to evaluate the Co contamination level (GB 5085.6-2007, China) [12], by only considering the total concentration of Co. Although Co toxicity has been studied, studies regarding the impact of Co species on the identification of solid waste hazardous characteristics are lacking. Currently, the systematic knowledge of the hazardous characteristics of leached Co residue is still incomplete, and a multivariate analysis is needed to better analyze the hazardous characteristics of leached Co residue by particularly considering Co species.

This study investigated the toxicity of Co residue leached from a metal chemical company in Guangdong province in China via a multivariable analysis and then compared its hazardous characteristics with the current identification standards and technical specifications in China. Particularly, different Co species were studied. The cumulative toxicity of Co was calculated and compared with or without considering the speciation. The leaching toxicity and toxic substance concentrations in the leached Co residue were tested to double confirm the hazardous characteristics of leached Co residue. Finally, the identification of hazardous characteristics of the leached Co residue was summarized by comparing with the current standards. As such, future suggestions were developed for the identification of such solid wastes.

2. Materials and Methods

2.1. Materials

The leached Co residue samples sourced from a metal chemical company (Yangjiang City, Guangdong Province, China) were generated in the leaching section. After washing and dewatering with a diaphragm filter press system, they were collected with an actual monthly output of about 6.2 tons. The composition of the leached Co residue is shown in Table S1 (Supporting Information). Eight different samples were obtained in December of 2019.

The hydrometallurgical process developed for the separation and recovery of Co from the Co intermediate (cobalt hydroxide) consisted of the following six major unit operations: leaching, extraction, synthesis, calcination (production of cobalt metal compounds), electrolysis (production of copper by-products), and washing. Figure 1 shows the production process flow chart. In general, the Co intermediate (cobalt hydroxide), sulfuric acid, bisulfite, and water were added to the reaction tank, and steam was directly introduced into the reaction tank to control the reaction temperature. After a period of reaction, the metal elements such as Co entered the leaching solution, and a small amount of acid leaching residue was generated. Then, hydrogen peroxide was added into the reaction tank. After the completion of the reaction, all reactants and products in the reaction tank were passed through the filter press, and lime water was added to adjust the pH. All the reactants and products were passed through the filter press again, with the filtrate proceeding to the next extraction process. The remaining filter residue entered the cleaning section. And, finally, the leached Co residue samples were generated by peeling from the plate and frame filter press.

2.2. Analytical Methods

2.2.1. Preliminary Screening of Hazardous Characteristics

Leaching tests can provide a basis for assessing leaching risks and thus for selecting appropriate treatment and disposal alternatives [13]. The inorganic and organic compounds leached from the leached Co residue samples were tested based on the regulations in the Chinese identification standards for hazardous waste identification for extraction toxicity (GB 5085.3-2007) [14]. The leaching solution samples were prepared according to the sulfuric acid and nitric acid method for the leaching toxicity of solid waste (HJ/T 299-2007). The pretreatment method using hexavalent chromium was adapted from the alkali digestion method in Appendix T of GB 5085.3-2007, and for the detection method, we referred to the diphenylcarbazide spectrophotometry technique for determining hexavalent chromium in solid waste (GB/T 15555.4-1995) [15]. Mercury, arsenic, and selenium were tested by microwave dissolution/atomic fluorescence spectrometry (HJ 702-2014) [16]. Lead, cadmium, arsenic, beryllium, nickel, manganese, and Co were detected by inductively coupled plasma atomic emission spectrometry (HJ 781-2016) [17]. For the methods of detecting fluoride and cyanide (cyanogen ion), we referred to Appendices F, G, and O in GB 5085.3-2007, respectively. For the detection of hexavalent chromium, we referred to the determination of hexavalent chromium in solid waste using diphenylcarbazide spectrophotometry (GB/T 15555.4-1995).

2.2.2. Sequential Extraction Analysis of Cobalt

The speciation of toxic elements was investigated by sequential extraction (fractionation) analysis, wherein they were categorized into several fractions based on a stepwise addition of reagents with increasing reactivity [18]. These fractions included the water-soluble, exchangeable, reducible, oxidizable, and residual forms. They are often attributed to the elements associated, bonded, or adsorbed on hydroxysulphates, exchangeable sites in clay minerals or carbonates, iron and manganese oxyhydroxides, organic matter/sulfide and silicate phases, respectively [19]. In this method, the existing forms of heavy metals are divided into cobalt oxide, cobalt sulfide, water-soluble Co, and other mineral inclusions [20]. Since both cobalt sulfate and cobalt chloride are soluble in water and can be extracted by aqueous solutions, they are expected to dissolve. The weak acid solution mainly extracts cobalt oxide, cobalt sulfide, cobalt hydroxide, and other mineral inclusions.

2.2.3. Other Analyses

X-ray diffraction (XRD) patterns were obtained with X-ray diffractometry(Philips, PANalytical B.V., Almelo, The Netherlands), using Cu Kα radiation (λ = 1.5406 Å) and a scan step size of 0.05° in the 2θ range from 10 to 80°. The surface morphology of the original powder was measured with a scanning electron microscope (SEM; JSM-7500F, JEOL Ltd., Tokyo, Japan) with an accelerating voltage of 1.5 kV.

2.2.4. Calculation of Cumulative Toxic Content

According to GB 5085.6-2007, when calculating the content of toxic substances in the solid waste, it is necessary to convert the content of inorganic elements into the content of their corresponding compounds. Under the most unfavorable hypothesis, the compounds with the largest molecular weight and the lowest identification standard value are selected to calculate the content of toxic substances. The integrated pollution index is used to evaluate the heavy metal contamination level. Solid waste meeting any of the following conditions is identified as a hazardous waste: (1) the total content of one or more highly toxic substances in Appendix A of the identification standard for toxic substances in hazardous wastes (GB 5085.6-2007) is ≥0.1%; (2) the total content of one or more toxic substances in Appendix B of the standard is ≥3%; (3) the total content of one or more carcinogenic substances in Appendix C of the standard is ≥0.1%; (4) the total content of one or more mutagenic substances in Appendix D of the standard is ≥0.1%; (5) the total content of one or more reproductive toxic substances in Appendix E of the standard is ≥0.5%; (6) the solid waste contains two or more different toxic substances listed in Appendix A to E of the standard. It is classified as hazardous waste if the following equation is satisfied:

$$\sum \left[\right({\displaystyle \frac{{\mathrm{P}}_{{\mathrm{T}}^{+}}}{{\mathrm{L}}_{{\mathrm{T}}^{+}}}}+{\displaystyle \frac{{\mathrm{P}}_{\mathrm{T}}}{{\mathrm{L}}_{\mathrm{T}}}}+{\displaystyle \frac{{\mathrm{P}}_{\mathrm{Carc}}}{{\mathrm{L}}_{\mathrm{Carc}}}}+{\displaystyle \frac{{\mathrm{P}}_{\mathrm{MUTA}}}{{\mathrm{L}}_{\mathrm{Muta}}}}+{\displaystyle \frac{{\mathrm{P}}_{\mathrm{Tera}}}{{\mathrm{L}}_{\mathrm{Tera}}}}\left)\right] \ge 1$$

(1)

where P_T⁺ denotes the content of highly toxic substances in solid waste;

P_T denotes the content of toxic substances in solid waste;

P_Carc denotes the content of carcinogenic substances in solid waste;

P_Muta denotes the content of mutagenic substances in solid waste;

P_Tera denotes the content of reproductive toxic substances in solid waste;

L_T⁺, L_T, L_Carc, L_Muta, and L_Tera denote the standard values of various toxic substances in (1)–(5), respectively.

3. Results and Discussion

3.1. Speciation Analysis of Cobalt

Figure 2 shows the phase analysis results of the state of the Co occurring in the leached Co residue samples based on Tessier extraction. According to Figure 2, the content of water-soluble Co accounted for only 0.1% of the total Co in the samples. Most of the Co was present as cobalt oxide, accounting for 92.9% of the total Co. The trace amount of Co was in the form of sulfur copper cobalt ore, accounting for 5% of the total cobalt, and the trace amount of Co was wrapped in calcium sulfate dihydrate and aluminum calcium iron silicate in the form of ultrafine cobalt oxide.

The structure and morphology of the cathode materials after leaching were characterized by X-ray diffraction (XRD) and a scanning electron microscope (SEM). The mineralogical analysis results indicated that gypsum (CaSO₄·2H₂O) and brushite (Ca(HPO₄)·2H₂O) were the major mineralogical phases in the residue (Figure S1 in the Supporting Information). Minor phases such as aluminum, calcium, iron, and silicate, and small amounts of muscovite, albite, calcite, dolomite, chlorite, talc, cobalt oxide, and thiocopper cobalt in the residue were also detected. According to the SEM results (Figure S2 in the Supporting Information), the Co in the sample was present as cobalt oxide and cobaltite, and the relative contents were 0.12% and 0.01%, respectively. The toxicity of Co cannot be evaluated solely based on the total Co concentration in the liquid phase but also requires the determination of the amount of Co precipitated/sorbed in leached Co residue and the speciation of the Co present in the bulk solution [8]. Combining the results shown in Figures S2 and S3, the predominance of Co in the oxide form was the determining factor of the nonhazardous characteristics of the leached Co residue samples. It has been widely reported that Co oxides are less soluble than the soluble CoCl₂ [10], so they are less mobile and toxic.

3.2. Cumulative Toxicity of Cobalt

Table 1. Comparison of calculated toxic substance contents in leached Co residue (%) with or without considering the speciation analysis of cobalt.

	Acutely Toxic Substance			Toxic Substances				Carcinogenic Substances				Reproductive Toxic Substance	Mutagenic Substance	Cumulative Toxicity (Dimensionless)
	Sodium Chromate	Mercuric Chloride	Silver Cyanide	White Spirit	Lead Fluoride	Manganese	Antimony Pentoxide	Cadmium Sulfate	Nickel Sulfide	Cobalt Sulfate	Beryllium Oxide	Lead Phosphate	Sodium Chromate
Considering Co species	0.0034	5.32 × 10−5	5.15 × 10−5	0.16	0.0064	0.0136	5.46 × 10−4	2 × 10−5	0.0287	0.0002	0.0050	2.21 × 10−4	1.46 × 10−3	0.4507
Not considering Co species	0.0034	5.31 × 10−5	5.15 × 10−5	0.162	0.0064	0.0136	5.46 × 10−4	2 × 10−5	0.0287	0.1490	0.0050	2.21 × 10−4	1.46 × 10−3	1.9383

shows a comparison of the calculation results of the toxic substance concentrations with or without considering the speciation analysis of cobalt. The results show that when considering the speciation analysis of cobalt, the cumulative toxicity of all toxic substances was 0.4507, which is lower than the limit in the standard for the identification of hazardous wastes (GB 5085.6-2007). However, without taking the Co speciation analysis into account, the concentrations of cobalt sulfate were much higher; the cumulative toxicity exceeded the standard limit. The regular monitoring of Co concentrations in cleaning agents and the surrounding environment should be conducted to ensure compliance with safety standards. Implementing effective waste management practices will also help prevent the accumulation of Co residues and mitigate environmental risks.

3.3. Leaching Toxicity and Toxic Substance Concentrations

Table 2. Extraction toxicity test results of formal solid waste samples (mg/L).

Sampling Date	Sample Number	Cr6+	As	Se	Fluoride	Ni	Co	Total Chromium
2 December 2019	1	ND	0.00396	0.00322	0.318	0.03	0.05	ND
6 December 2019	2	ND	0.00586	0.00348	0.226	0.03	0.10	ND
10 December 2019	3	ND	0.00754	0.00238	0.417	0.03	0.08	ND
14 December 2019	4	ND	0.00153	0.0044	0.430	0.04	0.05	ND
18 December 2019	5	ND	0.00355	0.00551	0.306	0.04	0.05	ND
22 December 2019	6	ND	0.00155	0.0071	0.616	0.04	0.08	ND
26 December 2019	7	ND	0.00246	0.00376	1.070	0.03	0.08	ND
30 December 2019	8	ND	0.00306	0.00314	1.420	0.04	0.05	ND
Detection limit		0.004	0.0001	0.0001	0.0148	0.02	0.02	0.02
Detection maximum		ND	0.00754	0.0071	1.420	0.04	0.10	ND
Standard limits		5	5	1	100	5	-	15

Note: ND means not detected.

shows the extraction toxicity test results of the formal solid waste samples. The results show that hexavalent chromium and total chromium were not detected in the eight leaching solution samples. The detection rates of arsenic, selenium, fluoride, nickel, and Co were 100%, and their detection ranges were 0.00153–0.00754, 0.00238–0.0071, 0.226–1.420, 0.03–0.04, and 0.05–0.10 mg/L, respectively, all of which are below the identification standard for hazardous waste leaching toxicity (GB 5085.3-2007). Therefore, the leached Co residue did not show the hazardous characteristics of leaching toxicity.

The toxic substance concentrations of the eight samples are shown in

Table 3. Results of toxic substances in samples of cobalt leaching mixed slag.

Sampling Date	Sample Number	White Spirit	Mg	Ni	Co	As	Cumulative Toxicity
		%	mg/kg
2 December 2019	1	0.195	105	180	468	28.8	0.410
6 December 2019	2	0.212	149	171	510	36.5	0.420
10 December 2019	3	0.072	188	227	576	19.4	0.424
14 December 2019	4	0.05	120	116	333	31.4	0.269
18 December 2019	5	0.104	146	156	486	34.5	0.356
22 December 2019	6	0.041	205	252	635	24.9	0.465
26 December 2019	7	0.078	215	276	732	17.8	0.499
30 December 2019	8	0.072	161	172	468	21.4	0.342
Detection limit		0.02	3.1	0.4	0.5	0.01

. According to the results, the detected values were 0.041–0.212 mg/kg for petroleum solvent, 105–215 mg/kg for manganese, 116–276 mg/kg for nickel, 333–732 mg/kg for Co, and 17.8–36.5 mg/kg for arsenic. These values are lower than the identification standard for the leaching toxicity of hazardous wastes (GB 5085.3-2007). Similarly, the leached Co residue did not show the hazardous characteristics of toxic substances.

This further proves that the toxicity of Co cannot be evaluated solely based on the total Co concentration but also requires the determination of the amount of Co precipitated or sorbed in the leached Co residue. When calculating the content of accumulated toxic substances, some heavy metals should be converted into the content corresponding to the toxic valence state and then calculated; otherwise, some solid wastes are misjudged as hazardous wastes. In addition, this study focused on a single case from a company in Guangdong province, which may limit the generalizability of its findings, as it did not account for environmental variables such as pH, temperature, or interactions with other elements, which can vary across regions and influence cobalt speciation, behavior, and toxicity in different industrial and environmental settings.

4. Conclusions

This study investigated the impact of Co species on the hazardous characteristics of leached Co residue by looking into a case study from Guangdong province, China. Through analysis via Tessier extraction, it was noted that most of the Co existed in cobalt oxide in the leached Co residue, accounting for 92.9% of the total cobalt. Further tests of leaching toxicity and toxic substance concentrations indicated that the leached Co residue did not show the hazardous characteristics of toxic substances. While calculating the cumulative Co toxicity, if not taking the Co speciation analysis into account, the concentrations of cobalt sulfate are much higher, all exceeding the standard limit. However, if taking the Co speciation analysis into account, the cumulative toxicity of all toxic substances was 0.269–0.499, which is lower than the limit in the standard for the identification of hazardous wastes. This study further proves that the toxicity of Co cannot be evaluated solely based on the total Co concentration but also requires the determination of Co speciation. Future research should focus on the speciation analysis of cobalt’s hazardous characteristics rather than total concentration measurements and should include studies on bioavailability, ecological impacts, and the development of standardized assessment protocols.