Heavy metal lead(Pb) removal in contaminated soils using citric acid and malic acid as washing agents

doi:10.21203/rs.3.rs-1862022/v1

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Heavy metal lead(Pb) removal in contaminated soils using citric acid and malic acid as washing agents

https://doi.org/10.21203/rs.3.rs-1862022/v1

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Soil washing is a rapid and efficient remediation technique for soil contaminated by heavy metals. In this study, Pb was removed from contaminated soil with citric acid (CA) and malic acid (MA). The effects of washing concentration, liquid-solid(L/S) ratio, washing time, and pH value on the removal rate and metal stability in lead-contaminated soil were investigated. Results showed that the mixed chelating agent's (MC) removal rates of the heavy metal lead from the soil by citric acid (0.3m) and malic acid (0.3m) at a certain volume ratio of 4:1 (CA: MA = 4:1) was up to 88.65%. After washing, the leaching concentration of lead in soil decreased by 63.39%, which was lower than the identification standard value of hazardous waste. The speciation analysis study indicated that the acid-soluble and reducible fractions in the soil with excellent bioavailability after MC washing were reduced by 83.23% and 71.99%, respectively. Which reduces the environmental risk greatly. Besides, heavy metal lead was also found to become more stable after the soil was washed by MC. This method is shown to help reduce the bioavailability and leachability of lead and has proven to environmentally friendly way to remediate lead-contaminated soil.

Soil washing

Condition optimization

Lead-contaminated soil

Citric and malic acids

Speciation analysis

Soil is a natural resource on which mankind depends for survival and is a significant component of the ecological environment(Tao et al., 2013). With the improvement of people's living standards, the issue of heavy metal contamination has become increasingly serious, not only threatening cultivated fields and soil agricultural products but also affecting human health(Luo et al., 2011). Unreasonable use of fertilizers and pesticides in the mining industry, electroplating industry, smelting industry, and agriculture is the primary sources of metal contamination in the soil. (Qin et al., 2021). Recently, the non-standard management of electronic waste leads to severe lead pollution. Lead has serious toxic effects on most living organisms, all kinds of mineral forms, and long-term durability in the natural environment. Human absorption of the heavy metal lead can result in irreversible impacts to organisms, including nerves, cardiovascular, digestive, hematopoietic, and endocrine systems. In addition, children are found most affected by lead contamination on their growth(Mathee et al., 2018). Therefore, Lead-contaminated soils have received widespread attention and many researchers have endeavored to seek solutions for remediations (Hernández-Soriano et al., 2011; Looney et al., 2006).

Many lead-contaminated soil remediation technologies have been applied on lead-contaminated sites(Ricketts et al., 2020), including physical remediation, chemical remediation, and bioremediation. For physical remediation, soil removal and soil replacement methods are commonly used. The soil removal method is effective but only suitable for polluted areas with low heavy metal content whilst, the soil replacement one can achieve a completely repaired status, but the operating cost was too high. Besides, bioremediation is recently developed but is also not been widely used in industry because the stability of bacteria is poor, the concentration range of adsorbed heavy metals is low, and the restoration cycle is relatively long. On the other characteristics of relatively simple operation procedure, low operation cost, no need to extend the monitoring time, and permanent removal of heavy metals. The soil was also reported to be reusable after suitable washing(Hou et al., 2014).

For chemical washing remediation technology, contaminated soil was mixed with a washing reagent and agitated for some time to remove contaminants that may impact the properties of the soil, Therefore, selection of suitable washing reagent is important to remediation of lead-contaminated soil so as to reduce the harmful effect of it to human health and the environment.

The frequently used agents mainly include (LMWOAs) washing agents (e.g. malic acid, tartaric acid, and citric acid), inorganic washing agents (e.g. HCl and H₂SO₄)(Tang et al., 2017), and surfactants (e.g. rhamnolipid and saponin), and synthetic organic chelating agents(e.g. saponin and ethylenediaminetetraacetic acid(EDTA), and diethylenetriaminepentaacetic acid (DTPA)(Ferraro et al., 2016; Yoo et al., 2017). Despite the high removal efficiency of inorganic agents and synthetic organic chelators, they still have some disadvantages(Jelusic and Lestan, 2014; Kim et al., 2016). For example, the inorganic washing agent will acidify the soil during the washing process, disrupting the physic-chemical properties of the soil and leading to nutrient loss from the soil(Cao et al., 2017). Synthetic organic chelators with relatively expensive prices are extremely poorly biodegradable and are prone not to be used in the washing process (Jez and Lestan, 2016). Therefore, the actual application of synthetic organic chelating agents is restricted. Surfactants can form micelles in the solution and perform well in removing organic pollutants, however, it is less effective in clearing heavy metals away from the soil(Ishiguro et al., 2016). On the other hand, LMWOAs has received more attention recently to remove heavy metals in soil because of their low price. Besides, they are only biodegradable itself but also promotes the desorption of heavy metals out of soil. They do not bring secondary pollution and only cause little damage to the soil(Chen et al., 2016). And therefore LMWOAs are promising washing agent to remediate contaminated soil.

LMWOAs include citric, malic, lactic, tartaric acids, and so on, which are primarily generated by plant root secretion, microbial metabolism, and soil substance breakdown(Tan et al., 2022). Among them, citric, malic, and tartaric acids are widely used in many studies because they are more easily obtained from the environment. Research studies show that LMWOAs play an important role in removing heavy metals form the soil. For instance, the transformation and migration of heavy metals that are greatly affected by LMWOAs in the soil were reported by some studies. It also shows that chelation, precipitation, and redox of organic acids with heavy metals all impact heavy's fixation metals in the soil(Gholizadeh and Hu, 2021). The citric acid in the LMWOAs changes the form of heavy metals in the soil by releasing H⁺, and at the same time responds with the heavy metal ions to form soluble complexes, which can effectively remove the acid-soluble fraction of the heavy metals in the soil, while the chemical process of malic acid is an uneven diffusion process. However, some researchers have concluded that individual CA did not operate well, with possible reasons that it activates the remaining heavy metals in the soil and decreases the residuary state in the comparatively steady state, and therefore the residual content of lead in soil increased after MC washing. To overcome the problems of soil acidification and other problems caused by addition of a single CA, a chelating reagent using chelation metals with an MC that was composed of a definite volume ratio of CA and MA was proposed and used in this study. CA plays an important role in acid solubility and helps improve the complexing ability. Both CA and MA are biodegradable and relatively cheap and therefore could be a cost effective and environmental-friendly method.

At present, the research on washing and remediation of lead-contaminated soil mainly focuses on the screening of single agents and the optimization of washing parameters, and the remediation of compound agents is rarely considered; The evaluation of remediation effect is mostly limited to the study of removal rate, and there are few reports on soil bioavailability, metal binding strength, and lead leaching toxicity after remediation.

The research aimed to determine the optimal parameters based on the removal rates of lead-contaminated soil in citric and malic acids under different washing conditions, and to compound the selected agents under the optimal parameter conditions. In addition, the speciation distribution of lead and the changes in metal stability and mobility in soil before and after washing with agents were also investigated. Finally, changes in the mineral composition, morphological characteristics and elemental anatomy of the soil were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Assessing the remediation effect of lead-contaminated soil.

2.1. Soil sample and agents

The soil samples were obtained from a lead-acid battery factory in Chongqing, China. After clearing impurities away, the soil was air-dried, ground, and then passed through a 100-mesh sieve. The soil was mixed well and stored in an airtight container. The basic physical and chemical properties of the soil are shown in Table 1. The citric acid and malic acid used in the experiments were all of analytical grade (Aladdin, Shanghai, China).

2.2. Soil washing experiments by different parameters

Washing concentration screening: The soil samples (2 g) were put in a series of centrifuge tubes (100 mL), with 50 mL of citric acid, and malic acid solutions of 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mol/L were added. The suspensions were shaken at 200 r/min on a shaking table for 12 hours and then centrifuged at 3,000 r/min for 20 min(Tan et al., 2022). The concentration of Pb in soil samples was adopted by an atomic absorption spectrometer (AAS-800).

Washing solid-liquid ratio screening: based on result from procedure (1), the agents optimum concentration was picked out for the solid-liquid ratio experimentation. The solid-liquid ratio was adjusted at 1:1, 1:2, 1:3, 1:5, 1:10, and 1:15. Refer to (1) for other steps.

Washing time screening: based on results from proedure (2), the most beneficial concentration and the solid-liquid ratio was picked for different time experimentations. The time was adjusted at 0, 2, 4, 6, 8, 10, 12, 16, and 24 h. Refer to (1) for other steps. The obtained kinetics were fitted with the pseudo-second-order kinetic and Elovich equations, and the equations are as follows:

The pseudo-second-order kinetic:\(\text{ }\frac{\text{t}}{{\text{Q}}_{\text{t}}}\text{=}\frac{\text{1}}{\text{k}{{\text{Q}}_{\text{e}}}^{\text{2}}}\text{+}\frac{\text{t}}{{\text{Q}}_{\text{e}}}\)

The Elovich:\({\text{Q}}_{\text{t}}\text{=}\text{Alnt+B}\)

where, \({Q}_{t}\) (mg/kg) and \({Q}_{e}\) (mg/kg) are the amounts lead desorbed on soils at time t and equilibrium; k (mg·mg^− 1·hr^− 1) in the pseudo-second-order kinetic equation represent the desorption rate constant; In the Elovich equation, A is the desorption constant related to the maximum desorption capacity; B is the desorption rate coefficient.

Washing pH value's selection: Results based on (1)(2)(3), the optimum concentration, solid-liquid ratio, and reaction and time were selected for different pH experiments. And the pH range was then adjusted to 2, 3, 4, 5, 6, and 7 by using adapting the NaOH solution.

2.3. Compound soil washing experiments of citric acid and malic acid

Determine the best washing parameters of citric acid and malic acid, compound washing with them under the best conditions and screen out the compound ratio with better removal effect of lead-contaminated soil. The compound ratio of citric acid and malic acid was shown in Table 2. Finally, the washed soil samples were dried, ground across 100 mesh sieve and made into sealed plastic mesh sieve and made into sealed plastic bags for the later application.

2.3. Speciation analysis of lead in soils

The soil fractions before and after washing were analyzed according to the modified BCR procedure (Rauret et al., 2000). Four operationally defined fractions were determined for metal: acid-soluble(F1), reducible(F2), oxidizable (F3), and residuary fractions (F4)(Gusiatin and Klimiuk, 2012). All experiments were conducted for three times at a same condition and results were averaged from three replicates(n = 3). The morphological extraction steps of lead in soil depicted in Table 3. The centrifuged solution was filtered by a 0.45µm microporous membrane and moved to 50 mL volumetric bottles. The lead concentration in various forms was determined by an atomic absorption spectrometer (AAS-800)(Li et al., 2021).

2.4 Metal stability and mobility analysis of lead in soils

The calculation of metal stability adopts the I_R value and was defined as:

I_R =\(\frac{\sum _{\text{i}\text{=1}}^{\text{k}}{\text{i}}^{\text{2}}{\text{F}}_{\text{i}}}{{\text{k}}^{\text{2}}}\)

And i is the index figure of the extraction stage, from 1 (weakest extractant) to most substantial extractant (k = 4 in BCR process). F_i is the element's percentage in the solid stage component i of the sum extracted volume (fractional content).

The I_R values can vary from 0–1: high values represent the stability of metals in the soil due to their appearance in residuary fractions; low values indicate distributions with an excellent proportion of soluble forms, and intermediate values indicate patterns that involve metal partitioning between all solid-phase components(Gusiatin and Klimiuk, 2012). Obtaining relative mobility and bioavailability of metals in soil, the M_F factor which is a ratio of the metal concentration in the roving component to the total of all components was applied:

M_F=\(\frac{{\text{F}}_{\text{1}}}{{\text{F}}_{\text{1}}\text{+}{\text{F}}_{\text{2}}\text{+}{\text{F}}_{\text{3}}\text{+}{\text{F}}_{\text{4}}}\text{100}\)%

Relatively high M_F values have been viewed as comparatively high mobile symptoms and biological validity of heavy metals in soil(Radziemska et al., 2019).

2.5. Characterization analysis

The soil samples were pretreated according to the “Solidity Waste-Extraction Procedure for Leaching Toxicity- Acetic Acid Buffer Solution method.”(China, 2007). The Pb concentration in the leaching solution was measured by atomic absorption spectrometer (AAS-800). Spectroscopy characteristics analysis of soils before and after washing was carried out. The crystal structure, surface morphology, and elements’ composition of the soil were analyzed by X-ray diffractometer (XRD; X’ Pert PRO), scanning electron microscope (SEM; FEI inspect F50), and energy-dispersive X-ray spectroscopy (EDS; EDAX Super Octane), respectively(Tan et al., 2022).

3.1 The effect of different parameters on soil washing

3.1.1 The effect of washing agent concentration

The effects of CA, and MA as agents on lead concentration and removal rate in the soil at different washing concentrations are shown in Fig. 1. According to Fig. 1, the removal rate rose quickly while the washing concentrations ranged from 0.01 to 0.1 mol/L, and when the concentrations reached 0.3 mol/L, the removal rates of CA and MA reached plateau gradually. For the reason that large amounts of organic acids bound with the heavy metals in the soils by chelation and continued to form heavy metal complexes, the concentration of organic acids that were raised at the initial phase of rinsing. The metal complexes were able to change cation with the soil so that the heavy metals are resolvable continuously. As a result, washing rate was raising quickly. Among the reagents, CA has the best washing effect on lead. Superior washing efficiency of CA was attributed to stable complex formation structures with lead ions. Following the result of the concentration screening experimentation, the agents demonstrated successful removal rate at a concentration of 0.3 mol/L. Consequently, a concentration of 0.3 mol/L was chosen for the following experimentations.

3.1.2 The effect of solid-liquid ratio

The effect of two agents on the removal of lead in soil with different solid-liquid(S/L) ratios at a concentration of 0.3 mol/L was shown in Fig. 2. As the liquid-solid (L/S) ratio increased, the removal rate of lead gradually increased. When the L/S ratio increased from 1 to 10, the removal rates of CA and MA for the two agents increased from 13.98%, 12.35–74.64% and 69.28%, respectively. Further increases in the L/S rate didn’t change the L/S rate of lead much. This is because the use of small amounts of agents limits the reaction of the lead-contaminated soil with the washing solution. When the ratio was raised, the contact sphere between the washing agent and the soil slightly increased, which when result in an increase in removal rate gradually. The larger the L/S ratio means the larger the contact area of the soil particles with the solution, which helps to complex the solution with heavy metals. However, some heavy metals exist in the crystal structure of the soil and are more difficult to be released. As a result, a optimum L/S ratio is reached. After that, the washing rate will no longer rise(Zhang et al., 2012). The L/S ratio is one of the important factors that affect the removal of heavy metals in soil washing technology. The heavy removal rate of metals can not only be improved by a suitable L/S ratio in the soil but also save economic costs and reduce site occupation. Since the S/L ratio is 1:10, the removal rate of the agents tends to be flat. To save water for soil washing, the L/S ratio of 10 was chosen as an ideal experimental condition in the next experiments.

3.1.3. The effect of washing time

It can be seen from Fig. 3 that within 0.5 ~ 2h, the removal rate of lead in the soil increased rapidly, and it is in the fast reaction stage. However, within 2 ~ 24h, the removal rate of lead in the soil became increasing slowly. And after 12h, the removal rate gradually became flattened. At this time, the removal rate of lead by the two organic acids reached 76.16% and 68.20%, respectively. Relevant research has shown that the more effective the functional groups contained in organic acids, the easier it is to associate with heavy metal ions since these functional groups offered a bigger specific surface area and adsorption sites(Tan et al., 2022). CA contains there carboxyl groups and therefore its removal rate is the best. Though MA contained two carboxyl groups, the former had two hydroxyl groups and the latter had only one, the former one is therefore had a higher removal rate. In the initial stage of washing, the weak acid-extracted and reducible heavy metals in the soil particles had weak binding power and were easy to be released and washed out; as the washing time increased, the relatively tightly bound parts of the heavy metals and soil particles were gradually washed. Therefore, the washing speed gradually slowed down. When the soil's heavy metal desorption reached saturation, its adsorption gradually increased. At this time, it was difficult to further increase the reagent rate by increasing the washing time. Eventually, as the reaction progresses, the organic acid and the lead ions in the soil gradually reached the complexation balance. Once the complexation balance was reached, the removal rate did not change much with the increase in washing time. the surface charge of the complexing agent decreased and the complexed lead ions were mutually exclusive. Washing time is a key factor in soil washing restoration technology. Too long washing time will increase energy consumption. A suitable washing time can not only reduce energy consumption but also shorten the repair time. Therefore, a contact time of 12h was chosen as the experimental condition.

To better understand the process of lead washing in soil, two kinetic equations (pseudo-second-order kinetics and Elovich kinetics) were picked out to analyze and assess the washing kinetics data. According to Fig. 4 and Table 4, where the citric acid washing-desorption kinetic process are shown, the pseudo-second-order kinetic model fitted the kinetic process better than the Elovich model with a higher correlation coefficient (R²). Some studies have pointed out that the desorption reaction fits well with the pseudo-second-order kinetic equation, indicating that the stoichiometric ratio of metal and coordination binding sites is 1:2, that is, 1 Pb²⁺ and 2 monodentate or monovalent coordination sites. Binding, which may be the cation exchange of 2 H⁺ or the coordination of 2 ligands or bidentate ligands(Lasheen et al., 2012); The kinetic results indicates that the desorption process belonged to the combined action of physical and chemical desorption, with the chemical desorption playing a main role. And for malic acid, the kinetic results demonstrated that Elovich model fitted better than the pseudo-second-order kinetic model. The study revealed that the data has a high degree of fit with Elovich, indicating that the leaching and desorption kinetic process are heterogeneous diffusion process.

3.1.4. The effect of pH

The pH of the soil solution is one of the decisive factors that affect the ability of the soil to adsorb heavy metal ions and the movement of heavy metal ions. Figure 5 shows that the pH value has a great influence on the ability of the two organic acids to remove heavy metal lead from the soil. When the pH value was between 2 and 7, as the pH value increased, the amount of negative charge on the soil surface increased accordingly, the adsorption force for lead ions is therefore increased, Correspondingly, the removal effect of the two acids on Pb decreased. This is because the repair effect at low pH mainly depends on the movement of protons. The stronger the acidity, the greater the number of metal ions that produce exchange reactions. Therefore, under strong acid conditions, the replacement and dissolution of H⁺ dominate. With the increased pH, the desorption effect of organic acid on soil Pb mainly depended on its complexing ability(Qin et al., 2004). Organic acids can not only provide a large amount of H⁺ but also provide certain functional groups. A high concentration of H⁺ can positively charge the surface of protonated soil particles, reduce the adsorption capacity of cationic heavy metals, and obtain desorption. As a result, functional groups can react with heavy metals in a complex way to remove heavy metals. This is consistent with the research finding that hydrogen ions (H⁺) freed by the carboxylic acid group played a substantial role in lead elimination (Hu et al., 2021). While the pH was 3, the removal rate of CA and MA to lead-contaminated soil were 76.875% and 69.73%, respectively. Based on the impact on soil properties and the removal efficiency, 3.0 was considered as a suitable pH value.

3.2. The effect of better compound agents on soil washing

It can be seen from Fig. 6 that the washing effects of CA and MA in different ratios after the single washing. The washing effects of CA and MA with different compound ratios were not the same, and the removal effect after compound washing was better than that of single washing. When the serial number was g (CA: MA = 4:1), the washing effect was optimal and the removal rate of lead-contaminated soil reached 88.65%. The main reason could be attributed to the more functional groups from MC. It is easier for it to associate with heavy metal ions, providing bigger specific side area and adsorption sites with the functional groups. After MC enters the soil, it takes priority to bond with heavy metals using a functional groups such as carboxyl and hydroxyl groups, which inhibit heavy's adsorption metals on soil particles' surface. CA and MA under different compound ratios may show different levels of synergy or antagonism, resulting in different removal effects. Accordingly, we chose to explore the washing repairing effect of a single CA, MA, and MC (CA: MA = 4:1) at this time.

3.3. Heavy metal fractionation in the soil analysis before and after washing

It was necessary to analyze the morphology and distribution of heavy metals in the soil after washing given to the follow-up health risk and remediation technology (Wang et al., 2020). The hazards of heavy metals in soil are not only related to their total quantity but also affected by the various morphological distributions of heavy metals. Compared with the original soil, the distribution of the metals was considerably changed after washing with agents (Fig. 7). In pristine soil, lead is mainly present in reducible F2 and acid-soluble F1(37.27%) fractions, and the two fractions account for 52.74% and 37.27%, respectively. the great amount of lead was weakly bound that making it easy to be extracted by washing agent(Guo et al., 2022). It was reported that changes in the distribution of metals can alter their environmental risks as different metal components have different chemical reactions. From our research, metals' proportions are found to varying degrees in acid-soluble, reducible and oxidizable states after washing. However, there was a degree of increase in residues, which may be because the residual agent in the soil changed the present form of lead and converted it into a stable residue state. Among them, the best removal of acid-soluble and reducible fractions would be optimal by the composite agent of MC, and its content was 143.75 and 339.81 mg/kg respectively, which were 83.23% and 71.99% lower than the original soil. The main reason is that the MC washing agent can not only offer a substantial acidic medium but also absorb plenty of ions contending with lead oxide radicals so that most of the alive lead and potential bioavailable lead in the soil is effectively removable. The presence of heavy metals in soils can impact their environmental behavior, with the acid-soluble and reducible fractions having relatively high migration capacity and bioavailability, which of high environmental risk and should be treated and addressed as first forms; the oxidizable and residual fractions exist in a relatively stable mineral lattice and were usually not easily released. The experimental results showed that washing not only made the content of metals in the soil were reduced but also the speciation ratio of metals in the soil change considerably. Before washing, the metals in the soil were primarily in the extremely reactive weak acid reducible and extractable forms, after washing the heavy metals in the soil were primarily in a relatively stable form, less mobile, and less biologically effective, which also greatly reduced the risk to the environment.

3.4. The effect of metal stability and mobility of lead in the soil before and after washing

Effective washing methods should not reduce the concentration of total metals simply to satisfy the quality standards of the soil, but should also eliminate the environmental hazards caused by the existence of flowing metal forms. Apart from the total metal content, the mobility and bioavailability of heavy metals in soil are also usually analyzed for evaluating the effects of soil remediation(Gusiatin and Klimiuk, 2012).

In this study, M_F and I_R indicators were used to evaluate the mobility and metal bond strength in the soil before and after washing(Fedotov et al., 2007). The I_R and M_F values of the contaminated soil and the original soil were determined under optimal conditions with CA, MA, and MC (Fig. 8). As can be seen from the Fig. 8, washing affects the change of metal binding in the soil, especially after the MC washing, the biggest value of I_R has changed from 0.2 to 0.43, which indicated that the heavy metal lead was more stable in the soil after the MC. It can be seen that the soil stability after CA treatment was stronger than MA. The size of the M_F value reflects the toxicity and bioavailability of heavy metals in the soil. As it can also be seen from the figure, toxicity and bioavailability of heavy metals in soil was reduced after washing with agents, and the bioavailability of heavy metals after MC washing was also the lowered, from 47.61–5.89%. However, the I_R value of heavy metals has a strong correlation with M_F. Generally, the lower the relative binding strength, the higher metal mobility. The I_R value change of heavy metals in the soil was less than the change of M_F value (Gusiatin and Klimiuk, 2012). M_F value refers only to changes in unstable components, indicating that M_F value was more sensitive than I_R value.

3.5. XRD, SEM, and EDS analysis before and after washing

XRD, SEM, and EDS analyses were conducted to investigate whether agents impact the crystal structure, surface morphology, and elements' composition of the soil. Alterations in soil structure before and after washing in a similar way of concern, particularly for industrial lands(Watermeyer et al., 2008). Figure 9 shows the XRD patterns of soils before and after washing with different agents and uses a PDF file-based matching program to identify the characteristic peaks of minerals. The results showed that the original polluted soil mainly contained SiO₂ (main characteristic peaks 20.856°, 26.638°, 50.137°, 59.952°, 68.305°) and PbO (main characteristic peaks were 29.335° and 35.307°). After washing with different agents, There was no new peak found in the residual solid by XRD analysis for soil samples, and the main peaks remained stable, which indicated that the washing process had little effect on soil mineral composition. The agents cannot destroy the soil crystal structure, because the chelation of soil heavy metals and organic acids can only form stable new ions, without forming new crystal structures. Besides, other forms of lead were not detected in the soil, indicating that the phase of other forms of lead in the soil may be amorphous, unable to form a crystalline structure, or in minor quantities.

The surface morphology and distribution of elements of the soil are indicated in Fig. 10 before and after treatment with agents. The morphology shows a rough surface, irregularly shaped structures with clearly visible contours (Fig. 10a). After washing with agents (Figs. 10b-d), large soil particles were decomposed into smaller crystals, and a better denseness between the soil samples was observed at the same time. This was because the morphology of lead elements on the surface of soil particles changed after agents were added to the soil, causing the particles to disperse gradually. Research has shown that washing will change the particle morphology of the soil. The contour of the soil particles was stilled clear after washing, which indicated that CA, MA, and MC washing did not change the soil microstructure. Hence, using agents to wash the soil is safe and does not lead to changes in soil texture. In addition, EDS spectra showed that the ratios of Pb, Fe, and Al in the soil reduced to some extent after MC that washed. As a result, it can be concluded that MC is a proper washing agent for Pb-contaminated soil remediation.

In conclusion, soil washing with MC seemed to be more favourable over citric and malic acid washes alone for rapid extraction of heavy metals lead from contaminated soils. At a condition of washing time 12h, liquid-solid ratio of 10, MC reagent concentration of 0.3mol/L, and the pH of 3, a soil lead removal rate of 88.65% was obtained. And after washing, the leaching concentration of lead in soil decreased by 63.39%, which was lower than the identification standard value of hazardous waste. It is found that MC with the synergistic effect of acidolysis or complexation has showed much better removal rate of heavy metals in soil. Besides, the active states, relative mobilities, and bioavailability of lead in soil were also significantly reduced after MC washing. In addition, MC addition also raised the stability of these elements, and decreased the acid-soluble and reducible fractions with high bioavailability in the soil by 83.23% and 71.99%, respectively. In summary, application of MC is economically feasible and help reduce the risk to soil health and therefore plays an important role in the remediation of lead-contaminated soil.

Funding

This work was financially supported by Chongqing Technology Innovation and Application Demonstration Project (cstc2018jszx-cyzdX0019).

Competing Interests

The authors declare that they have no known competing financial interests or personal relationships that have appeared to influence the work reported in this paper.

Data Availability Statement

All data, models, and code generated or used during the study appear in the submitted article.

Author Contributions

Manqing Zhao: Investigation, Methodology, Analysis, Writing-original draft; Huan Yan: Investigation, Analysis, Writing-revising; Chao Liu: Writing-revising & editing; Cong Li: Resources, Funding acquisition; Shijie Li: Investigation, Methodology; Huanfang Gao: Conceptualization, Resources, Funding acquisition, Writing-revising & editing and Supervision. Final manuscript read and approved by all authors.

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Table.1 Basic physical and chemical properties of soil

Soil properties and units	Values
Pb (mg/kg)	4818
Cu (mg/kg)	62.33
Mn (mg/kg)	380.00
Zn (mg/kg)	118.33
Rb (mg/kg)	77.66
Organic matter mass fraction (%)	1.01
Water content (%)	2.00
Lead (Pb) leaching concentration (mg/L)	9.05

Table.2 The compound ratio of citric acid and malic acid

The volume of citric acid /ml	Volume of malic acid/ml	Citric acid：Malic acid	Serial number
20	0		a
20	0		b
6.6	13.3	1:2	c
5	15	1:3	d
4	16	1:4	e
10	10	1:1	f
16	4	4:1	g
15	5	3:1	h
13.3	6.6	2:1	i

Table.3 Sequence Extraction Procedure

Step	Fraction	Extract agents	Extraction scheme
1	Acid-soluble fraction	0.11mg.L^-1HAC	Solid-liquid ratio (1g:40mL), temperature (25℃±5℃), oscillation frequency (180r/min±20r/min), time (16 hours), centrifuged at 3000 g for 20 min.
2	Reducible fraction	0.5mol.L^-1NH₂OH HCl	Solid-liquid ratio(1g:40mL), temperature (25℃±5℃), oscillation frequency (180r/min±20r/min), time (16 hours), centrifuged at 3000 g for 20 min.
3	Oxidizable fraction	30%H₂O₂, 1mol.L^-1NH₄AC	Add 10 mL H₂O₂, stand still at room temperature for 1 hour, steam to near dry at 85℃, and add 50 mL NH₄AC, temperature (25℃±5℃), oscillation frequency (180r/min±20r/min), time (16 hours), centrifuged at 3000 g for 20 min.
4	Residuary fraction	HCL, HNO₃, HF, HClO₄	0.25g solid residue from the previous step, mixed acid digestion.

Table.4 Parameters of washing kinetic equations of lead

Washing agents	Pseudo-second-order model			Elovich kinetic model
Washing agents	R²	Qe	K	R²	A	B
Citric acid	0.9670	81.6071±1.6497	0,0171±0.00269	0.8259	47.8481±4.3151	11.8426±2.0551
Malic acid	0.8983	72.7506±3.8003	0.0091±0.0028	0.9729	33.1354±1.6581	12.51977±0.7896

No competing interests reported.

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Heavy metal lead(Pb) removal in contaminated soils using citric acid and malic acid as washing agents

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Abstract

Figures

1. Introduction

2. Materials And Methods

2.1. Soil sample and agents

2.2. Soil washing experiments by different parameters

2.3. Compound soil washing experiments of citric acid and malic acid

2.3. Speciation analysis of lead in soils

2.4 Metal stability and mobility analysis of lead in soils

2.5. Characterization analysis

3. Results And Discussion

3.1 The effect of different parameters on soil washing

3.1.1 The effect of washing agent concentration

3.1.2 The effect of solid-liquid ratio

3.1.3. The effect of washing time

3.1.4. The effect of pH

3.2. The effect of better compound agents on soil washing

3.3. Heavy metal fractionation in the soil analysis before and after washing

3.5. XRD, SEM, and EDS analysis before and after washing

Conclusion

Declarations

References

Tables

Additional Declarations

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