Enhancing Roxarsone Degradation and In Situ Arsenic Immobilization Using a Sulfate-Mediated Bioelectrochemical System

Electrochemical sensor for detecting roxarsone in animal-derived foods using MXene and silver telluride

Detecting harmful feed additives in animal-derived foods is essential due to potential health and environmental risks. Given its toxicity, Roxarsone, a common organoarsenic compound, requires sensitive detection methods. This study introduces an electrochemical sensor using a glassy carbon electrode modified with 2D MXene (Ti3C2Tₓ) layered with silver telluride (Ag2Te). Characterization through XRD, FT-IR, and XPS confirmed the sensor's composition. Electrochemical assessments using cyclic voltammetry and differential pulse voltammetry showed notable electron transfer and catalytic efficiency improvements. The sensor exhibited a broad detection range for roxarsone (0.03-2310 μM) with an ultra-low detection limit (0.32 nM) and demonstrated excellent reproducibility (RSD < 3 %), stability, and selectivity. Real sample analysis in meat products confirmed its practical application, achieving high recovery rates. This work offers a robust approach for roxarsone detection, employing MXene and Ag2Te's synergistic properties to enhance food safety and environmental monitoring.

Mo-doped Co LDHs as Raman enhanced substrate for detection of roxarsine

BACKGROUND

Roxarsone (ROX) is widely used as a feed additive, which is indigestible after ingestion by poultry, and most of it can only be excreted into the natural environment and degraded into highly toxic and carcinogenic inorganic arsenic compounds, which pose a hazard to the ecosystem and human health. However, for roxarsone, traditional detection methods require complex and time-consuming procedures, so it is urgent to find a new fast detection method for detection of ROX.

RESULTS

In this work, we developed a novel Raman enhancement material and utilized the Surface-enhanced Raman scattering (SERS) technique to achieve rapid and sensitive detection of roxarsone. Specifically, Mo-doped cobalt layered double hydroxides (Co-LDHs) semiconductor material (abbreviated as CMM-100) was prepared by a simple method of using ion-assisted MOF etching. Under laser excitation at a wavelength of 532 nm, the CMM-100 showed excellent SERS property to various organic dye molecules such as methylene blue (MB), Toluidine Blue(TB), and Crystal Violet (CV). Especially, an enhancement factor (EF) of 1.4 × 106 was achieved for MB. Compared with the traditional method, this work utilized the fast and non-destructive SERS technology, which achieved a rapid detection of ROX. The detection limit was as low as 9.73 × 10-10 M, and the detection range was from 10-9 M to 10-3 M.

SIGNIFICANCE

In this work, SERS technology was adopted for the rapid and sensitive detection of ROX. This study provides a Raman-enhanced substrate named CMMs for detection of food additives, pesticides, biomolecules, etc., which also broadens the application areas of SERS materials.

Influence of dissolved organic matter on the anaerobic biotransformation of roxarsone accompanying microbial community response

Roxarsone (ROX), commonly employed as a livestock feed additive, largely remains unmetabolized and is subsequently excreted via feces. ROX could cause serious environmental risks due to its rapid transformation and high mobility in the anaerobic subsurface environment. Dissolved organic matter (DOM) is an important constituent of fecal organics in livestock waste and could affect the ROX biotransformation. Nonetheless, the underlying mechanisms governing the interaction between DOM and ROX biotransformation have not yet been elucidated in the anaerobic environment. In this study, the changes of ROX, metabolites, and microbial biomass in the solutions with varying DOM concentrations (0, 50, 100, 200, and 400 mg/L) under anaerobic environments were investigated during the ROX (200 mg/L) degradation. EEM-PARAFAC and metagenomic sequencing were combined to identify the dynamic shifts of DOM components and the functional microbial populations responsible for ROX degradation. Results indicated that DOM facilitated the anaerobic biotransformation of ROX and 200 mg/L ROX could be degraded completely in 28 h. The tryptophan-like within DOM functioned as a carbon source to promote the growth of microorganisms, thus accelerating the degradation of ROX. The mixed microflora involved in ROX anaerobic degrading contained genes associated with arsenic metabolism (arsR, arsC, acr3, arsA, nfnB, and arsB), and arsR, arsC, acr3 exhibited high microbial diversity. Variations in DOM concentrations significantly impacted the population dynamics of microorganisms involved in arsenic metabolism (Proteiniclasticum, Exiguobacterium, Clostridium, Proteiniphilum, Alkaliphilus, and Corynebacterium spp.), which in turn affected the transformation of ROX and its derivatives. This study reveals the mechanism of ROX degradation influenced by the varying concentrations of DOM under anaerobic environments, which is important for the prevention of arsenic contamination with elevated levels of organic matter.

Facet-dependent adsorption of aromatic organoarsenicals on hematite: The mechanism and environmental impact

Aromatic organoarsenic feed additives have been extensively used in poultry and livestock farming; however, a risk of releasing toxic inorganic arsenic exists when they are exposed to the environment. An in-depth understanding of the adsorption -migration behavior of aromatic organoarsenicals on environmental media is limited. In this study, p-arsanilic acid (p-ASA) and roxarsone (ROX) were considered as examples to systematically study their adsorption behaviors on the surface of hematite, a representative iron oxide in soil. By comparing the adsorption abilities and adsorption kinetics of hematite exposed with different facets (hexagonal nanoplates, HNPs, mainly exposed with {001} facets and hexagonal nanocubes, HNCs, exposed with {012} facets), combined with in situ shell-isolated nanoparticle enhanced Raman spectroscopy characterization and density functional theory simulation, the facet-dependent adsorption performance was observed and the mechanism revealed. The results showed that p-ASA formed a bidentate binuclear complex on HNCs and HNPs, whereas ROX formed monodentate mononuclear and bidentate binuclear configurations on the {001} and {012} facets, respectively. These differences not only lead to facet-dependent adsorption capacities but also affect their stability, as verified by sequential extraction experiments, affecting the environmental behavior and fate of aromatic organoarsenicals. This study not only provides insights into the environmental behavior of aromatic organoarsenicals but also offers theoretical support for the development of functional adsorbents and remediation strategies.

Insight into the transformation of 4-hydroxy-3-aminophenylarsonic acid (HAPA) and its mechanisms under simulated sunlight irradiation

Four-hydroxy-3-aminophenylarsonic acid (HAPA), the reduced product of roxarsone (4-hydroxy-3-nitro-phenylarsonic acid, ROX) under anaerobic conditions, is resistant to be biologically degraded under anaerobic/anoxic conditions. The transformation of HAPA in aquatic environment under sunlight irradiation is still unknown. In this study, the photodegradation of HAPA and the possible mechanism under simulated sunlight conditions were investigated. The result shows that under visible light irradiation, HAPA wasn't degraded. Under UV254 and UV302 irradiation, about 60% and 30% HAPA were decomposed, while nearly no HAPA was degraded under UV365 irradiation over a period of 240 min. UVC light was the main wavelength for the degradation of HAPA under sunlight conditions. HCO3- and NO3- slightly enhanced the photodegradation, but Cl- and SO42- had a marginal influence on the photodegradation. During the photodegradation, HAPA was decomposed into organic intermediates, inorganic arsenics, ammonia and undetermined arsenic species. Arsenite (As(III)) was the dominant inorganic arsenic species from the photodegradation of HAPA. The mechanism analysis shows that singlet molecular oxygen (1O2) has little influence on the decomposition of HAPA under UV irradiation, but significantly enhanced the conversion of As(III) to arsenate (As(V)).

Electrochemical determination of Roxarsone using preconcentration-based signal amplification on modified screen-printed electrode

A lab-made screen-printed electrode based on poly(ethylene terephthalate) (PET) substrate modified with a hybrid film containing gold nanoparticles-decorated graphene (AuNPs-GRA/PET-SPE) was employed for the voltammetric determination of Roxarsone (ROX) in chicken purge and river water samples. The electrode exhibited an increased electroactive area and enhanced charge transfer due to the nanostructured matrix. The electrochemical determination involved a preconcentration approach with a reduction step of ROX at a constant potential of -0.6 V, followed by voltammetric sweep towards the oxidation of the adsorbed hydroxylamine at 0.32 V. The methodology achieved a limit of detection of 60 nM and 97 nM for ROX in diluted river water and chicken purge samples, respectively. This effective methodology offers a promising tool for monitoring ROX levels in environmental and food samples.

Inhibitory effect of polyethylene microplastics on roxarsone degradation in soils

Roxarsone (3-nitro-4-hydroxyphenylarsonic acid, Rox(V)), an extensively used organoarsenical feed additive, enters soils through the application of Rox(V)-containing manure and further degrades to highly toxic arsenicals. Microplastics, as emerging contaminants, are also frequently detected in soils. However, the effects of microplastics on soil Rox(V) degradation are unknown. A microcosm experiment was conducted to investigate soil Rox(V) degradation responses to polyethylene (PE) microplastics and the underlying mechanisms. PE microplastics inhibited soil Rox(V) degradation, with the main products being 3-amino-4-hydroxyphenylarsonic acid [3-AHPAA(V)], N-acetyl-4-hydroxy-m-arsanilic acid [N-AHPAA(V)], arsenate [As(V)], and arsenite [As(III)]. This inhibition was likely driven by the decline in soil pH by PE microplastic addition, which may directly enhance Rox(V) sorption in soils. The decreased soil pH further suppressed the nfnB gene related to nitroreduction of Rox(V) to 3-AHPAA(V) and nhoA gene associated with acetylation of 3-AHPAA(V) to N-AHPAA(V), accompanied by a decrease in the relative abundance of possible Rox(V)-degrading bacteria (e.g., Pseudomonadales), although the diversity, composition, network complexity, and assembly of soil bacterial communities were largely influenced by Rox(V) rather than PE microplastics. Our study emphasizes microplastic-induced inhibition of Rox(V) degradation in soils and the need to consider the role of microplastics in better risk assessment and remediation of Rox(V)-contaminated soils.

Biomineralization behavior and mechanism of microbial-mediated removal of arsenate from water

The microbial-mediated removal of arsenate by biomineralization received much attention, but the molecular mechanism of Arsenic (As) removal by mixed microbial populations remains to be elucidated. In this study, a process for the arsenate treatment using sulfate-reducing bacteria (SRB) containing sludge was constructed, and the performance of As removal was investigated at different molar ratios of AsO₄^3- to SO₄^2-. It was found that biomineralization mediated by SRB could achieve the simultaneous removal of arsenate and sulfate from wastewater but only occurred when microbial metabolic processes were involved. The reducing ability of the microorganisms for the sulfate and arsenate was equivalent, so the precipitates produced at the molar ratio of AsO₄^3- to SO₄^2-of 2:3 were most significant. X-ray absorption fine structure (XAFS) spectroscopy was the first time used to determine the molecular structure of the precipitates which were confirmed to be orpiment (As₂S₃). Combined with the metagenomics analysis, the microbial metabolism mechanism of simultaneous removal of sulfate and arsenate by the mixed microbial population containing SRB was revealed, that is, the sulfate and As(V) were reduced by microbial enzymes to produce S^2- and As(III) to further form As₂S₃ precipitates. This research provided a reference and theoretical foundation for the simultaneous removal of sulfate and arsenic mediated by SRB-containing sludge in wastewater treatment.

Modification of TiO2 by Er3+ and rGO enhancing visible photocatalytic degradation of arsanilic acid

As a typical wide band gap photocatalyst, titania (TiO₂) cannot use the visible light and has fast recombination rate of photogenerated electron-hole pairs. Simultaneous introduction of erbium ion (Er³⁺) and graphene oxide (rGO) into TiO₂ might overcome these two drawbacks. In this study, Er³⁺ and rGO were co-doped on TiO₂ to synthesize Er³⁺-rGO/TiO₂ photocatalyst through a two-step sol-gel method. Based on the UV-visible diffuse reflectance spectra and photoluminescence spectrum, the introduction of Er³⁺ and rGO increased the visible light absorption efficiency and enhanced the migration of photogenerated electron. Pure TiO₂ has almost no photocatalytic activity for arsanilic acid (p-ASA) degradation under visible light irradiation. However, while doping with 2.0 mol% Er³⁺ and 10.0 mol% rGO, the p-ASA could be completely degraded within 50 min by the Er³⁺-rGO/TiO₂ photocatalyst under visible light irradiation, and most of produced inorganic arsenic was in situ removed by adsorption from the solution. The reactive oxygen species (ROS) reacting with p-ASA was determined and superoxide radical (O₂^•-) and singlet oxygen (¹O₂) were the dominant ROS for the oxidation of p-ASA and arsenite. This work provides an approach of introducing Er³⁺ and rGO to enhance the visible light photocatalytic efficiency of TiO₂.

Highly efficient persulfate catalyst prepared from modified electrolytic manganese residues coupled with biochar for the roxarsone removal

The contamination of organoarsenic is becoming increasingly prominent while SR-AOPs were confirmed to be valid for their remediation. This study has found that the novel metal/carbon catalyst (Fe/C-Mn) prepared by solid waste with hierarchical pores could simultaneously degrade roxarsone (ROX) and remove As(V). A total of 95.6% of ROX (20 mg/L) could be removed at the concentration of 1.0 g/L of catalyst and 0.4 g/L of oxidant in the Fe/C-Mn/PMS system within 90 min. The scavenging experiment and electrochemical test revealed that both single-electron and two-electron pathways contributed to the ROX decomposition. Spectroscopic analysis suggested the ROX has been successfully mineralized while As(V) was fixed with the surface Fe and Mn. Density functional theory (DFT) calculation and chromatographic analysis indicated that the As7, N8, O9 and O10 sites of ROX molecule were vulnerable to being attacked by nucleophilic, electrophilic and radical, resulting in the formation of several intermediates such as phenolic compounds. Additionally, the low metal leaching concentration during recycling and high anti-interference ability in various water matrices manifested the practicability of Fe/C-Mn/PMS system.

Synergistic effect of bioanode and biocathode on nitrobenzene removal: Microbial community structure and functions

This study aimed to reveal the synergistic effect of bioanode and biocathode on nitrobenzene (NB) removal with different microbial community structures and functions. Single-chamber bioelectrochemical reactors were constructed and operated with different initial concentrations of NB and glucose as the substrate. With the synergistic effect of biocathode and bioanode, NB was completely removed within 8 h at a kinetic rate constant of 0.8256 h^-1, and high conversion rate from NB to AN (92%) was achieved within 18 h. The kinetic rate constant of NB removal was linearly correlated with the maximum current density and total coulombs (R² > 0.95). Increase of glucose and NB concentrations had significantly positive and negative effects, respectively, on the NB removal kinetics (R² > 0.97 and R² > 0.93, respectively). Geobacter sp. and Enterococcus sp. dominated in the bioanode and biocathode, respectively. The presence of Klebsiella pneumoniae in the bioanode was beneficial for Geobacter species to produce electricity and to alleviate the NB inhibition. As one of the dominant species at the biocathode, Methanobacterium formicicum has the ability of nitroaromatics degradation according to KEGG analysis, which played a crucial role for NB reduction. Fermentative bacteria converted glucose into volatile fatty acids or H₂, to provide energy sources to other species (e.g., Geobacter sulfurreducens and Methanobacterium formicicum). The information from this study is useful to optimize the bioelectrocatalytic system for nitroaromatic compound removal.

Phenylarsonics in concentrated animal feeding operations: Fate, associated risk, and treatment approaches

Phenylarsonics are present as additives in animal feed in some countries. As only a small fraction of these additives is metabolized in animals, they mostly end up in the environment. A comprehensive investigation of the fate of these additives is crucial for evaluating their risks. This review aims to provide a clear understanding of the transformation mechanism of phenylarsonics in vivo and in vitro and to evaluate their fate and associated risks. Degradation of phenylarsonics releases toxic As species (mainly as inorganic arsenic (iAs)). Trivalent phenylarsonics are the metabolites or biotic degradation intermediates of phenylarsonics. The cleavage of As groups from trivalent phenylarsonics catalyzed by C-As lyase or other unknown pathways generates arsenite (As(III)). As(III) can be further oxidized to arsenate (As(V)) and methylated to methyl-arsenic species. The half-lives associated with abiotic degradation of phenylarsonics ranged from a few minutes to tens of hours, while those associated with biotic degradation ranged from several days to hundreds of days. Abiotic degradation resulted in a higher yield of iAs than biotic degradation. The use of phenylarsonics led to elevated total As and iAs levels in animal products and environmental matrices, resulting in As exposure risk to humans. The oxidation of phenylarsonics to As(V) facilitated the sorptive removal of As, which provides a general approach for treating these compounds. This review provides solid evidence that the use of phenylarsonics has adverse effects on both human health and environmental safety, and therefore, supports their withdrawal from the global market.

Zero-valent iron mediated alleviation of methanogenesis inhibition induced by organoarsenic roxarsone

Zero-valent iron (ZVI) can enhance anaerobic digestion, and has great potential to alleviate/eliminate methanogenesis inhibition. Little is known about the feasibility of utilizing ZVI to alleviate methanogenesis inhibition that is caused by typical animal feed additive roxarsone in livestock wastewater. In this study, the role of ZVI on alleviating roxarsone-induced methanogenic inhibition and its mechanisms were investigated. With the increase of roxarsone concentration from 5 to 50 mg/L, the inhibition of methanogenesis increased from 3.0% to 65.7%. This inhibition was alleviated by 80.7% and 57.2% when 1.0 and 10.0 g/L ZVI were added, respectively. Due to ZVI addition, an efficient arsenic immobilization onto ZVI (45.4-85.8%) was achieved mainly through the formation of FeAsO₄ precipitate and adsorption by ZVI. Under the function of ZVI, hydrogenotrophic methanogenic activity was obviously restored. The microbial community analysis indicates that the ZVI-regulated alleviation on the methanogenesis inhibition was attributed to the enrichment of Methanobacterium and Methanosarcina. The findings from this study demonstrate that ZVI addition is an effective way for treatment of organoarsenic-contaminated wastewater.

In-situ production and activation of H2O2 for enhanced degradation of roxarsone by FeS2 decorated resorcinol-formaldehyde resins

Fenton technology performs well in high-risk roxarsone (ROX) removal, but it is limited by the high H₂O₂ transportation and storage risks. Herein, FeS₂ decorated resorcinol-formaldehyde resins (FeS₂-RFR) were successfully prepared to in-situ produce and utilize H₂O₂ for efficient removal of ROX. Under solar light illumination, resorcinol-formaldehyde resins (RFR) efficiently generated a high concentration of H₂O₂, with a yield of 500 μmol g^-1 h^-1. FeS₂ can in-situ decompose H₂O₂ to generate ·OH, participating in the oxidation of ROX. As a result, the FeS₂-RFR catalyst degraded more than 97% of ROX within 2 h and ROX was selectively degraded into low-toxic As(V), which can be simply removed by traditional adsorption or precipitation processes. During the degradation of ROX, ·OH played a dominant role. Moreover, the cations (Na⁺, K⁺, and Ca²⁺), anions (SO₄^2-, Cl^-), and humic acid had no noticeable inhibition effect on ROX removal. Furthermore, FeS₂-RFR can still remove 70% of ROX even after three cycles, proving that this in-situ photo-Fenton system exhibited stability. This study innovatively proposed a double-active site FeS₂-RFR photocatalyst for in-situ production and activation of H₂O₂ and showed a sustainable and eco-friendly way for organoarsenic compounds degradation.

Roxarsone transformation and its impacts on soil enzyme activity in paddy soils: A new insight into water flooding effects

The aromatic arsenical roxarsone (ROX) has been used as feed additive for decades worldwide. The past or present application of animal manure containing ROX in paddy fields results in arsenic (As) accumulation in rice grain. However, the degradation and transformation mechanisms of ROX in paddy soil which determine As bioavailability and uptake by rice are still unclear. The current study investigated the variation of As speciation and soil enzyme activities in ROX-treated soils under flooded and non-flooded conditions for six months. Our results showed that 70.2% of ROX persisted in non-flooded paddy soils after 180 d while ROX degraded completely within 7 d in flooded soils. The rapid degradation of ROX under flooded conditions owed to the enhanced biotic transformation that was caused by the low Eh and the predominant presence of Clostridium spp. and Bacillus spp. ROX was not only transformed to As(III) and As(V) in non-flooded soils but also to 3-amino-4-hydroxyphenylarsonic acid and methyl arsenicals in flooded soils. The degradation products significantly inhibited soil enzyme activities for 7-30 d, but the inhibition effects disappeared after 90 d due to the sorption of transformed As products to amorphous Fe oxides. This study provides new insights into the flooding effect on ROX fate in paddy fields, which is important for the management of animal waste and risk control on polluted sites.

Response of anaerobic granular sludge to long-term loading of roxarsone: From macro- to micro-scale perspective

Extensive use of organoarsenic feed additives such as roxarsone has caused organoarsenicals to occur in livestock wastewater and further within anaerobic wastewater treatment systems. Currently, information on the long-term impacts of roxarsone on anaerobic granular sludge (AGS) activity and the underlying mechanisms is very limited. In this study, the response of AGS to long-term loading of roxarsone was investigated using a laboratory up-flow anaerobic sludge blanket reactor spiked with 5.0 mg L^-1 of roxarsone. Under the effect of roxarsone, methane production decreased by ∼40% due to the complete inhibition on acetoclastic methanogenic activity on day 260, before being restored eventually. Over 30% of the influent arsenic was accumulated in the AGS and the capability of AGS to prevent intracellular As(III) accumulation increased with time. The AGS size was reduced by ∼30% to 1.20‒1.26 mm. Based on morphology and confocal laser scanning microscopy analysis, roxarsone exposure stimulated the excretion of extracellular polymeric substances and the surface spalling of AGS. High-throughput sequencing analysis further indicated roxarsone initially altered the acidogenic pathway and severely inhibited the acetoclastic methanogen Methanothrix. Acetogenic bacteria and Methanothrix were finally enriched and became the main contributor for a full restoration of the initial methane production. These findings provide a deeper understanding on the effect of organoarsenicals on AGS, which is highly beneficial for the effective anaerobic treatment of organoarsenic-bearing wastewater.

Production of H2S with a Novel Short-Process for the Removal of Heavy Metals in Acidic Effluents from Smelting Flue-Gas Scrubbing Systems

Direct sulfidation using a high concentration of H₂S (HC-H₂S) has shown potential for heavy metals removal in various acidic effluents. However, the lack of a smooth method for producing HC-H₂S is a critical challenge. Herein, a novel short-process hydrolysis method was developed for the on-site production of HC-H₂S. Near-perfect 100% efficiency and selectivity were obtained via CS₂ hydrolysis over the ZrO₂-based catalyst. Meanwhile, no apparent residual sulfur/sulfate poisoning was detected, which guaranteed long-term operation. The coexistence of CO₂ in the products had a negligible effect on the complete hydrolysis of CS₂. H₂S production followed a sequential hydrolysis pathway, with the reactions for CS₂ adsorption and dissociation being the rate-determining steps. The energy balance indicated that HC-H₂S production was a mildly exothermic reaction, and the heat energy could be maintained at self-balance with approximately 80% heat recovery. The batch sulfidation efficiencies for As(III), Hg(II), Pb(II), and Cd(II) removal were over 99.9%, following the solubilities (K_sp) of the corresponding metal sulfides. CO₂ in the mixed gas produced by CS₂ hydrolysis did not affect heavy metals sulfidation due to the presence of abundant H⁺. Finally, a pilot-scale experiment successfully demonstrated the practical effects. Therefore, this novel on-site HC-H₂S production method adequately achieved heavy metals removal requirements in acidic effluents.