Arsenic-contaminated Soil Research Articles

Effects of arsenic on the biofilm formations of arsenite-oxidizing bacteria

Arsenite-oxidizing bacteria (AOB) play a key role in the biogeochemical cycle of arsenic in the environment, and are used for the bioremediation of As contaminated groundwater; however, it is not yet known about how arsenic affects biofilm formations of AOB, and how biofilm formations affect bacterial arsenite-oxidizing activities. To address these issues, we isolated seven novel AOB strains from the arsenic-contaminated soils. They can completely oxidize 1.0 mM As(III) in 22–60 h. Their arsenite oxidase sequences show 43–99% identities to those of other known AOB. Strains Cug1, Cug2, Cug3, Cug4, and Cug6 are able to form biofilms with thickness of 15–95 µm, whereas Cug8 and Cug9 cannot form biofilms. It is interesting to see that arsenite inhibited the biofilm formations of heterotrophic AOB strains, but promoted the biofilm formations of autotrophic strains in a concentration-dependent manner. The arsenite-oxidizing rates of Cug1 and Cug4 biofilms are 31.6% and 27.6% lower than those of their suspension cultures, whereas the biofilm activities of other strains are similar to those of their suspension cultures. The biofilm formation significantly promoted the bacterial resistance to arsenic. This work is the first report on the complex correlations among environmental arsenic, bacterial biofilm formations and bacterial arsenite-oxidizing activities. The data highlight the diverse lifestyle of different AOB under arsenic stress, and provide essential knowledge for the screening of efficient AOB strains used for constructions of bioreactors.

Remediation of arsenic-contaminated soils via waste-reclaimed treatment agents: Batch and field studies

The potential of three different waste-reclaimed treatment agents (WRTAs) for removing arsenic was evaluated via laboratory-scale batches, column experiments, and field tests. WRTA1 was synthesized using acid mine drainage sludge (AMDS), cement, and sand. WRTA2 and WRTA3 were synthesized by adding fly ash or Ca(OH)2, respectively, to WRTA1. The maximum adsorption capacities of WRTA1, WRTA2, and WRTA3 for As(III) using the Langmuir model were 16.6 mg g−1, 14.5 mg g−1, and 20.6 mg g−1, respectively, and for As(V) were 29.6 mg g−1, 27.3 mg g−1, and 31.3 mg g−1, respectively. The pseudo-second-order equation for the adsorption of As(III) and As(V) fit the kinetics data well. The column study showed that the WRTAs are very effective adsorbents and exhibit > 80% removal of As from the contaminated soil. The inhibition of As accumulation in plants (i.e., rice, chili, and sesame) by the WRTAs was evaluated and showed a 96% inhibition in As accumulation in chili roots for WRTA2. The suggested WRTAs are promising agents for the removal of As and heavy metals from heavily contaminated agricultural soil.

Tolerance and Bioaccumulation of Arsenate by Aspergillus Oryzae TLWK-09 Isolated from Arsenic-Contaminated Soils

Eleven indigenous arsenic-tolerant fungi were isolated from arsenic-contaminated mine tailing and identified by molecular biology methods. Among them, Aspergillus oryzae (denoted as A. oryzae TLWK-09) had high tolerance and bioaccumulation of As(V). The maximum tolerance to As(V) concentration of A. oryzae TLWK-09 reached 5000 mg/L. As(V) bioaccumulation on A. oryzae TLWK-09 in the aqueous system was investigated under different environmental conditions such as mycelia dosage, contact time, pH, and ionic strength. Bioaccumulation data of As(V) were fitted to Langmuir model, and the maximum uptake capacity of A. oryzae TLWK-09 for As(V) was 54.12 mg/g at 301 K. The morphological structures of mycelia changed obviously under As(V) stress by scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis. The analysis of Fourier transform infrared spectroscopy (FTIR) indicated the presence of carboxyl, hydroxyl, and amino groups on the fungal mycelia, which showed that these groups accounted for As(V) bioaccumulation. These results suggested that A. oryzae TLWK-09 could be an efficient and promising bioremediation material for As(V) pollution.

Effects of Phosphorus and Iron on the Phytotoxicity of Lettuce (Lactuca sativa L.) in Arsenic-contaminated Soil

Effects of Phosphorus and Iron on the Phytotoxicity of Lettuce (Lactuca sativa L.) in Arsenic-contaminated Soil

Impact of Redox Condition on Fractionation and Bioaccessibility of Arsenic in Arsenic-Contaminated Soils Remediated by Iron Amendments: A Long-Term Experiment

Iron-bearing amendments, such as iron grit, are proved to be effective amendments for the remediation of arsenic- (As-) contaminated soils. In present study, the effect of redox condition on As fractions in As-contaminated soils remediated by iron grit was investigated, and the bioaccessibility of As in soils under anoxic condition was evaluated. Results showed that the labile fractions of As in soils decreased significantly after the addition of iron grit, while the unlabile fractions of As increased rapidly, and the bioaccessibility of As was negligible after 180 d incubation. More labile fractions of As in iron-amended soils were transformed into less mobilizable or unlabile fractions with the contact time. Correspondingly, the bioaccessibility of As in iron-amended soils under the aerobic condition was lower than that under the anoxic condition after 180 d incubation. The redistribution of loosely adsorbed fraction of As in soils occurred under the anoxic condition, which is likely ascribed to the reduction of As(V) to As(III) and the reductive dissolution of Fe-(hydr)oxides. The stabilization processes of As in iron-amended soils under the anoxic and aerobic conditions were characterized by two stages. The increase of crystallization of Fe oxides, decomposition of organic matter, molecular diffusion, and the occlusion within Fe-(hydr)oxides cocontrolled the transformation of As fractions and the stabilization process of As in iron-amended soils under different redox conditions. In terms of As bioaccessibility, the stabilization process of As in iron-amended soils was shortened under the aerobic condition in comparison with the anoxic condition.

Tannic acid for remediation of historically arsenic-contaminated soils

ABSTRACTSoil washing effectively and permanently decreases soil pollution. Thus, it can be considered for the removal of the most toxic elements, for example arsenic (As). In this study, historically As-contaminated soils (2041–4294 mg/kg) were remediated with tannic acid (TA) as the washing agent. The scope of this study included optimization of the operational conditions of As removal, determination of As distribution in soil before and after double soil washing, and measurement of TA loss during washing. The optimum conditions for As removal were 4% TA, pH 4 and 24 h washing time. The average As removal after single and double washings was 38% and 63%, respectively. TA decreased As content in amorphous and poorly crystalline oxides by >90%. Although TA increased the amount of As in the easily mobilizable As fraction, the stability of As in washed soils increased, with reduced partition indexes of 0.52–0.66 after washing. The maximum capacity of the soils to adsorb TA (qmax) was 50.2–70.4 g C/kg. TA sorption was higher at alkaline than at acidic conditions. Only TA removes As from soils effectively if the proportion of As in amorphous and poorly crystalline oxides is high. Thus, it can be considered for remediation of historically contaminated soils.

Toxicity of arsenite to earthworms and subsequent effects on soil properties

Arsenic (As) is widely distributed in soil and is toxic to plants, animals and humans. In this study, earthworms (Eisenia fetida) were exposed to five concentrations of sodium arsenite (5, 10, 20, 40, and 80 mg As kg−1) in farm soils for 28 d. With increasing soil As(III) concentrations, As bioaccumulation in earthworms increased (maximum bioaccumulation factor 3.77), and levels of reactive oxygen species (ROS) and malondialdehyde (MDA) were elevated. The expression of nuclear factor erythroid 2- related factor 2 (Nrf2) and heme oxygenase -1 (HO-1) were upregulated by As in a dose-dependent pattern, and reached 5.93 and 2.94 times the control values for Nrf2 and HO-1 respectively, at 28 d in the 80 mg As kg−1 soil treatment. Similarly, DNA damage, as measured in earthworm sperm using the comet assay, increased with increasing As(III) concentrations, with ‘Olive tail moment’ values in the comet assay ranging from c. 0.5 in Control to c. 3.5 at 80 mg As kg−1 soil. In contrast, activity of the antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), decreased. These results indicate that As(III) caused oxidative stress that resulted in damage to lipids and DNA. Nrf2 and HO-1 protein expression was demonstrated in earthworms for the first time to our knowledge, and found to be a sensitive biomarker of arsenic contamination. The presence of earthworms was also found to change the distribution of As in soil, in particular, reducing the proportion in the residual fraction and increasing the proportion in As bound to Fe-oxides. This may result in increased bioavailability of bound arsenic. Soil organic matter, NH4+-N, NO3−-N and available K were indirectly changed by the As(III) through its toxicity to earthworms. This study helps to inform future assessments and biomonitoring of soil arsenic contamination.

Arbuscular mycorrhiza reduces the negative effects of M. phaseolina on soybean plants in arsenic-contaminated soils

All crops are negatively affected by several abiotic and biotic stresses, alone or jointly; however, some microorganisms, such as arbuscular mycorrhizal fungi (AMF), are able to alleviate them. Here, we investigated the effects of the AMF Rhizophagus intraradices (N.C. Schenck & G.S. Smith) C. Walker & A. Schüßler on soybean plants (Glycine max L.) grown in arsenic-contaminated soils and infected by the fungus Macrophomina phaseolina (Tassi) Goid., (charcoal rot). Two pots experiments were carried out in a glasshouse, and three levels of As (0, 25, and 50mg As kg−1) were evaluated. Plant and mycorrhizal parameters, disease severity, glomalin content, and arsenic content in roots and leaves were analyzed. Both arsenic and the pathogen negatively affected soybean biomass and morphological parameters. Moreover, both stresses adversely affected mycorrhizal symbiosis. Low levels of AMF colonization and vitality were observed in high As concentration and in pathogen presence; however AMF inoculation not only reduced the disease but also lowered arsenic accumulation rate in soybean biomass. On the other hands, disease severity was reduced by arsenic. Total glomalin content, produced by the AMF was increased in arsenic-enriched substrates, but was not modified in the presence of the pathogen. Increases in glomalin production could be one of the reasons by which soybean plants accumulate low arsenic amounts while the competition between AMF and the pathogen plays an important role in reducing the disease severity.

Arsenic Speciation and Extraction and the Significance of Biodegradable Acid on Arsenic Removal-An Approach for Remediation of Arsenic-Contaminated Soil.

A series of arsenic remediation tests were conducted using a washing method with biodegradable organic acids, including oxalic, citric and ascorbic acids. Approximately 80% of the arsenic in one sample was removed under the effect of the ascorbic and oxalic acid combination, which was roughly twice higher than the effectiveness of the ascorbic and citric acid combination under the same conditions. The soils treated using biodegradable acids had low remaining concentrations of arsenic that are primarily contained in the crystalline iron oxides and organic matter fractions. The close correlation between extracted arsenic and extracted iron/aluminum suggested that arsenic was removed via the dissolution of Fe/Al oxides in soils. The fractionation of arsenic in four contaminated soils was investigated using a modified sequential extraction method. Regarding fractionation, we found that most of the soil contained high proportions of arsenic (As) in exchangeable fractions with phosphorus, amorphous oxides, and crystalline iron oxides, while a small amount of the arsenic fraction was organic matter-bound. This study indicated that biodegradable organic acids can be considered as a means for arsenic-contaminated soil remediation.

Remediation of Arsenic-Contaminated Soil Using Alkaline Extractable Organic Carbon Solution Prepared from Wine-Processing Waste Sludge

ABSTRACTPast studies have shown that dissolved organic carbon (DOC) washing can effectively remove heavy metals from contaminated soil. In this study, we used alkaline DOC solutions for remediation of arsenic (As)-contaminated soil (with an initial As concentration in the topsoil of 390 mg kg−1). The removal of As and the change in soil nutrients during DOC washing were studied for 60 min at pH 10 with a 60:1 liquid/soil ratio (v/m). Approximately 88% of As was removed by washing the soil twice using a 3000 mg L−1 DOC solution at 25°C. Following this treatment, the pH of the soil had increased from 5.6 to 9.2; organic carbon content had increased from 3.5% to 4.1%; cation exchange capacity, ammonium-N, and available phosphorus had increased to 2.3, 1.4, and 6.6 times their original levels, respectively; and exchangeable K, Na, Ca, and Mg had increased to 91, 6.1, 4.2, and 2.2 times their original levels, respectively. A sequential extraction investigation revealed that residual As and easily exchangeable As in the fraction were initially 10.2% and 9.2%, respectively, but that the former became the maximum remainder (64%) after the ultimate DOC washing.

Fenton reagent reduces the level of arsenic in paddy rice grain

Hydroponic and pot experiments were conducted to examine the effects of Fenton reagent on paddy rice plant growing in arsenic-contaminated soils. Fenton reagent significantly reduced arsenic phytotoxicity, uptake by the plants and accumulation in rice grain. This is attributed to oxidation of As3+ to As5+ by hydroxyl radicals and immobilization of arsenate by reacting with precipitating Fe3+ to form practically insoluble compounds. Although this process enhanced the formation of Fe-enriched coatings on root surface, it appears that root plaque had limited effects on inhibiting As uptake since most of the young roots were not covered by iron plaque. It is more likely that As immobilization in the bulk soils play a major role in reducing As flux towards rhizosphere. The findings have implications for understanding As behavior in paddy field receiving rainwater-borne hydrogen peroxide and developing cost-effective techniques for reducing As level in rice grain produced from As-contaminated soils.

Genomic Characterization of the Arsenic-Tolerant Actinobacterium, <i>Rhodococcus erythropolis</i> S43

Rhodococcus erythropolis S43 is an actinobacterium isolated from an arsenic-contaminated soil sample, collected from an old smelter site, including an arsenic smelter, in Germany. This strain has unique features as compared to the other members of the species, namely resistance to elevated concentrations of arsenic. Here, we present the microbiological features and genomic properties of this biotechnologically relevant strain. The 6,812,940 bp draft genome is arranged into 264 scaffolds of 848 contigs. It possesses 62.5% of CG content and comprises 6,040 coding sequences and 49 tRNA genes. Bioinformatic genome analysis showed the presence of arsenic-resistance genes. A complete ars operon was found containing the arsACDR cluster coding for ArsA (efflux pump ATPase), ArsC (arsenate reductase), ArsD (chaperone) and ArsR (ars operon regulator). Our results show that the arsC mRNA level significantly increased in response to arsenite and arsenate exposure, suggesting its involvement in the arsenic resistance phenotype of strain S43. In addition, this strain showed to have a plethora of genes coding for proteins involved in oxidative-stress response, including catalase, super-oxide dismutase, glutathione peroxidase-related genes, thioredoxin and thioredoxin reductase, suggesting it is highly tolerant to oxidative conditions. Finally, genes for radiation resistance, biodesulfurization, and oil and phenol degrading pathways were also detected. Altogether this data make R. erythropolis S43 a good candidate microorganism for bioremediation of highly contaminated environments and other industrial applications.

Sea Level Rise Induced Arsenic Release from Historically Contaminated Coastal Soils.

Climate change-induced perturbations in the hydrologic regime are expected to impact biogeochemical processes, including contaminant mobility and cycling. Elevated levels of geogenic and anthropogenic arsenic are found along many coasts around the world, most notably in south and southeast Asia but also in the United States, particularly along the Mid-Atlantic coast. The mechanism by and the extent to which arsenic may be released in contaminated coastal soils due to sea level rise are unknown. Here we show a series of data from a coastal arsenic-contaminated soil exposed to sea and river waters in biogeochemical microcosm reactors across field-validated redox conditions. We find that reducing conditions lead to arsenic release from historically contaminated coastal soils through reductive dissolution of arsenic-bearing mineral oxides in both sea and river water inundations, with less arsenic release from seawater scenarios than river water due to inhibition of oxide dissolution. For the first time, we systematically display gradation of solid phase soil-arsenic speciation across defined redox windows from reducing to oxidizing conditions in natural waters by combining biogeochemical microcosm experiments and X-ray absorption spectroscopy. Our results demonstrate the threat of sea level rise stands to impact arsenic release from contaminated coastal soils by changing redox conditions.

Spectroscopic Diagnosis of Arsenic Contamination in Agricultural Soils.

This study investigated the abilities of pre-processing, feature selection and machine-learning methods for the spectroscopic diagnosis of soil arsenic contamination. The spectral data were pre-processed by using Savitzky-Golay smoothing, first and second derivatives, multiplicative scatter correction, standard normal variate, and mean centering. Principle component analysis (PCA) and the RELIEF algorithm were used to extract spectral features. Machine-learning methods, including random forests (RF), artificial neural network (ANN), radial basis function- and linear function- based support vector machine (RBF- and LF-SVM) were employed for establishing diagnosis models. The model accuracies were evaluated and compared by using overall accuracies (OAs). The statistical significance of the difference between models was evaluated by using McNemar’s test (Z value). The results showed that the OAs varied with the different combinations of pre-processing, feature selection, and classification methods. Feature selection methods could improve the modeling efficiencies and diagnosis accuracies, and RELIEF often outperformed PCA. The optimal models established by RF (OA = 86%), ANN (OA = 89%), RBF- (OA = 89%) and LF-SVM (OA = 87%) had no statistical difference in diagnosis accuracies (Z < 1.96, p < 0.05). These results indicated that it was feasible to diagnose soil arsenic contamination using reflectance spectroscopy. The appropriate combination of multivariate methods was important to improve diagnosis accuracies.

Combination of microbial oxidation and biogenic schwertmannite immobilization: A potential remediation for highly arsenic-contaminated soil

Here, a novel strategy that combines microbial oxidation by As(III)-oxidizing bacterium and biogenic schwertmannite (Bio-SCH) immobilization was first proposed and applied for treating the highly arsenic-contaminated soil. Brevibacterium sp. YZ-1 isolated from a highly As-contaminated soil was used to oxidize As(III) in contaminated soils. Under optimum culture condition for microbial oxidation, 92.3% of water-soluble As(III) and 84.4% of NaHCO3-extractable As(III) in soils were removed. Bio-SCH synthesized through the oxidation of ferrous sulfate by Acidithiobacillus ferrooxidans immobilize As(V) in the contaminated soil effectively. Consequently, the combination of microbial oxidation and Bio-SCH immobilization performed better in treating the highly As-contaminated soil with immobilization efficiencies of 99.3% and 82.6% for water-soluble and NaHCO3-extractable total As, respectively. Thus, the combination can be considered as a green remediation strategy for developing a novel and valuable solution for As-contaminated soils.

Use of Endophytic and Rhizosphere Bacteria To Improve Phytoremediation of Arsenic-Contaminated Industrial Soils by Autochthonous Betula celtiberica.

The aim of this study was to investigate the potential of indigenous arsenic-tolerant bacteria to enhance arsenic phytoremediation by the autochthonous pseudometallophyte Betula celtiberica The first goal was to perform an initial analysis of the entire rhizosphere and endophytic bacterial communities of the above-named accumulator plant, including the cultivable bacterial species. B. celtiberica's microbiome was dominated by taxa related to Flavobacteriales, Burkholderiales, and Pseudomonadales, especially the Pseudomonas and Flavobacterium genera. A total of 54 cultivable rhizobacteria and 41 root endophytes, mainly affiliated with the phyla Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria, were isolated and characterized with respect to several potentially useful features for metal plant accumulation, such as the ability to promote plant growth, metal chelation, and/or mitigation of heavy-metal stress. Seven bacterial isolates were further selected and tested for in vitro accumulation of arsenic in plants; four of them were finally assayed in field-scale bioaugmentation experiments. The exposure to arsenic in vitro caused an increase in the total nonprotein thiol compound content in roots, suggesting a detoxification mechanism through phytochelatin complexation. In the contaminated field, the siderophore and indole-3-acetic acid producers of the endophytic bacterial consortium enhanced arsenic accumulation in the leaves and roots of Betula celtiberica, whereas the rhizosphere isolate Ensifer adhaerens strain 91R mainly promoted plant growth. Field experimentation showed that additional factors, such as soil arsenic content and pH, influenced arsenic uptake in the plant, attesting to the relevance of field conditions in the success of phytoextraction strategies.IMPORTANCE Microorganisms and plants have developed several ways of dealing with arsenic, allowing them to resist and metabolize this metalloid. These properties form the basis of phytoremediation treatments and the understanding that the interactions of plants with soil bacteria are crucial for the optimization of arsenic uptake. To address this in our work, we initially performed a microbiome analysis of the autochthonous Betula celtiberica plants growing in arsenic-contaminated soils, including endosphere and rhizosphere bacterial communities. We then proceeded to isolate and characterize the cultivable bacteria that were potentially better suited to enhance phytoextraction efficiency. Eventually, we went to the field application stage. Our results corroborated the idea that recovery of pseudometallophyte-associated bacteria adapted to a large historically contaminated site and their use in bioaugmentation technologies are affordable experimental approaches and potentially very useful for implementing effective phytoremediation strategies with plants and their indigenous bacteria.

Soil moisture could enhance electrokinetic remediation of arsenic-contaminated soil.

Electrokinetic remediation (EKR) is the most efficient technique for remediation of fine-grained soil. The primary removal mechanisms of heavy metal in EKR are the electromigration and electroosmosis flow under appropriate electric gradients. Most EKR studies have researched the variation according to the electrolyte and electric voltage. Also, EKR could be influenced by the migration velocity of ions, while few studies have investigated the effect of moisture content. In this study, soil moisture was controlled by using tap water and NaOH as electrolytes to enhance electromigration and electroosmosis flow. In both electrolytes, the higher moisture content led to the more As removal efficiency, but there were no differences between tap water and NaOH. Therefore, tap water was the most cost-effective electrolyte to remove As from fine-grained soil.

An evaluation of different soil washing solutions for remediating arsenic-contaminated soils

Soil washing is a promising way to remediate arsenic-contaminated soils. Most research has mostly focused on seeking efficient extractants for removing arsenic, but not concerned with any changes in soil properties when using this technique. In this study, the removal of arsenic from a heavily contaminated soil employing different washing solutions including H3PO4, NaOH and dithionite in EDTA was conducted. Subsequently, the changes in soil physicochemical properties and phytotoxicity of each washing technique were evaluated. After washing with 2 M H3PO4, 2 M NaOH or 0.1 M dithionite in 0.1 M EDTA, the soil samples’ arsenic content met the clean-up levels stipulated in China’s environmental regulations. H3PO4 washing decreased soil pH, Ca, Mg, Al, Fe, and Mn concentrations but increased TN and TP contents. NaOH washing increased soil pH but decreased soil TOC, TN and TP contents. Dithionite in EDTA washing reduced soil TOC, Ca, Mg, Al, Fe, Mn and TP contents. A drastic color change was observed when the soil sample was washed with H3PO4 or 0.1 M dithionite in 0.1 M EDTA. After adjusting the soil pH to neutral, wheat planted in the soil sample washed by NaOH evidenced the best growth of all three treated soil samples. These results will help with selecting the best washing solution when remediating arsenic-contaminated soils in future engineering applications.

Phytoextraction of arsenic-contaminated soil with Pteris vittata in Henan Province, China: comprehensive evaluation of remediation efficiency correcting for atmospheric depositions.

Research on the appropriate method for evaluating phytoremediation efficiency is limited. A 2-year field experiment was conducted to investigate phytoremediation efficiency using the hyperaccumulator Pteris vittata on an arsenic (As)-contaminated site. The remediation efficiency was evaluated through the removal rate of As in soils and extraction rate of heavy metals in plants. After 2years of remediation, the concentration of total As in soils decreased from 16.27mgkg-1 in 2012 to 14.58mgkg-1 in 2014. The total remediation efficiency of As was 10.39% in terms of the removal rate of heavy metals calculated for soils, whereas the remediation efficiency calculated from As uptake by P. vittata was 16.09%. Such a discrepancy aroused further consideration on the potential input of As. A large amount of As was brought in by atmospheric emissions, which possibly biased the calculation of remediation efficiency. In fact, considering also the atmospheric depositions of As, the corrected removal rate of As from soil was 16.57%. Therefore, the results of this work suggest that (i) when evaluating the phytoextraction efficiency, the whole input and output cycle of the element of interest in the targeted ecosystem must be considered, and (ii) P. vittata has the potential to be used to remediate As-contaminated soils in Henan Province, China.

Synergistic effects of the combination of oxalate and ascorbate on arsenic extraction from contaminated soils

Arsenic is often associated with iron oxides in soils due to its high affinity with iron oxides and the abundance of iron oxides in the environment. Dissolution of iron oxides can subsequently release arsenic associated with them into the environment, which results in the increase of arsenic mobility in the soil environment. In this study, arsenic extraction from soils via the dissolution of iron oxides was investigated using oxalate, ascorbate, and their combination in order to effectively remediate arsenic-contaminated soils. Oxalate mainly extracted iron from soils via a ligand-promoted reaction, while ascorbate extracted iron mainly via a reductive reaction. Arsenic extractions from soils by oxalate and ascorbate were shown to behave similarly to iron extractions, indicating the concurrent release of arsenic adsorbed on iron oxides upon the dissolution of iron oxides. The combination of oxalate and ascorbate greatly increased arsenic extraction, indicating the synergistic effects of the combination of oxalate and ascorbate on iron and arsenic extraction from soils. Oxalate and ascorbate are naturally-occurring organic reagents that have chelating and reducing capacity. Therefore, the use of oxalate and ascorbate is environmentally friendly and effective for the remediation of arsenic-contaminated soils.