Environmental Risk Assessment of Reclaimed Water Purification Using an Agent Prepared from Waste Acid Resulting from Titanium Dioxide Industry

Abstract

The production of titanium dioxide in China generates substantial waste acid and ferrous sulfate, which are repurposed into polyferric sulfate for industrial wastewater treatment. However, this water purification agent contains heavy metals like Ti, V, Mn, Cr, Co, Cu, Ni, Zn, Ba, and Pb, posing unrecognized environmental risks. This study identified these risks through pollutant screening and the process analysis of a Jiangsu-based titanium dioxide enterprise, evaluating the potential impacts on water quality and worker health. The results show that concentrations of manganese and titanium in the polyferric sulfate reached up to 163 mg/L and 631 mg/L, respectively. Notably, the non-carcinogenic hazard quotients (HQs) for cobalt, vanadium, and manganese were 307, 5.6, and 2.6, all exceeding the safe limit of 1, with cobalt presenting a particularly significant risk due to its low reference dose (RfD) of 0.0003 mg/kg-d. This study concludes that national standards should be revised to include limits for these pollutants to ensure safer practices in industrial wastewater treatment.

1. Introduction

In industrial production, the generation of waste acid is a common phenomenon. These waste acids primarily originate from chemical reactions such as nitration, esterification, sulfonation, alkylation, and catalysis, as well as processes like titanium dioxide (TiO₂) production, steel pickling, and gas drying [1]. According to published statistics [2], China produces approximately 100 million tons of waste acid annually, with over 95% being waste sulfuric acid, making it the predominant type of waste acid. The main sources of waste sulfuric acid include the TiO₂ industry, non-ferrous metal smelting, and steel pickling. Notably, the TiO₂ industry produced nearly 30 million tons of waste sulfuric acid in 2021, accounting for about 30% of the total waste acid volume.

According to data from the National Chemical Industry Productivity Promotion Center, by 2021, China’s total production capacity for TiO₂ exceeded 3.5 million tons, with actual output surpassing 3 million tons. In TiO₂ production, the sulfuric acid method is widely adopted due to its inexpensive raw materials, mature technology, simple operation, and low investment costs, representing 94.85% of production methods [3,4]. However, this process consumes significant resources, especially water and sulfuric acid, leading to the generation of “three wastes” (wastewater, waste gas, and waste residue) and by-products. For every ton of TiO₂ produced, approximately 4 tons of sulfuric acid are consumed [3], resulting in large quantities of ferrous sulfate and waste sulfuric acid. Currently, the primary methods for handling TiO₂ waste acid are neutralization and concentration recovery [5]. However, both methods have limitations. Neutralization produces gypsum, which is difficult to dispose of, while concentration recovery can increase processing costs and potentially degrade TiO₂ quality due to impurity accumulation. Additionally, the ferrous sulfate generated during TiO₂ production has low purity and contains multiple impurities, making it challenging to utilize directly [6]. To effectively recycle the solid waste resources generated during TiO₂ production, scholars have proposed producing polyferric sulfate from TiO₂ waste acid and ferrous sulfate [7]. This method offers easy access to raw materials, relatively simple processes, mature technology, minimal secondary pollutants, and significant economic and environmental benefits. However, the environmental risks associated with using these wastewater treatment agents in downstream industries have not received adequate attention.

Therefore, this study aimed to assess the environmental risks of reclaimed water purification agents prepared from waste acid and ferrous sulfate in the TiO₂ industry. Specifically, we focus on identifying potential environmental risks through pollutant screening and a process analysis of a Jiangsu-based TiO₂ enterprise. Our research builds upon the existing literature by providing a detailed evaluation of heavy metals in reclaimed water purification agents and their potential impacts on water quality and worker health. Unlike previous studies that primarily focused on recovery methods, our work emphasizes the comprehensive risk assessment of these agents, highlighting previously underrecognized environmental hazards. Additionally, we propose recommendations for revising national standards to ensure safer practices in industrial wastewater treatment.

2. Materials and Methods

2.1. Study Area

A titanium dioxide enterprise in Jiangsu Province uses the sulfuric acid method to manufacture titanium dioxide. During this process, the enterprise generates titanium waste acid and ferrous sulfate. The company then utilizes these by-products as raw materials to produce polyferric sulfate (regenerated water purification agent), which it sells to downstream enterprises for industrial wastewater treatment. This study focuses on the regenerated water purification agent produced by the enterprise.

2.2. Experimental Methods and Quality Control

2.2.1. Experimental Methods

The pretreatment and detection of all samples were carried out in the CMA certified laboratory of Nanjing Institute of Environmental Science, Ministry of Ecology and Environment. The contents of mercury, arsenic, antimony, and zinc were determined using the “Solid Waste—Determination of Mercury, Arsenic, Selenium, Bismuth, Antimony—Microwave Dissolution/Atomic Fluorescence Spectrometry” standard (HJ 702-2014) [8]. The detection limits of mercury, arsenic, antimony, and zinc were 0.00002 mg L⁻¹, 0.00010 mg L⁻¹, 0.00010 mg L⁻¹, and 0.01 mg L⁻¹, respectively. The contents of lead, nickel, chromium, cobalt, manganese, titanium, and vanadium were determined using the Solid Waste—Determination of 22 Metal Elements—Inductively Coupled Plasma Optical Emission Spectrometry standard (HJ 781-2016) [9]. The detection limits of lead, nickel, chromium, cobalt, manganese, titanium, and vanadium were 0.03 mg L⁻¹, 0.02 mg L⁻¹, 0.02 mg L⁻¹, 0.01 mg L⁻¹, 0.02 mg L⁻¹, 0.02 mg L⁻¹, and 0.0001 mg L⁻¹, respectively.

2.2.2. Quality Control

The production of polyferric sulfate is by intermittent reaction. The amount produced for a single reaction is about 20 tons, the duration is 90 to 120 min, and the unloading time is 10 to 20 min. The actual daily production of polyferric sulfate is about 600 tons. Based on the requirements of the “Technical Specification for Sampling and Sample Preparation of Industrial Solid Waste” standard (HJ/T 20-1998) [10], and therefore, sampling was performed in batches. One sample was collected every day for 10 consecutive days, accounting for a total of 10 samples. One parallel sample for on-site quality control was collected each on the third and seventh days. Three parallel samples for laboratory quality control were allocated to each batch of samples. Based on the results of the quality control, the deviation of detection between the field parallel samples and the laboratory parallel samples was less than 5%.

2.3. Risk Assessment Model

The exposure period of adults in sewage treatment plants is long, and the exposure frequency is high. The carcinogenic risk and non-carcinogenic effects of pollutants are generally evaluated based on the exposure of adults. With the addition of the reclaimed water purifier, heavy metals from the purifier enter the sewage. Without protective equipment, or when used incorrectly, workers’ health may be affected through the oral intake of splashed wastewater. In this paper, the model recommended in the “Technical Guidelines for Soil Pollution Risk Assessment of Construction Land” standard (HJ 25.3-2019) [11] was used to assess the environmental risks.

For the non-carcinogenicity of a single pollutant, considering the exposure hazard of the adult population, the exposure amount through the oral route is calculated as follows [11]:

$${CGWER}_{nc}={\displaystyle \frac{{GWCR}_a\times {EF}_a\times {ED}_a}{\begin{array}{c}{BW}_a\times {AT}_{nc}\end{array}}}$$

(1)

where ${CGWER}_{nc}$ is oral exposure (L kg⁻¹ d⁻¹), ${\mathrm{G}\mathrm{W}\mathrm{C}\mathrm{R}}_a$ is daily oral intake of splashed wastewater (L d⁻¹), ${EF}_a$ is adult exposure frequency (d a⁻¹), ${ED}_a$ is adult exposure period (a), ${BW}_a$ is average adult weight (kg), and ${AT}_{nc}$ is the average time of non-carcinogenic effect (d).

The hazard quotient for exposure is calculated as follows [11]:

$${HQ}_{cgw}={\displaystyle \frac{{CGWER}_{nc}\times C_{gw}}{\begin{array}{c}{RfD}_o\times WAF\end{array}}}$$

(2)

where ${HQ}_{cgw}$ is hazard quotient of oral exposure, ${\mathrm{C}}_{gw}$ is concentration of the pollutant (mg L⁻¹⁾, ${RfD}_o$ is oral intake reference dose (mg kg-d), ${ED}_a$ is adult exposure period (a), and $\mathrm{W}\mathrm{A}\mathrm{F}$ is reference dose distribution ratio of exposure to wastewater.

3. Results and Discussion

3.1. Identification and Analysis of Pollution Characteristics of Waste Acid and Ferrous Sulfate

Ilmenite is milled by a ball mill to a powder of required particle size, and then reacted with 98% concentrated sulfuric acid. After sedimentation, bleaching, washing, and other processes, a large amount of waste sulfuric acid is produced. Heavy metals and other pollutants in ilmenite enter the waste acid [2,12,13]. The mass fraction of sulfuric acid in the waste acid is about 22%. The heavy metal pollutants in the waste acid are presented in

Table 1. Contents of heavy metals in waste acid and ferrous sulfate heptahydrate.

	Waste Acid	FeSO4	GB/T 534 [14]	GB 8978 [17]	GB/T 10531 [18]	GB 36600 [19]
	mg L−1	mg kg−1	%	mg L−1	%	mg kg−1
Ba	ND	5~13.8	-	-	-	-
V	162~168	22.2~30	-	-	-	752
Cd	ND	0.6~0.9	-	-	0.0001	-
Hg	0.00122~0.00191	0.022~0.037	0.01	0.05	0.00002	-
Co	6.54~7.58	16.2~21.2	-	-	-	70
Cr	ND	4.7~5.2	-	-	0.0010	-
Mn	428~500	395~538	-	2	-	-
Ni	10.6~13.3	17.9~19.8	-	1	-	900
Pb	0.59~0.85	25.6~33	0.02	1	0.0004	-
As	0.444~0.708	1.78~1.9	0.001	0.5	0.0002	-
Ti	849~887	2840~4190	-	-	-	-
Sb	0.0174~0.0308	ND	-	-	-	-
Cu	ND	ND	-	-	-	-
Se	ND	ND	-	-	-	-
Zn	43.2~52.6	60.8~75.7	-	2	-	-

Note: ‘-’ indicates that the standard does not specify a limit for this heavy metal. ‘ND’ indicates not detected.

. The maximum detected concentrations of arsenic, lead, and mercury were 0.708 mg L^−1, 0.85 mg L⁻¹, and 0.00191 mg L⁻¹, respectively. These concentrations are lower than the limit requirements of 0.001%, 0.02%, and 0.01% for arsenic, lead, and mercury, respectively, in first-class sulfuric acid based on the “industrial sulfuric acid” standard (GB/T 534-2014) [14]. The minimum concentration of first-class sulfuric acid in “industrial sulfuric acid” (GB/T 534-2014) is 92.5%. With the accumulation of low-concentration waste acids, therefore, arsenic, lead, and mercury may exceed the product quality limits in industrial sulfuric acid, creating an environmental risk [15,16]. Antimony, cobalt, nickel, zinc, vanadium, manganese, and titanium were detected in waste acid, but these heavy metals are not required in “industrial sulfuric acid” (GB/T 534-2014). The maximum detected concentrations of nickel, zinc, and manganese in waste acid were 13.3 mg L⁻¹, 52.6 mg L⁻¹, and 500 mg L⁻¹, respectively, which were much higher than the limit requirements of 1 mg L⁻¹, 2 mg L⁻¹, and 2 mg L⁻¹ based on the Integrated Wastewater Discharge Standard (GB8978-1996) [17]. The maximum detection content of manganese exceeded 250 times its limit value, based on GB8978-1996. Heavy metals such as antimony, cobalt, vanadium, and titanium are pollutants in waste acid that existing product quality standards do not focus on. The maximum detected contents of antimony, cobalt, vanadium, and titanium are 0.0308 mg L⁻¹, 7.58 mg L⁻¹, 168 mg L⁻¹, and 887 mg L⁻¹, respectively. Hexavalent chromium, selenium, cadmium, barium, copper, beryllium, and thallium were not detected in waste acid. In view of this, the production of sulfuric acid from waste acid using the concentration method for the manufacture of titanium dioxide will affect the quality of the final product. Selling concentrated sulfuric acid poses a great environmental risk. The environmental protection department should, therefore, include waste acid into the scope of key supervision.

During the purification of titanium liquor, ferrous sulfate heptahydrate is produced through processes such as vacuum crystallization and cooling crystallization. The concentrations of heavy metal contaminants in ferrous sulfate heptahydrate are presented in Table 1. The mass fraction of ferrous sulfate heptahydrate is about 90%, which basically meets the requirements, based on the Water Treatment Agent Ferrous Sulphate standard (GB/T10531-2016) [18], for the concentration of ferrous sulfate heptahydrate in Class I products. The maximum detectable levels of mercury, cadmium, arsenic, and chromium in ferrous sulfate heptahydrate were 0.037 mg kg⁻¹, 0.9 mg kg⁻¹, 1.9 mg kg⁻¹, and 5.2 mg kg⁻¹, respectively, all of which were less than the limit values of 0.00002%, 0.0001%, 0.0002%, and 0.0010% for Class I products, based on GB/T10531-2016. The detected levels of lead were 25.6 mg kg⁻¹ and 33 mg kg⁻¹, respectively, both exceeding the limit value for Class I products by 0.0004%, based on GB/T10531-2016, and the maximum exceedance of lead was 8.25 times. The maximum detected levels of nickel, cobalt, and vanadium in ferrous sulfate heptahydrate were 19.8 mg kg⁻¹, 21.2 mg kg⁻¹, and 30 mg kg⁻¹, respectively, all of which were less than the screening values of 900 mg kg⁻¹, 70 mg kg⁻¹, and 752 mg kg⁻¹ for Class II land based on the Soil environmental quality Risk control standard for soil contamination of development land (GB 36600-2018) [19]. The maximum concentrations of barium, zinc, manganese, and titanium in ferrous sulfate heptahydrate, all of which are pollutants not focused on by existing product quality standards, were 13.8 mg kg−1, 75.7 mg kg⁻¹, 538 mg kg⁻¹, and 4190 mg kg⁻¹, respectively. Copper, selenium, and antimony were not detected in ferrous sulfate heptahydrate.

The pollutants contained in waste acid and ferrous sulfate heptahydrate are mainly derived from ilmenite [16]. The concentration of lead exceeded the corresponding product quality standards, and the concentrations of heavy metals such as nickel and vanadium exceeded the limits set by the Integrated Wastewater Discharge Standard (GB8978-1996) and by the screening values of second-class land set by the Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard (GB 36600-2018). The concentrations of heavy metals such as manganese, zinc, and titanium are high, but they have neither been included in the existing product quality standards nor been subject to key control by the management department. For example, a novel process has been developed for the recovery of valuable metals like Mn from chlorination titanium-white waste acid (CTWA), achieving over 97% recovery efficiency and producing battery-grade MnSO4 products [20]. In summary, the aforementioned factors should be focused on when assessing the environmental risk of reclaimed water purification agent.

3.2. Analysis of Product Quality and Pollution Characteristics of Reclaimed Water Purification Agent

The production process of polymerized ferric sulfate is mainly divided into dissolution and oxidation polymerization, and the raw and auxiliary materials mainly include waste acid, ferrous sulfate heptahydrate, oxygen, catalyst, and so on [5]. Ferrous sulfate heptahydrate is first put into the dissolution tank as raw material, and 22% dilute sulfuric acid is added. The ferrous sulfate heptahydrate is then dissolved by slow heating while stirring, and when it has completely dissolved, a refined ferrous sulfate solution is obtained by separation. The refined solution is then sent to the polymerization reactor where oxygen and a catalyst are added to oxidize ferrous sulfate to ferric sulfate using a circulation pump and reactor. The reaction is stopped when the concentration of ferrous iron drops to the specified concentration. The iron sulfate solution in the polymerization reactor is sent to the maturation tank, where liquid polymerized iron sulfate is obtained after the maturation reaction [21]. The production process of recycled polymeric ferric sulfate is known to release pollutants mainly from waste acid and ferrous sulfate heptahydrate [22]. Based on the analysis of the waste acid and ferrous sulfate heptahydrate, the potential characteristic pollutants in polymerized ferric sulfate include mercury, arsenic, antimony, zinc, lead, nickel, chromium, cobalt, manganese, titanium, vanadium, and so on. Ten samples of recycled polymerized ferric sulfate were collected and analyzed based on the requirements of the Technical Specification for Sampling and Preparation of Industrial Solid Waste standard (HJ/T20-1998). The results are presented in

Table 2. Contents of heavy metals in reclaimed water purification agent (mg L⁻¹).

	Hg	As	Zn	Pb	Ni	Cr	Ti	Co	Mn	V	Sb
Minimum value	0.000060	0.090	11.60	2.21	3.24	6.05	537.00	3.45	123.00	16.10	0.00154
Median value	0.000060	0.103	12.50	2.28	3.50	6.95	605.50	3.77	140.00	19.05	0.00263
Mean value	0.000068	0.100	12.64	2.30	3.54	7.03	600.10	3.74	138.40	19.15	0.00270
SD	0.000010	0.005	0.81	0.08	0.23	0.66	27.57	0.16	11.42	1.96	0.00086
Maximum value	0.000090	0.105	14.50	2.51	3.90	8.27	631.00	3.99	163.00	22.60	0.00478
Limits	0.5	5	50	10	50	25	-	-	-	-	-

.

The concentrations of heavy metals in the regenerated polymeric ferric sulfate were measured and compared against the standard limits specified in GB 14591-2016 [23] for Class II liquid products. The mean concentrations of mercury, arsenic, zinc, lead, nickel, and chromium were 0.000068 mg/L, 0.10016 mg/L, 12.64 mg/L, 2.304 mg/L, 3.54 mg/L, and 7.025 mg/L, respectively, all of which are below the corresponding standard limits. For maximum concentrations, the levels of mercury, arsenic, zinc, lead, nickel, and chromium were 0.00009 mg/L, 0.105 mg/L, 14.5 mg/L, 2.51 mg/L, 3.9 mg/L, and 8.27 mg/L, respectively, also meeting the GB 14591-2016 standard, which specifies maximum limits of 0.00005%, 0.0005%, 0.0050%, 0.0010%, 0.0050%, and 0.0025% for mercury, arsenic, zinc, lead, nickel, and chromium, respectively. Similar studies have reported comparable results in the context of heavy metal concentrations in recycled water purification agents [24]. For instance, a study on the recovery and reuse of iron-containing waste solutions from titanium dioxide production showed that the concentrations of heavy metals in the recovered product met the relevant environmental quality standards, thus confirming the feasibility of this recycling approach [20]. In summary, the concentrations of these heavy metals in the regenerated polymeric ferric sulfate are relatively low, indicating minimal environmental risk when used for industrial wastewater treatment. Beyond the six primary heavy metals, the recycled water purification agent contains other elements such as cobalt, manganese, titanium, vanadium, and antimony. While these elements are present, they are not highlighted in the GB 14591-2016 standard for polymeric ferric sulfate, and thus lack corresponding standard limit values. Future research should focus on establishing appropriate limits for these additional elements to ensure comprehensive safety and efficacy.

Titanium in polymerized ferric sulfate generally exists as a TiO₂-nH₂O gel, and based on the requirements of the Water Treatment Agent Ferrous Sulphate (GB 10531-2016) Class II liquid product standard, the mass fraction of TiO₂ should be less than 1%. The detected concentration of titanium in the regenerated polymerized ferric sulfate ranged from ~537 to 791 mg L⁻¹. This corresponds to a mass fraction of ~0.089% to 0.13% of TiO2, which is in line with the requirements of the GB 10531-2016 Class II liquid product standard, suggesting that the risk of titanium to the environment, in terms of the comprehensive utilization of polymerized ferric sulfate, is acceptable.

3.3. Environmental Impact and Risk Analysis of Recycled Water Purifiers

3.3.1. Analysis of the Impact of Reclaimed Water Purifiers on the Effluent Quality

According to Technical Specification for Solid Waste Recycling Pollution Prevention and Control (HJ 1091-2020) [25], the recycling of solid waste as a product should conform to the national, local, or industrial product quality standards and to the relevant national pollution control standards or technical requirement specifications. When there is no national pollution control standard or technical specification, the environmental risk should be evaluated qualitatively and quantitatively using the characteristic pollutants in the recycled solid waste as the object of evaluation. The evaluation should include an account of pollutant migration and transformation during solid waste recycling and the use of recycled products [6,7].

The environmental safety of polyferric sulfate is directly related to the scene (exposure environment). The release of pollutants in water purification agents, and the migration and transformation of the released pollutants, differ based on the exposure environment, and this influences the effect on human health and the environment. Polymerized ferric sulfate is mainly used to treat industrial wastewater such as electroplating wastewater, tannery wastewater, dyeing and dyeing wastewater, steel industry wastewater, and paper wastewater [26,27]. The optimal dosage of water purification agent was determined according to different turbidities of raw water. In general, the reference dose of polyferric sulfate is 300 to 500 mg L⁻¹. When the turbidity of industrial waste acid is high, the dose of polyferric sulfate is appropriately increased to 1000 mg L⁻¹. Polymerized ferric sulfate can simultaneously reduce turbidity and remove heavy metal contaminants from water. Flocs have a large number of hydroxyl groups on their surface, which are formed during coagulation and precipitation, and adsorb heavy metals from the water through heavy metal–ligand surface complexation. The complexes are then removed through coagulation, precipitation, and filtration processes [28].

To assess the maximum potential impact of application of the reclaimed water purifier on the effluent water quality, the assessment was carried out as follows: (1) the amount of polymeric ferric sulfate was assumed to be the maximum application amount of 1000 mg L⁻¹, as per the literature; (2) heavy metals were assumed not to be removed by coagulation and precipitation after the application of polymeric ferric sulfate; and (3) the maximum detectable concentrations of antimony, cobalt, vanadium, and manganese were used. Based on the most unfavorable principle, the use of recycled water purifiers may lead to an increase in the effluent quality indicators for industrial wastewater treatment plants, that is, 0.00000478 mg L⁻¹, 0.0226 mg L⁻¹, 0.00416 mg L⁻¹, and 0.163 mg L⁻¹, respectively, for antimony, vanadium, cobalt, and manganese, all of which are much less than the corresponding discharge standards of 0.3 mg L⁻¹, 1 mg L⁻¹, 1 mg L⁻¹, and 2 mg L⁻¹, respectively.

In general, the effluent quality of the sewage treatment plant will fluctuate due to changes in the quality and quantity of the incoming water, and the magnitude of the fluctuation can be expressed by the volatility [12,16,21]. Based on automatic monitoring data of four major water pollutants released by the Ministry of Ecology and Environment of the National Key Pollutant Discharge Units of Urban Sewage Treatment Plants in 2020, the fluctuation coefficients of chemical oxygen demand, ammonia nitrogen, total nitrogen and total phosphorus in the discharge outlets of more than 3000 national key pollutant discharge units are 150%, 200%, 130%, and 170%, respectively. The use of reclaimed water purifying agent will cause a fluctuation in effluent quality. The potential maximum fluctuation rates are 0.0016%, 2.26%, 0.42%, and 8.15%, respectively, which are relatively small. The use of reclaimed water purifier, therefore, has little effect on the indexes of antimony, vanadium, cobalt, and manganese in effluent water quality.

3.3.2. Human Health Risk Analysis

The primary role of risk characterization is the calculation of carcinogenic and non-carcinogenic risks of single, and all concerned, pollutants [29]. Risk assessment models are used to calculate the carcinogenic risk and hazard quotients of pollutants based on the dose–effect relationship. This includes the carcinogenic risk and hazard quotient of a single pollutant in water and the total carcinogenic risk and hazard index of a single pollutant in water. In the industrial wastewater treatment process, polymeric ferric sulfate may affect not only the effluent quality but also the health of operators. The human health risk assessment model recommended by the “Technical Guidelines for Soil Pollution Risk Assessment of Construction Land” standard (HJ 25.3) can be used to assess additional risks during the use of recycled polyferric sulfate. Based on HJ 25.3, the risk is unacceptable when the hazard quotient of a single pollutant exceeds 1.

The non-carcinogenic risk of heavy metals is related not only to the ADD but also to the RfD of each heavy metal (Formula (2)). The greater the toxicity of heavy metal elements, the smaller the RfD; on the contrary, RfD is larger. Antimony, vanadium, cobalt, and manganese are mainly non-carcinogenic. The heavy metal RfDs are presented in

Table 3. Fluctuation rate of concentration of Sb, Co, V, and Mn.

	Concentration mg L−1	Fluctuation Rates %	Emission Limits mg L−1	Standards
Sb	0.00478	0.00159	0.3	GB 30770-2014 [30]
V	22.6	2.26	1	GB 26452-2011 [31]
Co	4.16	0.416	1	GB 25467-2010 [32]
Mn	163	8.15	2	GB 8978-1996 [17]

.

During the application phase, the reclaimed water purifier may endanger the health of workers through dermal contact or oral ingestion, The non-carcinogenic hazard quotient calculation results are detailed in

Table 4. Calculation results of non-carcinogenic hazard quotients.

	RfDo (mg/kg-d)	${\mathit{C}\mathit{G}\mathit{W}\mathit{E}\mathit{R}}_{\mathit{n}\mathit{c}}$		${\mathit{H}\mathit{Q}}_{\mathit{c}\mathit{g}\mathit{w}}$		HI
		Dermal	Ingestion	Dermal	Ingestion
Sb	0.0004	1.67 × 10−10	1.11 × 10−2	1.33 × 10−8	2.65 × 10−1	2.65 × 10−1
V	0.009	1.67 × 10−10	1.11 × 10−2	1.61 × 10−5	5.57 × 101	5.57 × 101
Co	0.0003	6.67 × 10−11	1.11 × 10−2	9.25 × 10−7	3.07 × 102	3.07 × 102
Mn	0.14	-	1.11 × 10−2	-	2.58 × 101	2.58 × 101

. According to the assessment results, the non-carcinogenic hazard quotients for vanadium and manganese were 5.6 and 2.6, respectively. They are both above the single-contaminant hazard quotient limit (HQL) of 1 and within the unacceptable range, with the main route of exposure being oral, accounting for over 99% of the total hazard. The main exposure mode of cobalt is also oral, and the total non-carcinogenic hazard quotient reaches 307, far exceeding the HQL of 1 for a single pollutant. This may cause serious damage to human health. The non-carcinogenic hazard quotient of antimony is 0.3, which is less than the HQL of 1 for a single pollutant, mainly because its concentration in the reclaimed water purification agent is low, that is, only 0.00478 mg L⁻¹. The severity of non-carcinogenic single pollutant effects based on dermal contact and oral exposure, therefore, were ranked as cobalt > vanadium > manganese > antimony. Cobalt is the least abundant at 4.16 mg L⁻¹, but its environmental risk is much greater than that of vanadium and manganese, mainly because the RfD of cobalt is only 0.0003 mg kg-d. Cobalt, therefore, has a lower risk threshold and is more likely to cause non-carcinogenic effects and endanger the health of the operator.

The non-carcinogenic effects of antimony, vanadium, cobalt, and manganese contained in the reclaimed water purifier during subsequent water treatment after dosing are 0.00026, 0.056, 0.31, and 0.026, respectively, based on the dermal and oral exposure routes, which are all less than the HQL of 1 for a single contaminant, and the environmental risk is acceptable. When replacing virgin water purifiers with reclaimed water purifiers in industrial wastewater treatment plants, therefore, operators should be supervised to take protective measures, such as wearing the correct mask and work clothes, especially during the dosing process.

4. Conclusions

Through the collection of waste acid and ferrous sulfate heptahydrate for pollutant screening and a production analysis of polymeric ferric sulfate, the potential pollutants are characterized as Ti, V, Mn, Cr, Co, Cu, Ni, Zn, Ba, Pb, and so on. The existing product quality standard for polymeric ferric sulfate, that is, Water Treatment Agent Polymeric Ferric Sulphate (GB 14591-2016), only has requirements and regulations for mercury, arsenic, zinc, lead, nickel, and chromium. The test results show that polymeric ferric sulfate also contains antimony, vanadium, cobalt, manganese, and titanium, with concentrations of manganese and titanium reaching 163 mg L⁻¹ and 631 mg L⁻¹, respectively, and posing certain environmental risks. This study can be used as a reference for environmental management when revising the standard of Water Treatment Agent Polymeric Ferric Sulphate (GB 14591-2016). The use of polymerized ferric sulfate in industrial wastewater treatment has an impact on the effluent and may lead to fluctuations in water quality of up to 8% mg L⁻¹, which places certain demands on the downstream water treatment system. The non-carcinogenic hazard quotients of vanadium, manganese, and cobalt are 5.6, 2.6, and 307, respectively, which are all higher than the HQL of 1 for single pollutants and within the unacceptable range. Cobalt, in particular, may induce serious physical damage at relatively low levels and should be included as a key concern during revision of pollutants in GB 14591-2016. In industrial wastewater treatment plants, therefore, where recycled water purifiers are used to replace virgin water purifiers, the operators should take additional protective measures, such as wearing the correct masks and overalls.