Pilot Test of Soil Washing for Arsenic-Contaminated H₂SO₄ Plant Soil Using Discarded H₂SO₄

Abstract

This study investigates an innovative soil washing process designed to remediate arsenic (As) contamination in sulfuric acid (H₂SO₄) plant soil by using discarded H₂SO₄ solution in situ. The pilot-scale process comprises five key steps: screening and rinsing of oversized sand, washing the soil with H₂SO₄, phase separation, recycling the washing solution, and water recovery. This research explored the optimal washing parameters for the process and further researched the reuse of the H₂SO₄ solution across multiple batches. The pH of the washing solution, critical at a threshold of 6.5, was identified as a key factor for effective recycling. Approximately 75% of the H₂SO₄ solution was successfully recycled. In terms of economic analysis, the total operational cost of the soil washing process was significantly lower than in previous studies. Overall, these findings demonstrate the feasibility of using discarded H₂SO₄ as a washing agent for As-contaminated soil. The integration of automated pH-based monitoring technology streamlines the washing process, providing a cost-effective and effective As removal remediation strategy, making it a viable option for large-scale applications in soil remediation.

1. Introduction

Arsenic (As) is a naturally occurring toxic metalloid present in the environment [1]. Human activities, such as mining, smelting, and pesticide application are major contributors to arsenic-contaminated soils [2,3]. As a highly toxic element, arsenic can lead to severe health issues, including disruptions in cellular metabolism, respiratory problems, cancer [4], and various pathological changes [1]. Given that soil is in constant interaction with water and air [5], arsenic-contaminated soil not only poses significant risks to human health but also threatens ecosystems by disrupting soil health and biodiversity [6]. According to the U.S. Geological Survey (USGS), China has consistently ranked first in global arsenic production [7]. In particular, regions in the southwest China, such as Yunnan Province, where significant realgar (As₄S₄) ore deposits exist, are heavily affected [8]. The application of As₄S₄ in sulfuric acid (H₂SO₄) production generates significant ecotoxicological threats via arsenic emission. In essence, As₄S₄ can releases As during roasting processes via Equation (1) [9]. Volatile arsenic trioxide (As₂O₃) either escapes as emissions or condenses within equipment. Subsequently, during flue gas scrubbing with water or lime slurry, As₂O₃ reacts to form soluble arsenite (H₃AsO₃) or solid arsenites Ca₃(AsO₃)₂ (Equations (2) and (3)) [10]. Moreover, solid residues from roasting contain unoxidized As₄S₄ and Fe(III)–arsenic phases like scorodite (FeAsO₄·2H₂O). These arsenic-containing roasting residues are often directly stockpiled on soil, where rainwater leaches soluble arsenate (H₂AsO₄⁻) through acid dissolution via Equation (4). In neutral–alkaline soils, arsenate (HAsO₄²⁻) adsorbs onto iron oxides, but under reducing conditions (e.g., flooded rice paddies), reductive dissolution of Fe(III) minerals releases As contaminating crops and groundwater, ultimately entering the food chain [11]. Therefore, it is an urgent need to implement appropriate measures to remediate arsenic-contaminated soils.

As₄S₄ + 7O₂ → 2As₂O₃(g) + 4SO₂

(1)

As₂O₃ + 3H₂O → 2H₃AsO₃

(2)

3Ca(OH)₂ + 2H₃AsO₃ → Ca₃(AsO₃)₂↓ + 6H₂O

(3)

FeAsO₄ + 2H⁺ → Fe²⁺ + H₂AsO₄⁻

(4)

Traditionally, the remediation of As-contaminated soils has relied on methods such as excavation and replacement [12], in situ capping [13], solidification/stabilization (S/S) techniques [14,15], bio-remediation [16,17], plant extraction technology [18], soil washing [6], ultrasonically aided technique [2,19], high intensity magnetic separation (HIMS) [20], and electrokinetic remediation [21,22]. Nevertheless, incomplete removal of metals through adsorption and coprecipitation, the possible mobilization of metals in changing chemical conditions (such as a decrease in pH), and/or solubilization of As-containing phases [23] still occur. Moreover, sulfate and acid rain can degrade S/S materials, compromising their durability and contributing to persistent As contamination [24]; such durability concerns also increase remediation costs. In contrast, soil washing has emerged as a promising alternative due to its ability to completely remove As contaminants, its high heavy metal treatment efficiency, broad soil–contaminant compatibility, and minimized waste generation [2,25,26]. In recent years, inorganic acids [27], organic acids [28], surfactants [25], and alkaline solvents [29] have been used to extract As in the soil minerals by the dissolution of minerals and anionic exchange [2,30]. However, the extensive use of reagents would increase the cost of soil remediation; moreover, these additional reagents may change the bioavailability of As in the soil, leading to potential environmental risk. Although there is a certain research foundation for soil washing technology in soil remediation, reducing the As content in soil is difficult because of the bad economy and automaticity of the remediation process [27,31,32].

In this research, we conducted a pilot-scale soil washing test to reduce As levels in contaminated H₂SO₄ plant soil to achieve standard B of the soil quality assessment for exhibition sites in China (80 mg kg⁻¹) by utilizing discarded H₂SO₄ in situ. The process can be dynamically optimized to ensure maximum arsenic removal while minimizing chemical usage by using a feedback loop that adjusts the concentration of H₂SO₄ and the washing duration based on pH readings. This study successfully improved the automation and cost-efficiency of the technology compared to previous pilot tests, which involved complex procedures and high processing costs.

2. Materials and Methods

2.1. Soil Samples

Soil samples were collected from the 0 to 30 cm surface layer at a plant site in Luliang, Yunnan. The use of As₄S₄ by H₂SO₄ plants generates a significant amount of arsenic (As) waste, leading to arsenic contamination in the plant soil. For standard pedological analysis, the soil pH was measured in a 1:2.5 (w/v) soil to 0.01 M CaCl₂ water solution suspension. Soil samples were analyzed for organic matter by modified Walkley–Black titrations, cation exchange capacity (CEC) by the ammonium acetate method, and soil texture by employing the pipette method. The characteristics of soil sample are present in

Table 1. Characteristics of soil samples collected from the contaminated plant site in Yunnan, China.

Parameters	As-Contaminated Soil
Land use Organic matter (%)	Plant–soil < 0.1
CEC (cmol/kg)	1.6
pH	7.70
Soil texture	Sand
Sand (%)	98.2
Silt (%)	1.4
Clay (%)	0.4
As (mg kg−1)	165.1
Ca (mg kg−1)	5.4 × 105
Fe (mg kg−1)	6.8 × 103

.

2.2. Chemicals and Reagents

Potassium chloride (KCI, purity, 99%), hydrochloric acid (HCI, purity, 36–38%), nitric acid (HNO₃, purity, 65–68%), sulfuric acid (H₂SO₄, purity, 95–98%), polyacrylamide (PAM, purity, 99%), sodium hydrogen phosphate (Na₂HPO₄·12H₂O purity, 99%), sodium dihydrogen phosphate (NaH₂PO₄·2H₂O, purity, 99%), and sodium hydroxide (NaOH, purity, 99%) were purchased from Beijing Chemical Reagents Company (Beijing, China). All solvents, chemicals, and reagents were used as received without any further purification. The Direct-Q3 UV Milli-Q Water purification system (Merck Millipore, Shanghai, China) was employed to prepare Ultrapure water (18.2 MΩ cm) throughout the experiments. The solution pH was adjusted with H₂SO₄ and NaOH solutions.

2.3. As Sequential Extraction Analysis

The As sequential extraction analysis is aimed to further verify the feasibility of the washing process.

• Add 25 mL of 0.25 M KCl to 2.5 g of sample in a 250 mL volumetric flask to extract the soluble fraction of arsenic species and stir the slurry for 2 h;
• Extract the adsorbed fraction of arsenic species by adding 0.1 M Na₂HPO₄ (25 mL, pH 8.0) and stirring for 20 h;
• Extract the carbonate fraction by adding 1 M sodium acetate (25 mL) and stirring for 5 h, and add 0.1 M Na₂HPO₄ (25 mL) and stir for 20 h;
• Extract the operationally defined crystalline mineral fraction of crystalline oxide and amorphous aluminosilicates by adding aqua regia (30 mL HCl and 10 mL HNO₃) and stirring for 1 h.

The stirring condition of the above arsenic sequential extraction procedures was 120 rpm. An aliquot of 10 mL of supernatant was taken and centrifuged at 3200 rpm for 20 min, and then filtered with a 0.45 μm filter.

The amount of the soluble fraction, adsorbed fraction, and carbonate-bound fraction is 96.6 percent in all As sequential extraction fractions. In addition, As content of the residual fraction is 6.6 mg·kg⁻¹, which is lower than 20 mg·kg⁻¹ which is the criterion referenced (

Table 2. As sequential extraction of soil samples collected from the contaminated plant site.

As Sequential Extraction	As Content in Soil (mg·kg−1)	Percent of ①~④Σ (%)
① soluble fraction	143.2	74.0
② adsorbed fraction	36.3	18.8
③ carbonate-bound fraction	7.3	3.8
④ residual fraction	6.6	3.4
①~④Σ	193.5	100.0
Total As	165.0

). Thus, the washing process with H₂SO₄, the acid from the contaminated plate, is available.

2.4. As Determination

The As content in soil was measured by X-ray fluorescence (XRF, Axios, Panalytical Netherland, Almelo, The Netherlands). The soluble As concentration in washing water was analyzed with inductively coupled plasma-optical emission spectroscopy (ICP-OES; Optima 8300, Perkin Elmer, Waltham, MA, USA). ICP-OES measurement of As at a wavelength of 193.696 nm may have interferences with V and Ge. The concentration of V was measured at 0.15 ppm, Ge was not detected, and, thus, would likely have a limited impact on As quantification [33].

2.5. Criterion

The As content in treated soil should be less than 20 mg·kg⁻¹ (in reference to the Standard A of soil quality assessment for exhibition sites in China). The As concentration in treated water should be less than 0.5 mg·L⁻¹ (in reference to the integrated wastewater discharge standard in China).

3. Soil Washing Process and Equipment

The flowchart of the pilot-scale soil remediation process (Figure 1) outlined here is a batch process consisting of five main steps:

3.1. Step1. Screening and Rinsing of Oversized Sand

Batches of 200 kg of As-contaminated soil (air-dried, with an average humidity of 4.8 ± 0.4%) fell from a feed hopper (a), which has a 3 m³ capacity, and were precisely transferred to the screening machine (b). This machine separated oversized sand from the soil using a 2 mm sieve. The oversized sand was then sent to the sand washer (c), where 10 L of fresh tap water was used for rinsing. The cleansed oversized sand was piled, awaiting backfilling.

3.2. Step2. Soil Washing and Phase Separation

Soil from Step 1, with a diameter less than 2 mm, was transferred to the first washing machine (d) via a screw conveyor and was washed with H₂SO₄ from a dosing tank (e). The initial washing process parameters were set at a 0.4% (V/V) H₂SO₄ concentration, a 5:1 water-to-soil ratio, and a 60 min washing time. Consequently, 1000 L of tap water and 4 L of concentrated sulfuric acid were mixed in the washing machine and agitated for 60 min at 25 rpm. Approximately 75% of the washing H₂SO₄ solution was recycled to the first washing machine for the subsequent batch of soil washing, leveraging the increased availability of H⁺ ions for washing in the lower pH solution. After soil washing, the mixture of soil and H₂SO₄ solution was sent to the high-efficiency settling tank (f), where it was separated into a solid phase (washed soil) and a liquid phase (washing H₂SO₄ solution).

3.3. Step3. H₂SO₄ Recycling and Treatment of Washed Soil

The washed soil was then transferred to the second washing machine (g) for further treatment using a pump, while the washing H₂SO₄ solution was sent to the buffer tank for treatment in Step 4. The washed soil in the second washing machine was rinsed with 1000 L of fresh tap water for 30 min at 20 rpm to further reduce the As concentration in the soil and increase the pH of the washing water, ensuring safe backfilling. After rinsing, the mixture from the second washing machine was transferred to a settling tank (h) for phase separation, with the washed soil and rinsing water being sent to the buffer tank in Step 4 for subsequent treatment. The washed soil was then pumped into a belt filter press, reducing the soil’s humidity to less than 80%. Finally, piles of remediated soil, mixed with the cleansed oversized sand from Step 1, were air-dried and backfilled.

3.4. Step4. Treatment of Washing Solution

The washing water in the buffer tank (j) from Step 1, Step 2, and Step 3 was close to neutral pH, in which condition the FeSO₄ was easier to be oxidized to Fe³⁺ by O₂ in the aeration tank. After adding 2.5 kg of FeSO₄ from the dosing tank into 2000 L washing water, the suspension flowed into the aeration tank, where it was oxidized in 30 min for Fe²⁺ to transform completely. Then, polyacrylamide (PAM) was added into the aeration tank (k) for As and Fe precipitating. Finally, the supernate in the aeration tank fell into the water tank (m) for recovery in Step 5 and the precipitate was sent to the plate-and-frame filter press (l); after that it would be hazardous waste that should be disposed of specially.

3.5. Step5. Recycling of Washing Water

The treated washing water in the water tank (m) was recycled. It was used in the first washing machine (d) in Step 2 to wash the subsequent batch of soil with H₂SO₄. Moreover, it was utilized in the second washing machine (g) in Step 3 to rinse the subsequent batch of soil, effectively serving as fresh tap water.

4. Results and Discussion

4.1. Parameters in the Washing Process

The recommended washing process parameters, including washing time, water/soil, and H₂SO₄ concentration, were determined to be 60 min, 0.4% H₂SO₄, and 5:1, respectively (Figure 2). More than 90% of the arsenic (As) was removed within 100 min of the washing period, and the washing period beyond 60 min did not significantly increase As removal (Figure 2A). Given that a longer washing period increases operating costs, 60 min was selected for this study. As shown in Figure 2B, although the As content in the washed soil under all water-to-soil conditions was below 20 mg/kg (referencing Standard A of soil quality assessment for exhibition sites in China), the washing efficiency of the H₂SO₄ solution significantly decreased at a 6:1 water-to-soil ratio due to a sharp reduction in As concentration compared to other conditions. Therefore, a 5:1 water/soil ratio was chosen. Additionally, the concentration of As in the washing solution showed a good linear correlation with H₂SO₄ concentrations up to 0.6% H₂SO₄ (Figure 2C). Considering the cost of the agent in the project, a 0.4% H₂SO₄ concentration was chosen, as the As content had remained below 20 mg/kg.

It is economically and simply feasible to treat As pollution using discarded H₂SO₄ from plants. However, the changes in soil physicochemical properties and phytotoxicity are recommended to assess [6,34]. pH is considered to be a key factor in regulating these properties [35]. Indeed, the results demonstrate near-neutral soil pH following the washing in Step 3. As it is presented, the residual acidic leachate reduces to <25% after solid–liquid separation in Step 2, and the rinsing (1000 L tap water) process in the Step 3 further removes arsenic while neutralizing pH. In practice, the washed plant soil was used to backfill the plant building on-site, and the pH of the washed soil ranged from 6 to ~7.5 (Figure 3). It should be noted that the acid washing may influence the composition of microbial communities, affect trace metal concentrations and/or speciation, and alter nutrient accessibility. Subsequent plant extraction technology and bio-remediation synergistic application could be used to enhance soil fertility. For this study, >90% efficiency of As removal was achieved, and neutral pH was maintained after the remediation process.

4.2. H₂SO₄ Recycling and Water Recovery

4.2.1. Reuse of H₂SO₄ in the Washing Process

The 0.4% H₂SO₄ solution used for soil washing remained effective for the next batch, so the potential for reusing the H₂SO₄ solution was explored in Step 3, and four batches were selected. The initial batch had an As concentration of 22.1 mg·L⁻¹ at a pH of 6.34. With increasing reuse, the As concentration also increased, while the pH fluctuated minimally until the fifth batch. It was evident that As removal was inhibited in the fifth batch due to the As concentration being equal in the fourth and fifth batches. Additionally, the pH began to rise from 6.5 after the fifth batch. In the sixth and seventh batches, the As concentration decreased gradually as the pH increased. Since some H₂SO₄ solution was lost during the soil washing process (Step 2) due to soil absorption, we controlled the recycling to an average of 75% (V/V) of the H₂SO₄ solution in the washing process.

The pH of the solution in the first washing machine (Figure 3) could serve as an effective monitoring factor for the reuse of H₂SO₄. If the pH was below 6.5, the washing solution could be used for the next batch; otherwise, it could not. Therefore, the process of H₂SO₄ recycling could be automated by monitoring the pH. This critical pH of 6.5 aligns with the research of Zhu, Zhang [36] who found that the As concentration decreased most rapidly in solution at around pH 6.5 during the mixing and precipitation of AsO₄³⁻ with Ca²⁺. As Table 1 indicates, the plant soil was sandy and contained 5.4 × 10⁵ mg/kg of Ca; considering the karst geological features of the sample collection site Yunnan, it is reasonable to infer that the predominant AsO₄³⁻ was adsorbed with Ca²⁺, and abundant CaCO₃-containing minerals partially neutralized sulfuric acid during washing. To be sure, this phenomenon is specific to samples of this study and not universally applicable. The result demonstrates this soil washing process has efficacy for alkaline soils. For non-alkaline soils the leachate pH rises more slowly, and more washing cycles may be achieved without changing the washing regent (when pH <6.5 after multiple washing cycles). Although soil organic matter has been reported to play a significant role in retaining heavy metals [37], organic matter content represents less than 0.1% in the soils from this study (Table 1), indicating that it likely removes less As than phases such as Fe-(oxyhydr)oxides [38].

4.2.2. Remediation of Washing Water for Water Recovery

The wastewater with about 45 mg·L⁻¹ As concentration from a high-efficiency settling tank (Figure 1) was treated by precipitation using FeSO₄ for water recovery. Firstly, the pH of wastewater was controlled to more than 7 by adding some Ca(OH)₂ into the buffer tank. Secondly, different concentrations of FeSO₄ shown in Figure 4 were injected into the wastewater for the oxidation process. Finally, As(V) and Fe(III) were coprecipitated during sufficient aeration in the aeration tank. However, the aim to realize As concentration below 0.5 mg·L⁻¹ (referencing the integrated wastewater discharge standard in China) was not simple because of the dosage of FeSO₄, although pH and aeration were easily controlled. The results shown in Figure 4 indicate that 0.5 and 1.0 kg FeSO₄ in 10 m³ solution could not decrease the As concentration under 0.5 mg·L⁻¹, but 1.25 kg and 1.5 kg FeSO₄ could. However, as the aeration time increases, the concentration of As in the recovered solution with 1.25 kg FeSO₄ rises again, probably due to desorption. Ultimately, this study set 1.5 kg FeSO₄ as the optimal condition for this technology.

4.3. As Speciation Transformation

Prior to acid washing, arsenic in contaminated soil primarily exists as adsorbed species on Fe/Al oxides and clay minerals (dominantly arsenate, As(V)) or as coprecipitates within amorphous Fe/Mn (oxy)hydroxides (e.g., FeOOH-As). During the H₂SO₄ washing step (Step 2), acidic dissolution (0.4% V/V H₂SO₄, Ph ≈ 2) liberates oxide-bound arsenic through proton-promoted desorption and partial matrix dissolution, converting solid-phase As into soluble arsenate anions (H₂AsO₄⁻, Equation (4)) in the leachate. A minor fraction of As(III) may oxidize to As(V) under acidic–aerobic conditions. Subsequent rinsing with water (Step 3) removes residual soluble arsenic but may leave re-adsorbed As(V) on mineral surfaces in the washed soil. Crucially, arsenic in the collected acidic leachate undergoes oxidative precipitation in Step 4, where aeration converts Fe²⁺ to Fe³⁺ (Equation (5)), followed by hydrolysis to form amorphous ferric oxyhydroxides (Fe(OH)₃, Equation (6)). These freshly formed solids effectively sequester dissolved arsenic via coprecipitation and adsorption, transforming mobile As(V) into stable ferric arsenate complexes (Equation (7)) [39]. Consequently, over 90% of the initial arsenic is ultimately immobilized in the hazardous iron–arsenate sludge, while the remediated soil retains only low-bioavailability residual phases (e.g., lattice-bound As in silicates or recalcitrant adsorbed arsenate).

4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O

(5)

Fe³⁺ + 3H₂O → Fe(OH)₃↓+ 3H⁺

(6)

≡FeOH + H₂AsO₄⁻ → ≡FeHAsO₄⁻ + H₂O

(7)

4.4. Process Costs

Although the total costs of the soil washing process including the equipment costs, personnel costs, material costs and profit were too difficult to evaluate during the pilot test, the costs of energy, materials, and wastes could be listed exactly in

Table 3. Material and energy costs of the soil remediation process for one ton of soil (five soil batches); solid waste generated and cost of its disposal.

Consumables	Consumption/Generation Per ton of Soil	Unit Cost	Costs Per Ton of Soil
Energy
Apparatus	245 KW h	0.9 CNY KW h−1	220.5 CNY
Materials
H2SO4	10 L	14 CNY L−1 a	140 CNY
Water	1000 L	4.4 CNY m−3	4.4 CNY
Lime	2.5 kg	1 CNY kg−1 a	2.5 CNY
FeSO4	1.5 kg	0.4 CNY kg−1 a	0.6 CNY
PAM	0.5 kg	3.6 CNY kg−1 a	1.8 CNY
Wastes
Solid waste	30 kg	1.8 CNY kg−1	54 CNY

^a Internet source(http://www.alibaba.com) average price of 10 sellers.

. Firstly, we calculated the total electric energy consumption for all the apparatus used: belt transporter, hopper, screening machine, sand washer, washing machine, aeration tank, plate-and-frame filter press, belt filter press, pH meter, and pumps.

The costs of all materials used in the soil washing process and wastewater treatment were measured and summarized in Table 3. The costs of all chemicals and materials were obtained from the internet (http://www.alibaba.com), while water pricing data were provided by local suppliers. The costs of H₂SO₄, CNY 140, were dominated in the total materials costs of CNY 150 for one ton of soil, but as referred to Section 4.1, H₂SO₄ from the discarded materials of H₂SO₄ plant was free in the pilot test. Thus, H₂SO₄ is a good choice for treating As pollution soil by washing in or near the H₂SO₄ plant. Compared with the previous pilot-tests, the total costs of CNY 150 for materials were inexpensive. David Voglar and Domen Lestan charge CNY 274 for materials per ton of metal contaminated garden soil using ethylene diamine tetraacetic acid (EDTA) in 2012 [40], and they cost CNY 231 with materials for one ton of soil using EDTA by the updated washing process in 2013 [41].

The total cost was CNY 423 lower than methods reported by Voglar and Lestan (CNY 580 in 2012 and CNY 515 in 2013). However, as displayed in Table 3, the energy cost was more than half of the total. Therefore, it would improve the economics of soil washing by simplifying the process and utilizing appropriate equipment.

5. Conclusions

The soil washing process for As-contaminated plants in our study aimed to make this technology more automatic and economical. H₂SO_4, which is the discarded material of plants, was used as a washing agent to treat pollution to reduce the cost of materials and then, based on the washing agent, the recommended washing process parameters including washing time, water/soil, and H₂SO₄ concentration were determined to be 60 min, 0.4% H₂SO₄, and 5:1, respectively. In addition, the H₂SO₄ recycling and water recovery induced the usage of H₂SO₄ and water as much as possible within H₂SO₄ by recycling four batches until they reached pH 6.5 and water treating until As concentration fell below 0.5 mg·L⁻¹. Moreover, the pH of the solution could be a great monitoring factor in the reuse of H₂SO₄ and recovery of water to realize automation. Finally, calculating the cost of energy, materials, and wastes revealed that materials cost of CNY 150 and total costs of CNY 423 were lower than those reported in previous pilot tests by David Voglar and Domen Lestan.