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. 2025 Dec 25;11(1):1200–1212. doi: 10.1021/acsomega.5c07082

Geochemical Drivers Govern Redox-Mediated Arsenic Transformation in Multivariate Sourced Organic-Amended Paddy Soils

Muhammad Mahroz Hussain †,‡, Xing Yang §,∥, Irshad Bibi †,⊥, Muhammad Shahid #, Hailong Wang ∇, Shengsen Wang ‡, Sabry M Shaheen ∥,*, Nabeel Khan Niazi †,*, Jörg Rinklebe ∥,*
PMCID: PMC12809813  PMID: 41552602

Abstract

Sustainable remediation of toxic metal­(loid)-contaminated paddy soils using biowastes is of great importance from both agricultural and environmental perspectives. The redox-mediated interactions between organic amendments, with multivariate sources (i.e., biogas slurry (BGS), rice husk-biochar (RH-BC), cow dung (CD)) and geochemical drivers may influence arsenic (As) mobilization under dynamically changing redox situations, such as in paddy soils. Here, we explored the impact of BGS, RH-BC, and CD on the mobilization pathway of As in a contaminated paddy soil under a wide range of soil redox potentials (E h: −252 mV to +512 mV), using an automated biogeochemical microcosm system. The partial least-squares-path model (PLS–PM) was used to identify geochemical drivers, which govern As mobilization under reduced and oxidized conditions. Results revealed that As mobilization in the unamended (control) soil was higher under reduced conditions (E h ≤ +100 mV; dissolved As = 4.2–9.2 mg L–1) than that in oxidized conditions (E h ≥ +100 mV; dissolved As = 3.8–6.7 mg L–1). With CD addition, the concentration of dissolved soil As decreased significantly by 19–62% at E h < 0 mV, followed by RH-BC (12–54%) and BGS (34–49%) compared to control. Temporal increase in pH under moderately reduced conditions (E h > +100 mV) led to a maximum decrease in dissolved As concentration with CD (36–77%), and it ranged from 18 to 75% and 29 to 49% for RH-BC and BGS, respectively, over control. These findings highlight that the addition of BGS, RH-BC, and CD, particularly CD, to As-contaminated paddy soil can decrease As mobilization under slightly reduced to oxidized conditions, which occur in the natural paddy soil-rice system. This research advanced our understanding to employ multivariate tools to identify the most potent organic amendment to immobilize As under paddy soil conditions.

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1. Introduction

Soil contamination with arsenic (As) is a global environmental, agricultural, and human health concern, especially under alternative redox cycles in rice paddy soils. , The fate and partitioning of As dynamically change under reduced soil conditions due to recurring release and binding cycles of As, with subsequent change in As speciation where arsenite (As­(III)) is dominant over arsenate (As­(V)) in paddy soils. , Arsenic is a Class-I human carcinogen, and it enters the food chain either through drinking As-contaminated well water or via consumption of As-containing food (e.g., rice grain). , The distribution and (im)­mobilization of As is influenced by soil pH, redox potential (E h), soil organic matter, labile carbon pool, microbes, and Fe/Mn oxides. In paddy soils, microbial consumption of oxygen (O2) may lead to anoxic conditions (E h = −100 to −400 mV), where As could be released in solution phase via reductive dissolution of Fe/Mn oxides. , There are other related processes that can modulate the soil E h like microbes-mediated reduction of electron acceptors (NO3 –, and SO4 2–) along with the reduction of CO2 via methanogenesis and organic acid formation, ultimately aiding in As species transformation.

Utilization of waste biomass as organic amendments for contaminated soils offers a cost-effective, sustainable and eco-friendly approach to immobilize contaminants, thus reducing human health risk. Origin of organic amendments played an integral role in modulating soil biogeochemical processes like nutrients release, mediating redox-induced transformation and carbon cycling. Various organic amendments, with lignocellulosic origin such as farmyard manure, biochar (BC), compost, and manures have shown potential to immobilize As in soil and enhance soil carbon content and nutrition capacity. , However, application of these organic amendments under paddy soil conditions often has a paradoxical effect: they act as a carbon substrate, stimulating microbial activity which consumes oxygen and promotes anaerobic conditions, thereby triggering the reductive dissolution of Fe oxides and releasing As in soil porewater.

Incorporation of biogas slurry (BGS) and cow dung (CD) is a common practice among farmers in developing countries such as Pakistan, Bangladesh, and India, where these materials serve as key sources of organic matter and nutrients. Additionally, BC has been reported to improve soil nutrient retention, soil physicochemical attributes, and contaminant (e.g., As, Cd, and Cr) immobilization. , Although immobilization of As using various BCs, inorganic and organic ameliorants has been reported in upland soils, its fate remains poorly understood under dynamically altering soil redox conditions, especially in amended paddy soils (e.g., Yang et al.). For instance, Hussain et al. reported that rice husk-biochar (RH-BC) and CD decreased exchangeable soil As (20% and 17%, respectively) among various ameliorants in two texturally different As-contaminated soils from Punjab, Pakistan, under paddy soil conditions. Yang et al. examined the effect of phosphorus-rich animal-derived biochar on (im)­mobilization of As under controlled redox conditions (E h = +100 and −200 mV) and demonstrated that As in solution phase was decreased by 38.7% and 35.4%, respectively, than that of the control.

The immobilization of As using organic ameliorants may also be due to the associated increase of carbon compounds and humic acids/fluvic acids, which can form stable complexes with As, thus reducing its mobility. , These studies investigated the effect of amendments on As immobilization via multiple experimental setups, while their pathway remained unexplored, particularly under varying redox conditions. The partial least-squares-path model (PLS–PM) is an emerging multivariate tool to explore the pathway of As binding and release under oxidized and reduced conditions (Huang et al.). These authors observed the influence of straw humification (anaerobic) and ferrihydrite addition on the repartitioning and distribution of As in paddy soils.

While previous research has provided valuable insights, there are critical knowledge gaps that should be addressed before field-scale application of multisourced organic amendments for immobilization of As under paddy soil conditions. Numerous studies have investigated the role of single amendments, particularly biochars, under dynamic redox potentials (e.g., Yang et al.). Furthermore, organic amendments such as biogas slurry (BGS) and cow dung (CD) have not been systematically evaluated and compared against lignocellulosic biochar under dynamically changing redox conditions that mimic the fluctuating redox potential such as in paddy fields.

Based on the above knowledge, here we hypothesized that redox-mediated interactions between organic amendments of varying sources including RH-BC (lignocellulosic biomass), CD (avian origin), and BGS (lignocellulosic and avian) and geochemical drivers may decrease As mobilization in paddy soil under changing redox conditions, where distinct pathways are governed by amendment-specific As biogeochemical processes. The objectives of this study were to (i) explore redox-induced release and immobilization of As in a paddy soil treated with RH-BC, BGS and CD, using a state-of-the-art automated microcosm system; (ii) examine the redox-mediated interactions between amendments (RH-BC, BGS and CD) and other soil attributes (e.g., pH, Fe, Mn, SO4 2–, and dissolved organic carbon (DOC)), (iii) determine the impact of these amendments on geochemical fractionation of As in treated paddy soils, and (iv) identify possible pathways of As release and binding via PLS–PM model.

2. Materials and Methods

2.1. Soil and Amendment Collection

A composite surface (0–20 cm depth) soil sample was collected from a paddy field in the Khudpur village, Lahore (31°24′24.4′′N; 74°04′48.6′′E) in Punjab province of Pakistan. The study site is in a semiarid climate with moderate rainfall near to River Ravi flood plain in Punjab, Pakistan. Farmers usually adopt recommended agricultural practices, such as tillage and fertilizer application with rice as a major crop along with some vegetables. Soil has a sandy clay loam texture with slightly alkaline pH (7.71), and organic carbon content, cation exchange capacity, and total soil As concentration of 0.66%, 15 cmolc kg–1, and 129 mg kg–1, respectively. The RH-BC was prepared under oxygen-limited conditions at 500 °C with a residence time of 2 h. The CD and BGS were collected from the agronomy farm area of the University of Agriculture Faisalabad (Pakistan).

Soil and amendment samples were air-dried, ground, and sieved (<2 mm) prior to characterization of various relevant physicochemical properties. The pH and electrical conductivity (EC) of the soil and amendments were measured using 1:5 solid to deionized water suspension. Soil texture was determined by the hydrometer method, while soil organic carbon (SOC) content was measured following Walkley and Black method. Organic amendments were digested using hydrogen peroxide and nitric acid at 120 °C; the digestate was diluted with deionized water to make a final volume of 25 mL and As concentration in the digestate was analyzed using a hydride generation atomic absorption spectrometer (HG-AAS; Agilent AA 240 with VGA-77; Australia). The Olsen phosphorus (P) method was used to determine the content of soil extractable P by 0.5 M sodium bicarbonate (NaHCO3) as an extractant and analyzed using UV–visible spectrophotometer at 410 nm. Total sodium (Na) and potassium (K) concentrations were measured using a five-channel flame photometer (BWB Model BWB-XP, 5 Channel Flame Photometer, England). Relevant soil properties and amendments are presented in Tables and S1.

1. Relevant Soil Properties and Total Elemental Concentrations in the Studied Soil and Organic Amendments .

  pH EC (mS m–1) Al (mg/kg) As (mg/kg) Fe (mg/kg) Mn (mg/kg) sulfur (mg/kg) Zn (mg/kg) total carbon (%)
Soil
untreated soil 7.7 12.3 20,332 ± 645 129 ± 2.9 27,013 ± 56 543 ± 4.5 124 ± 1.3 62 ± 3.5 1.15 ± 0.01
cow dung-treated soil 7.9 38.2 19,610 ± 880 83 ± 1.3 27,176 ± 322 536 ± 11.3 198 ± 2.4 56 ± 3.3 1.46 ± 0.01
rice husk-biochar-treated soil 7.7 22.6 18,437 ± 197 84 ± 0.8 26,076 ± 149 531 ± 4.7 152 ± 0.08 51 ± 0.3 1.60 ± 0.03
biogas slurry-treated soil 7.9 21.1 20,318 ± 1309 78 ± 0.3 27,651 ± 375 531 ± 1.2 167 ± 4.3 49 ± 0.5 1.37 ± 0.01
Amendments
cow dung 8.5 12.5 4400 ± 35 BDL 4982 ± 32 159 ± 1.2 5813 ± 163 77 ± 12.3 28 ± 0.12
rice husk-biochar 7.7 14.8 2357 ± 46 BDL 12,834 ± 287 497 ± 0.2 2148 ± 75 78 ± 4 12 ± 0.2
biogas slurry 10.4 76.0 14,976 ± 899 BDL 18,936 ± 396 284 6.6 3439 ± 33 83 ± 2.1 11 ± 0.1
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BDL: below detection limit..pH = Soil pH; EC = Electrical conductivity; Al = Aluminum; As = Arsenic; Fe = iron; Mn = Manganese; Zn = Zinc.

Geochemical fractionation of As after microcosm experiment with and without amendments was done using an 8-step sequential extraction method as reported by El-Naggar et al. In the first step (Step I), the soluble + exchangeable As pool was extracted using 1 M ammonium acetate (NH4OAc), followed by the extraction of carbonate-bound soil As fraction with 1 M NH4OAc (Step II). The manganese oxides-bound As fraction (Step III) was extracted using 0.1 M hydroxylammonium chloride (NH2OH-HCl) + 1 M NH4OAc, while in Step IV organic-bound soil As fraction was extracted by 0.025 M NH4EDTA. In Step V, sulfide-bound As fraction was extracted using H2O2 and NH4OAc. In Step VI and Step VII, the amorphous and crystalline Fe-oxide-bound As fractions were extracted using 0.2 M NH4-oxalate and 0.1 M ascorbic acid in 0.2 M NH4-oxalate buffer, respectively. Residual As fraction was extracted after digestion with aqua regia (Step VIII). Geochemical fractions determined via sequential extraction are operationally defined and may have some limitations. For instance, chemical extractants are not perfectly specific such as reagent targeting amorphous Fe oxides could also partially dissolve crystalline Fe oxides. Arsenic can also reprecipitate in other residual phases before the next extraction step. Furthermore, the sequential extraction method is also limited to distinguish between oxidation states of As or As bound with different minerals (viz As­(V) or As­(III) sorbed to ferrihydrite or coprecipitated scorodite). ,

2.2. Soil Incubation with Organic Amendments

One kilogram of the studied As-contaminated soil (total As = 129 mg kg–1) was mixed with CD, RH-BC, and BGS at 2% (w/w). Organic amendments-untreated (control) and -treated As-contaminated soils were flooded with water and incubated in a 2 L polyvinyl plastic container for 45 days at 25 ± 2 °C. Soil moisture content was maintained during the incubation period at flooded conditions, mimicking the paddy soil conditions. After incubation, soil samples were air-dried, crushed, sieved, and used for the redox experiment. The untreated and treated incubated soils were characterized for relevant properties and elemental concentrations (Table ). Soil samples were digested using concentrated HCl and HNO3 (aqua regia) in a microwave digester (Milestone; ETHOS EASY, Germany) and analyzed for total As and other elements (Fe, S, and Mn) according to the US-EPA method.

2.3. Automated Biogeochemical Microcosm Experiment

2.3.1. Experimental Setup

To emulate flooding of paddy soil under controlled redox conditions, an automated biogeochemical microcosm system (MC) was employed, as described in detail by Yu and Rinklebe. This system has been used for studying the redox biogeochemical behavior of different soil metal­(loid)­s. ,, In this experiment, a total of 16 microcosm vessels (four replicates for each of the above-mentioned three treatments plus control) were used, and each vessel was filled with 210 g of soil (amended and unamended) and 1680 mL of deionized water to achieve a 1:8 (soil: water) ratio in slurry. Each vessel was sealed, airtight, and continuously stirred with a mechanical stirrer to get a homogeneous slurry. Each vessel received 5 g of glucose and powdered straw to prevent carbon limitation of microbial metabolism, with this dosage sufficient to sustain activity without interfering with amendment-specific redox effects. This dosage was deliberately selected to simulate the pulse of organic carbon (e.g., root exudates, manure) typically found in paddy soils during carbon residue decomposition, i.e., a critical driver of transient redox shifts. The microcosm system is equipped with automated N2 flushing to lower E h and oxygen/synthetic air flushing to increase E h within the system. The pH and E h values were recorded using a data logger every 10 min. The measured E h values were calculated and standardized relative to the standard hydrogen electrode according to the procedure of Wolkersdorfer. In this study, the redox windows were maintained from −250 to +500 mV (Figure S1).

2.3.2. Slurry Sample Preparation and Chemical Analysis

A slurry sample was collected from each microcosm and transferred to 50 mL plastic tubes under anaerobic conditions inside the glovebox (MK3 Anaerobic Workstation, Don Whitley Scientific, Shipley, U.K.). Thereafter, the slurry samples were centrifuged at 5000 rpm for 25 min and passed through a 0.45 μm filter inside the glovebox to get the dissolved fraction. Filtered samples were subsampled for subsequent analysis into four different plastic tubes (15 mL): one for elemental concentrations using the inductively coupled plasma-optical emission spectrometry (ICP-OES, Ultima 2, Horiba Jobin Yvon, Unterhaching, Germany) and the second subsample for the analyses of total nitrogen (TN), total carbon (TC) and total organic carbon (TOC) by a CNS analyzer (Analytik Jena, Germany). The remaining samples were subjected to a UV–vis spectrophotometer (CADAS 200, Germany), which was used to analyze the specific UV absorbance (SUVA) that is an indicator for the DOC composition, and it was measured at 254 nm wavelength. Dissolved concentration of Fe2+ was determined on a UV–vis spectrophotometer using 1,10-phenanthroline method. The ion chromatograph (IC 790, Metrohm, Filderstadt, Germany) was used to measure the dissolved concentrations of Cl– and SO4 2– in the soil.

2.4. Quality Control/Quality Assurance and Statistical Analysis

The quality assurance of the measured elements in soil was achieved by maintaining quadruplicate analysis with blank and standard solutions from Merck during the measurement. Certified reference soil materials (BRM12, TMC, and TML) were exploited to guarantee a very high quality assurance. The recovery rate of total As was 93.1–95.2%. The relative standard deviation (RSD) was less than 15%. Statistical analysis and data processing of all of the results recorded by the data logger were completed using Sigma Plot version 10 and Minitab-17 software. The E h-pH average, maximum, and minimum values along with other chemical attributes were presented as mean of four replicates with standard error. RStudio was used to carry out regression-based correlation between key factors (Figure S2).

Partial least-squares-path model (PLS–PM) was used to define interactive factors using RStudio, where factors (parameters) were taken as soil quality (pH and E h), metal contents (Fe and Mn), sulfur (S and SO4 2–), organic matter (DOC and SUVA), and As. Further mechanism was developed using Mermaid Live Editor v10.6.1 software. RStudio (Packages: ggplot2, reshape2, vegan, pheatmap, ggpubr, RColorBrewer, dplyr, tidyr) was used to determine the correlation among the variables among amendments. The one-way ANOVA with p-values explained the contribution of amendments under both redox windows (Figures and S3).

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Boxplots with ANOVA p-values for key variables comparing treatments. Different colors showed different treatments and boxplots showed the values comparison among different treatments.

3. Results and Discussion

3.1. Arsenic Fractions in the Amended Soil

The addition of CD, BGS, and RH-BC to soil significantly reduced As in the soluble + exchangeable fraction (% of total As) compared to the control (Figure ). The proportion of As in the soluble + exchangeable fraction could lead to an increase in As mobilization and bioavailability to soil microbes and plants, which may inhibit plant growth (e.g., rice). The amendments also decreased the proportion of As in the carbonate fraction compared to its proportion in the control soil (Figure ). However, the amendment addition increased the percentage of As in the Mn-oxide-bound, organic-bound, and amorphous Fe oxides associated fractions compared to control (Figure ). Organic-bound As fraction was found to be maximum with CD (21%) > RH-BC (18%) > BGS (17%) (Figure ). Increase in organic-bound As fraction in the treated soil compared to control could be due to the presence of organic molecules in these organic amendments, such as fulvic acid, humic acid, and other low-molecular carboxylic acids that could bind As.

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Geochemical distribution of arsenic (% of total content) in the un-treated (control), biogas slurry-, rice husk-biochar-, and cow dung-treated- soil.

Sulfide-bound As fraction showed a slight increase in all three amended soils, which spanned 14–17%, slightly higher in control than the RH-BC-treated soil. The amorphous Fe oxides-bound As fraction was low in the unamended control (0.77%), and it ranged from 8.6 to 10.4% in other treatments. The high amorphous Fe-bound As fraction in BGS treatment could be explained due to the high Fe content in this amendment (Table ). The CD, RH-BC, and BGS decomposition possibly played an important role in As distribution in different soil fractions based on the Fe/Mn contents, which can release As in solution under reduced conditions.

This was also supported by the PLS–PM data showing the direct and indirect effects of As interacting factors dictating As binding and release (Figure ). The As fractionation patterns are directly governed by the distinct chemical signatures of each amendment. High Fe content in BGS: 18,936 mg/kg promote the formation of amorphous Fe-oxide-bound As, as dissolved Fe2+ reprecipitates as fresh, high-surface-area Fe oxides that effectively scavenge As from porewater. Conversely, the exceptionally high S content in CD: 5813 mg/kg provides the necessary substrate for microbial sulfate reduction, leading to the precipitation of immobile. As-sulfide minerals like phases (e.g., orpiment, realgar) and explaining the significant shift into the sulfide and residual fractions. Arsenic fractionation and mobilization in different treatments could be influenced by soil E h-pH interactions along with other elemental interactions, as will be discussed in the following section.

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Partial least-squares-path models (PLS–PM)direct, indirect, and total effects on As mobilization under the influence of multisourced organic amendments. Interdependence was analyzed using inner and outer models of PLS–PM, and a total of 10 sample observations were taken and divided into major geochemical drivers to regulate As mobility.

3.2. Effect of Ameliorants on the E h and pH Changes

The untreated and treated soils showed a wide range of E h changes, i.e., control (−252 to +512 mV), BGS (−188 to +506 mV), RH-BC (−218 to +484 mV), and CD (−223 to +481 mV) (Figure and Table ). The BGS, RH-BC, and CD showed varying trend in the pH, i.e., 7.6–5.6, 8.0–5.8, and 7.8–5.6, respectively, while the control showed 7.6–5.7 pH (Figure ). There was a positive and significant correlation between E h and pH in the untreated soil and the three treatments (r = 0.70–0.83; p < 0.05) (Table S3 and Figure S2). Interaction between the varying sourced amendments unveiled that there is total negative effect of soil quality (E h, pH) and organic matter parameters though it is in order of BGS (β = −0.5182) < RH-BC (β = −0.555) < CD (β = −0.6005) (Table S4b) as explained with direct negative effects in same order via PLS–PM interpretations (Figure ). Predominantly, surface functional groups are different in multivariate originated organic amendments, which could possibly alter the soil E h and pH values, though it is an important future research direction to explore the respective roles in defining their role under a wide range of E h/pH interactions. The addition of organic ameliorants could regulate soil pH in various pathways, such as protonation of the biomolecules, thus consuming H+ and decarboxylation of organic anions during organic C mineralization, resulting in the generation of OH– ions. Decrease in soil pH after flooding could possibly be ascribed to the decomposition of organic matter that might stimulate the organic acids production, such as humic acid, fulvic acid, and some other low-molecular organic acids. Fluctuation in E h and pH of soil could also be associated with the organic amendments addition ratio that can modulate the soil biogeochemical attributes through stimulating the carbonic acid formation under anaerobic conditions. Here, the role of gas emissions can be another reason for the change in the soil E h and pH values as described by our previous work which explained the role of various organic amendments in As immobilization associated with dramatic change in the soil redox and pH change observed periodically.

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E h–pH diagram gainded by the automated biogeochemical microcosm system (MCs) and the concentrations of dissolved As, iron (Fe), manganese (Mn), chloride (Cl), sulfur (S), sulfate (SO4 2–), dissolved organic carbon (DOC), and specific UV absorbance (SUVA) in the control, biogas slurry-, rice husk-biochar-, and cow dung-treated soils under different redox conditions. In total, 10 samples per MC were collected at different redox windows (E h = −252 to +512 mV). Error bars indicated the standard deviation of four independent MCs (n = 4).

2. Dissolved Concentration of Elements and Controlling Factors along with E h and pH in the Soil Slurry of the Untreated and Amendments-Treated Soils .

    control soil
 biogas slurry (BGS)-treated soil
 rice husk-biochar (RH-BC)-treated soil
 cow dung (CD)-treated soil
parameter unit min max mean SD min max mean SD min max mean SD min max mean SD
E h mV –252 512 191 225.7 –188 506 202 211.4 –218 484 202 203.5 –223 481 179 213.2
pH   5.65 7.58 6.26 0.59 5.75 7.58 6.34 0.46 5.81 7.95 6.63 0.63 5.57 7.81 6.58 0.75
As mg L–1 3.8 9.2 6.40 1.84 2.16 6.05 6.47 1.39 1.31 6.94 6.70 2.05 1.19 6.34 6.62 1.76
TFe   0.09 24.63 6.76 10.15 0.40 41.39 4.20 15.14 0.05 34.27 4.51 12.01 0.27 45.84 3.77 15.34
Fe2+   0.10 0.12 0.11 0.008 0.10 0.14 0.11 0.013 0.10 0.13 0.11 0.010 0.10 0.15 0.12 0.017
Mn   0.10 24.07 7.88 9.35 0.10 23.43 12.85 8.27 0.11 23.16 11.77 9.47 0.11 22.49 12.19 9.04
S   4.45 10.74 9.77 1.74 8.50 16.43 11.36 2.61 7.73 15.23 11.08 2.22 7.58 16.70 11.26 2.79
SO4 2–   19.41 41.83 13.08 7.70 30.7 58.97 12.74 11.20 31.63 55.85 12.40 8.33 28.92 58.28 11.12 10.09
Cl–   46.48 102.53 32.81 20.18 52.3 96.57 47.22 16.02 55.71 117.41 45.97 22.58 79.87 153 45.76 24.33
TC   310.2 3791.6 2217.6 11.80 697.2 3722 2270.3 11.4 762.7 3778 2254.9 10.10 401.3 3705.3 2070.8 12.2
IC   3.41 67.7 30.02 18.6 9.86 47.01 32.06 13.91 9.31 41.16 23.99 9.95 11.05 49.99 31.28 14.5
DOC   279.5 3750.2 72.15 244.6 651.8 3705.1 74.03 460.13 731.9 3760.7 85.19 294.3 361.6 3687.6 112.5 359.4
SUVA m–1 mg–1 L 0.21 0.34 0.30 0.038 0.25 0.33 0.29 0.031 0.21 0.33 0.30 0.038 0.27 0.35 0.32 0.023
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As = Arsenic; TFe = Total dissolved iron; S = Sulfur; Mn = Manganese; SO4 2– = sulfate; Cl– = chloride; DOC = Dissolved organic carbon; IC = Inorganic carbon; TC = Total carbon; TN = Total nitrogen; SUVA = Specific UV absorbance; SD = standard deviation.

3.3. Effect of CD, RH-BC, and BGS on Arsenic Mobilization under Different Redox Conditions

3.3.1. Arsenic Mobilization and E h/pH Changes

Results of Figure and Table revealed that mobilization of As in the control soil and in the CD-, RH-BC-, and BGS-treated soils was higher under reducing conditions than under oxidizing conditions. For example, the concentration of dissolved As in control soil was higher under reducing conditions (E h ≤ +100 mV; As: 4.2–9.2 mg L–1) that of under oxidizing (E h ≥ +100 mV; 3.8–6.7 mg L–1) conditions. Therefore, there was a significantly (p < 0.05) negative correlation between E h and concentration of dissolved As, notably in control (r = −0.921) and BGS amended soils (r = −0.921), albeit it was weakened in RH-BC (r = −0.646) and CD (r = −0.696) treatments (Table S3). The higher mobilization of As under reducing/acidic conditions than under oxidizing/alkaline conditions could be due to the reduction of As­(V) to As­(III). Also, the added organic amendments could act as an electron donor and may enhance the transformation of As­(V) to As­(III), thus increasing its mobilization under reducing conditions. Moreover, the reductive/acid dissolution of Fe oxides and the associated release of As to soil solution could contribute to the high concentrations of dissolved As under reducing/acidic conditions, as also agreed with Rinklebe et al. Application of organic amendments alters soil E h and pH, thus changing As and other elemental biogeochemical cycling (Figure ). There was an indirect effect of E h/pH in all applied amendments on As immobilization; however, maximum was observed with CD (β = −0.7904), followed by RH-BC (β = −0.78) (Figure ).

The CD, RH-BC, and BGS amendments decreased the concentration of dissolved As under the studied redox windows, as compared with the control (Figure ). The application of CD decreased the concentration of dissolved As at E h < 0 mV by 19–62%, followed by RH-BC (12–54%) and BGS (34–49%), compared to the control (Table ). With a temporal increase in pH under moderately reducing conditions (E h > +100 mV), a decrease in the concentration of dissolved As was found to be maximum with CD (36–77%), and it ranged from 18–75% to 29–49% with RH-BC and BGS, respectively, compared to control (Table ). Specifically, a decrease in the concentration of dissolved As (5.3 mg L–1) was observed with BGS amended soil at E h = +100 mV, whereas a maximum decrease in dissolved As concentration was found at E h = 200 and 300 mV (Figure ). At oxidizing conditions (E h = +400 and +500 mV), the CD-treated soil showed the lowest concentrations of dissolved As (1.2 and 1.3 mg L–1, respectively) (Figure ). Under oxidizing conditions, the concentration of dissolved As decreased by 19% with CD application compared to that under reducing conditions (Table ).

3. Impact of Biogas Slurry, Rice Husk-Biochar, and Cow Dung on Changes (%) in Dissolved Arsenic (As) in the Treated Soil Compared to the Control.
  concentration of dissolved As (mg L–1) and change in As (%) compared to control
targeted E h (mV) control soil + biogas slurry change in As (%) soil + rice husk biochar change in As (%) soil + cow dung change in As (%)
initial sample (+350) 4.2 2.3 –45.3 ± 8.1 3.2 –22.9 ± 4.1 2.8 –34.5 ± 6.2
–250 8.1 5.0 –38.3 ± 6.9 3.7 –54.1 ± 9.7 3.1 –62.1 ± 11.2
–200 9.2 5.4 –41.8 ± 7.5 5.4 –41.2 ± 7.4 4.8 –47.8 ± 8.6
–100 8.4 6.0 –28.0 ± 5.0 6.3 –25.0 ± 4.5 5.6 –33.5 ± 6.0
0 7.9 5.1 –34.9 ± 6.3 6.9 –12.2 ± 2.2 6.3 –19.7 ± 3.5
100 7.8 5.0 –36.3 ± 6.5 6.6 –15.7 ± 2.8 5.2 –32.8 ± 5.9
200 6.7 4.7 –29.7 ± 5.4 5.5 –18.0 ± 3.2 4.2 –36.6 ± 6.6
300 6.2 3.7 –40.5 ± 7.3 4.8 –23.1 ± 4.1 3.2 –48.7 ± 8.8
400 5.3 2.7 –49.8 ± 9.0 1.3 –75.2 ± 7.5 1.2 –77.6 ± 14.0
500 3.8 2.2 –43.1 ± 7.7 1.3 –64.5 ± 6.6 1.3 –65.7 ± 11.8
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Results also showed that CD followed by RH-BC application reduced the concentrations of dissolved As compared to the control under both oxidizing and reducing conditions. In the current study, an increase in the concentration of dissolved As (E h = −200 to +100 mV) and dissolved As pool in As-contaminated soil with RH-BC could be attributed to electrostatic repulsion between the negatively charged biochar surface and oxyanions of As. Although, change in the pH and E h of the soil-solution phase creates metastable redox window (in between E h = −200 to +200 mV) which supports the conversion of As­(V) into As­(III) species and vice versa; however with CD and RH-BC application it allows swift binding of As due to presence of various functional groups and higher C contents (Table ). Both CD and RH-BC have high carbon contents (Table ), which can possibly enhance the As immobilization by the formation of stable As–C complexes. Van Vinh et al. reported that the surface of pinecone biochar remained negatively charged under alkaline (pH > 9) conditions, while it was positively charged under acidic conditions, thus enhancing the soil cation exchange capacity, which contributed to higher As immobilization. However, research is still warranted to explore the solid-phase As speciation in CD, BGS, and RH-BC amendments-treated paddy soils under varying E h conditions to develop better understanding of the potential mechanisms of As (im)­mobilization under redox changes (see Figure ).

3.3.2. Arsenic Mobilization Affected by the E h-Induced Changes in Geochemical Drivers

Higher concentrations of dissolved Fe were observed in all treatments as compared to the control (Figure ). Dissolved Fe concentration increased rapidly between E h = −200 and 0 mV in the BGS-, RH-BC-, and CD-treated soils. The concentrations of dissolved Fe– rapidly decreased between E h = +300 and +500 mV, implying that the application of CD, BGS, and RH-BC could stimulate Fe oxidation under moderately to strongly oxidizing conditions. , A significant increase in the dissolved Mn concentrations was observed under reducing conditions, while their concentrations decreased under oxidizing conditions (Figure ). This could possibly be due to the reduction of Fe3+ and Mn4+ to Fe2+ and Mn2+ under reducing conditions (E h = −400 to −200 mV), which can enhance the solution phase Fe and Mn concentrations, as found in this study, that might interfere with As mobilization/immobilization. , At the second sampling (−250 mV E h), the concentrations of dissolved Fe and Mn were relatively lower than that at the third sampling (E h = −200 mV), which could be due to the increased microbial-induced dissolution of Fe/Mn oxides under reducing conditions which can increase As mobility by releasing adsorbed As and converting it to more mobile As­(III) species. In this study, until the second sampling (50 h; E h = −250 mV), relatively lower Fe and Mn concentration was detected than that at the third sampling (200 h; E h = −200 mV), which could be linked to the prolonged reducing conditions, thus releasing higher Fe and Mn in the solution phase after 200 h of incubation. An increase in E h could create an oxidizing environment, thereby immobilizing dissolved Fe and Mn through the formation of stable Fe and Mn/oxides. With organic amendments, both Fe and Mn concentrations decreased during the slightly oxidized conditions at E h = +100 mV, signifying that the organic amendments application might accelerate the oxidative transformation of Fe2+ and Mn2+ (Figure and Table ).

The reductive dissolution of Fe/Mn oxides might lead to the higher dissolved Fe and Mn contents together with As and DOC under reducing conditions, i.e., (E h = −300 to 0 mV). Therefore, positive and significant correlations (p < 0.05) were found between As and Fe (r = 0.81), As and Mn (r = 0.81), and As and SO4 2– (r = 0.86) (Table S3), indicating that dissolution of Fe/Mn oxides-induced As desorption could play a vital role in As release into solution phase under low E h in the current study. , The slightly acidic soils in the CD, RH-BC, and BGS amended soils and prevailing reducing conditions could possibly cause reductive dissolution of As-bearing Fe/Mn oxides and mobilization of As.

Maximum SO4 2– concentration was observed at E h ranging from −200 to +200 mV (at 50–350 h of incubation time); it was found to be low under the oxidizing conditions (E h = +200 to +400 mV). However, under reducing (E h = −200 mV) conditions, dissolved SO4 2– concentration increased significantly, which may indicate the generation of aqueous sulfide. Under oxidizing conditions, As can form insoluble mineral As complexes such as orpiment and realgar. Arsenic release in the solution phase can be mediated via low SO4 2– contents presence by limiting the formation of stable sulfide-As complexes. The concentration of SO4 2– was observed at the highest under oxidizing conditions (E h = +200 to +450 mV) with CD showing no significant impact on aqueous SO4 2– concentration (Figure and Table ). The increased SO4 2– concentration in soil solution could inhibit the reduction of Fe3+ by Fe-reducing microbes at pH > 5.0, ultimately impacting the As release from Fe reductive dissolution. The pronounced immobilization of As under reducing conditions, particularly with CD amendment, is a pivotal finding of this study. The strong negative total effect via PLS–PM of soil quality on As (β: ∼ −0.79 for CD) was significantly mediated through the Sulfur Cycle (Soil Quality → Sulfur Cycle: −0.662; Sulfur Cycle → As: −0.178). This pathway strongly suggests that CD enhanced the overall soil environment, which in turn stimulated microbial sulfate reduction. This biochemical process generates sulfide (S2–), leading to the precipitation of As as insoluble arsenopyrite (FeAsS) or other thioarsenate complexes, which is a primary mechanism for As sequestration under anaerobic conditions. The PLS–PM modeling also supported that the highest redox-driven As release was observed with BGS amendments (E h → As: β = −0.78) after control, where the ″electron shuttle″ mechanism was consistent with rapid OC mineralization creating reducing conditions that mobilized As largely by Fe/Mn oxide dissolution (Fe/Mn contents → As: β = 0.95) (Figure ). In contrast to improved sulfur cycling (sulfur → As: β = 0.24), RH-BC treatments showed more mild effects (E h → As: β = −0.70), indicating partial As sequestration via thiol complexation on biochar surfaces. Due to humic acid competitive sorption and microbial sulfate reduction, which produced competing As–S-DOC complexes, CD amendments showed the highest organic matter–As interactions (Organic Matter → As: β = −0.40). Although each treatment displayed the anticipated negative E h-As association, the secondary control mechanisms varied according to the chemistry of the amendment (Table S4B).

In the initial samples (0 to 50 h; E h + 350 to −250 mV), the highest DOC concentration was observed in RH-BC, followed by CD and BGS compared to the control (Figure ). Under strongly reducing conditions (E h = −250 to −100 mV), DOC concentration reduced; and under slightly reducing to strongly oxidizing conditions (E h = 0 to +400 mV), DOC concentration decreased dramatically. SUVA254 has been used as a substitute test for DOC aromaticity since it represents the ″average″ absorptivity of all the carbon molecules (high and low carbon compounds) that demonstrate DOC content in dissolved fractions. Also, the presence of DOC might play an important role in enhancing microbial-induced release of As from binding sites of soil Fe/Mn oxides. Weishaar et al. reported that SUVA values could provide an indication of the DOC composition in soil, which is also reported by Shaheen et al. SUVA254 ranged from 0.208 to 0.343 (m–1 mg–1 L) in control, 0.247 to 0.325 (m–1 mg–1 L) in the BGS treatment, and for RH-BC- and CD-treated soils it spanned 0.208 to 0.333 and 0.2743 to 0.3450 (m–1 mg–1 L), respectively (Figure ). A negative correlation was observed between E h and DOC concentration (r = −0.676; p < 0.05) (Table S3), suggesting OC degradation under reducing conditions. Under reducing conditions, the decomposition and hydrolysis of organic matter may lead to an accumulation of water-soluble intermediate metabolites, thereby increasing the concentration of DOC. Also, the dissolution of Fe- and Mn oxides under reducing conditions might lead to minimal adsorption sites for As sequestration, resulting in a decrease of microbial activity and an increase in the DOC concentration. Under oxidizing conditions, increasing the oxygen supply in the system can lead to rapid degradation of DOC by increasing microbial activity, thereby reducing the concentration of DOC. It has been reported that CD and BC could be the potential sources of DOC in soil solution under various E h conditions. In this study, the highest DOC content was in CD followed by BGS- and RH-BC-treated soils (Table ). Therefore, the release of DOC from these amendments could not be ruled out in soil solution. ,

Additionally, a negative correlation between SUVA254 and E h (r = −0.62; p < 0.05) was observed in all soils, including the control and amended soils (Table S3). A lower SUVA254 value indicates that the degree of aromaticity of organic carbon in soil was lower under strong oxidizing conditions. In contrast to the SUVA254, under these conditions, the proportion of complex carbon molecules, such as humic acid and fulvic acid, was higher under oxidizing conditions than under reducing conditions. This suggests relative enrichment of aromatic, high-molecular-weight humic-like substances in the DOC pool under anaerobic environments. We propose two primary mechanisms. First, under reducing conditions, the rapid microbial mineralization of labile, aliphatic organic compounds (e.g., simple sugars and acids) leaves behind a selectively preserved pool of recalcitrant, aromatic compounds, thereby increasing the relative aromaticity SUVA254 of the remaining DOC. Second, the reductive dissolution of Fe (oxyhydr)­oxides, a key process in anaerobic soils, can release mineral-bound aromatic DOC (e.g., humic and fulvic acids) that was previously sorbed to particle surfaces, further contributing to the elevated SUVA254 values in the solution phase. Conversely, the decrease in SUVA254 under oxic conditions points to the photochemical or oxidative degradation of complex aromatic molecules into simpler, more aliphatic compounds over time. This shift in DOC composition has a direct impact on As mobility by competing for adsorption sites on mineral surfaces, thereby hindering readsorption and facilitating the persistence of mobilized As in solution. Furthermore, this DOC can form soluble aqueous complexes with As, enhancing its transport potential. These findings underscore that the nature of DOC, beyond its mere quantity, is a paramount factor controlling As biogeochemistry in dynamic paddy soil environments. It has been reported that under reducing conditions, the DOC concentration may be characterized to represent the low-molecular-weight organic compounds (e.g., oxalic and citric acids), and under oxidizing conditions, it could be used to represent high-molecular-weight organic compounds (e.g., humic and fulvic acids) as it is attributed to the available humic fraction in the solution phase. Organic ameliorants may reduce the dissolved As concentration by employing changes in soil buffering capacity, CO2 evolution from carbon mineralization, and oxic/anoxic microbial-associated elemental cycling. Under dynamic redox conditions of a paddy field, microbial activity can be a primary driver to influence As speciation and mobility. The influence of microbes extends beyond As transformation, profoundly altering the soil’s geochemical matrix as observed in the current study. Redox conditions shift to anaerobic upon flooding, and heterotrophic microbes decompose organic amendments, consuming oxygen and releasing DOC. This process drops E h, triggering the microbial reductive dissolution of Fe­(oxyhydr)­oxides, which releases their sorbed As load. Simultaneously, specialized sulfate-reducing bacteria (SRBs) utilize sulfate and DOC, generating sulfide (S2–). , This sulfide can then precipitate As as immobile arsenopyrite-like minerals or form thiolated As species with altered As transformation patterns. Furthermore, microbial processing modulated the quality of DOC, often decreasing the specific UV absorbance (SUVA254), indicating a shift from complex, aromatic humic substances to simpler, more bioavailable organic acids. These low-SUVA254 compounds can further fuel microbial metabolism or be complex with metals, creating a complex feedback loop that governs the entire As cycle within the paddy environment. Microorganisms are central mediators in the environmental methylation of As, a process that transforms inorganic As into mono- (MMA) and dimethylated (DMA) As species, fundamentally altering its mobility, toxicity, and environmental fate. This biomethylation is primarily driven by a diverse array of bacteria, fungi, and archaea through the enzymatic activity of As­(III)-S-adenosylmethionine methyltransferases (ArsM in microbes and AS3MT in animals). The process involves the sequential addition of methyl groups from S-adenosylmethionine (SAM) to As­(III), yielding the more volatile and often less toxic methylated arsenicals. The resulting methylated As species are generally considered less acutely toxic than inorganic As­(III) but can be more readily absorbed by crops such as rice, posing a significant food chain exposure risk. Furthermore, under certain conditions, methylated As species can be degraded back to inorganic forms by other microbial consortia, highlighting the dynamic and complex role microorganisms play in the global As cycle. PLS–PM analysis robustly confirms that the amendments influenced As mobility primarily through indirect biogeochemical pathways rather than through direct effects. A key finding is the potent negative indirect effect of Soil Quality on As across all treatments. This signifies that the amendments’ primary mode of action was by enhancing overall soil health, which in turn created an environment conducive to As sequestration. This critical pathway was significantly mediated through the Sulfur Cycle. The substantial negative coefficients from the Soil Quality → Sulfur Cycle (e.g., β = −0.662 for CD) indicate that improved soil conditions promoted microbial sulfate reduction. This process generates sulfide (S2–), which can immobilize As through the precipitation of arsenopyrite (FeAsS) or other insoluble thioarsenic complexes, explaining the subsequent negative effect of the Sulfur Cycle on As (β: −0.178 for CD).

Furthermore, the strong positive indirect path from Metal Content (predominantly Fe/Mn oxides) →Organic Matter (Paths: 0.826 for CD, 0.919 for BGS) reveals a crucial interaction. The model suggests that the dissolution of metal oxides released inherent organic matter or provided surfaces for new organic complexation. The net effect on As, however, is determined by the nature of this organic matter as explained by the SUVA254 and DOC content of the current study. While the path from Organic Matter → As was nonsignificant, this likely reflects a balance of opposing processes: (1) the formation of organo-As complexes that can enhance mobility, and (2) the role of organic matter in stabilizing soil aggregates and providing energy for reductive processes that ultimately sequester As. The less negative total effect of Soil Quality on As in the BGS treatment (−0.705) aligns with its tendency to release more labile organic carbon, potentially tilting this balance toward mobilization via microbial respiration and reductive dissolution, as observed in our data. Thus, the PLS–PM model effectively disentangles the network of interactions, revealing that the superior efficacy of CD stems from its ability to optimally enhance soil quality and foster a sulfur cycle that drives As toward stable precipitates

4. Environmental Implications

This study provides fundamental insights into the biogeochemical cycling of As by elucidating the complex interplay among Fe/Mn oxides, SOM, sulfur, and As under dynamic and preset redox conditions. The research reveals critical mechanistic pathways controlling As mobilization and transformation in oxic/reduced environments, particularly how multioriginated organic amendments can influence the As mobilization and geochemical binding subsequently in recurring redox windows. Furthermore, the elucidation of As mobilization pathways involving release from Fe/Mn oxide dissolution and binding with S-DOC complexes via thiolation is the most prominent roadmap with RH-BC and CD amendments, as observed via PLS–PM-machine learning interlinked models. Integration of CD sequestered 62% and 78% of As under oxidizing and reducing conditions, respectively, followed by RH-BC, while BGS released maximal As into the solution phase due to interrupted Fe/Mn transformation patterns, suggesting its use should be carefully managed or avoided in high-As paddies to prevent peak exposure. For effective remediation, it is suggested to integrate CD amendment with water management practices such as implementing controlled intermittent flooding. This strategy leverages the dual benefit of CD’s ability to bind As under both oxidizing and reducing conditions, thereby mitigating the release pulses that typically occur during redox transitions. PLS–PM models provide a predictive framework to tailor amendment selection based on specific soil properties (e.g., initial Fe/S content), enabling stakeholders to design targeted remediation protocols that maximize As immobilization and ensure safer food production. Beyond the biogeochemical benefits, this study highlights the significant economic and practical advantages of these amendments, particularly CD. Its widespread availability and low cost present a clear economic benefit over the more processed RH-BC and BGS. Critically, the application of these amendments integrates seamlessly into existing farming practices without necessitating additional field preparation, thereby minimizing labor and resource barriers to adoption. In conclusion, CD is a cost-effective and environmentally friendly organic ameliorant to reduce As mobilization even under dynamic redox conditions to remediate As-contaminated soils. These mechanistic insights are particularly significant for predicting the behavior of arsenic in environments exposed to redox fluctuations, such as wetlands, paddy fields, and other aquifer systems, with important implications for environmental management and public health.

5. Conclusions

The current study reveals that the biogeochemistry of As is complex under a wide range of predefinite redox conditions, especially with the addition of different multivariate organic materials such as biogas slurry (BGS), rice husk biochar (RH-BC), and cow dung (CD). This study shows that the concentration of dissolved As was 3-fold higher (9.2 mg L–1) under reducing (E h = −100 to −250 mV) than that under oxidizing (3.8 mg L–1) (E h +100 to 500 mV) conditions. The addition of CD, RH-BC, and BGS immobilized As and decreased its release under both reducing and oxidizing conditions (19–62%, 12–54% and 34–49%, respectively, compared to control), while the impact was more obvious under oxidizing environments. Under reducing conditions (−250 to 0 mV), the concentration of dissolved As decreased by 62.1%, and under strongly oxidizing conditions (E h = 100–400 mV), it decreased by 77.6% compared to control with CD amendment.

These findings offer sound applicable knowledge on how well CD, RH-BC, and BGS and similar organic wastes can be effectively and safely applied to remediate As-contaminated paddy soils. These outcomes offer solutions for environmental implications for policy-making and local agricultural practices regarding the safe management of organic wastes and As-polluted rice paddy soils, which can be an aid for the mitigation of the As release from paddy soils, minimizing ecological and human health risks. Furthermore, this study can serve as a foundation for future research, focusing on refining CD, RH-BC, and BGS as tools for improving the health of contaminated paddy soils. Therefore, this study is of great environmental concern. However, future research is warranted to (1) better understand amendments-induced changes on As speciation, methylation, and thiolation under dynamic redox conditions, (2) clarify the complex As–Fe–S cycling in As-contaminated soil under dynamic redox conditions, and solid-phase As speciation using X-ray absorption spectroscopy (XAS) might be a suitable tool to be exploited. In addition, the role of soil microorganisms should be elucidated to understand possible pathways related to the shifting of the aerobic microbial population to the anaerobic population and identification. Further, the impact of different soil types and the stability of organic amendments shall be explored in the future.

Supplementary Material

ao5c07082_si_001.docx (12.3MB, docx)

Acknowledgments

We thank the team of the Laboratory of Soil- and Groundwater-Management, University of Wuppertal, especially Dipl.-Chem. Claus Vandenhirtz for chemical analyses. I.B. is thankful to Alexander von Humboldt Foundation, Germany, for Renewed Research Stay Fellowship (Ref 3.5-PAK/1164117) at the University of Bremen.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c07082.

  • Physicochemical properties of arsenic-contaminated soil collected from Khudpur, Punjab, Pakistan; sequential extraction method for geochemical arsenic fractionation from amended paddy soil; Pearson correlation matrix between arsenic (As) and controlling factors (Eh, pH, Fe, Mn, DOC, SUVA, Cl–, SO4 2–) for all treatments; partial least-squares-path model (PLS–PM) direct, indirect, and total effects on arsenic release and sequestration; E h–pH diagrams for microcosm vessels under control and amended conditions; pair-wise relationships between key competing factors governing arsenic mobilization from PLS–PM analysis; and variable dependence between redox windows, amendments, and As, Fe, DOC, SUVA interactions. (DOCX)

M.M.H. conducted the experiment, laboratory analyses, and data processing, creating the figures and tables, writing the original draft, writing corrections according to the advices of the coauthors; X.Y. conducted the experiment, laboratory analyses, and writingreview and editing; M.S. and I.B.: accurateness of calculations, correction, review and editing; H.W.: proofreading, review and editing; S.W.: proofreading, review and editing; S.M.S.: accurateness of calculations, correction, review and editing; N.K.N.: scientific concept, research idea and facilities, supervision, correction and editing; J.R.: research idea, conceptualization, supervision, facilities, correction and editing

The authors declare no competing financial interest.

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