Methanogenic inhibition by roxarsone (4-hydroxy-3-nitrophenylarsonic acid) and related aromatic arsenic compounds

Defect-enabling zirconium-based metal-organic frameworks for energy and environmental remediation applications

This comprehensive review explores the diverse applications of defective zirconium-based metal-organic frameworks (Zr-MOFs) in energy and environmental remediation. Zr-MOFs have gained significant attention due to their unique properties, and deliberate introduction of defects further enhances their functionality. The review encompasses several areas where defective Zr-MOFs exhibit promise, including environmental remediation, detoxification of chemical warfare agents, photocatalytic energy conversions, and electrochemical applications. Defects play a pivotal role by creating open sites within the framework, facilitating effective adsorption and remediation of pollutants. They also contribute to the catalytic activity of Zr-MOFs, enabling efficient energy conversion processes such as hydrogen production and CO2 reduction. The review underscores the importance of defect manipulation, including control over their distribution and type, to optimize the performance of Zr-MOFs. Through tailored defect engineering and precise selection of functional groups, researchers can enhance the selectivity and efficiency of Zr-MOFs for specific applications. Additionally, pore size manipulation influences the adsorption capacity and transport properties of Zr-MOFs, further expanding their potential in environmental remediation and energy conversion. Defective Zr-MOFs exhibit remarkable stability and synthetic versatility, making them suitable for diverse environmental conditions and allowing for the introduction of missing linkers, cluster defects, or post-synthetic modifications to precisely tailor their properties. Overall, this review highlights the promising prospects of defective Zr-MOFs in addressing energy and environmental challenges, positioning them as versatile tools for sustainable solutions and paving the way for advancements in various sectors toward a cleaner and more sustainable future.

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)).

Diclofenac-Impregnated Mesoporous Carbon-Based Electrode Material for the Analysis of the Arsenic Drug Roxarsone

This paper describes a novel electrode material, diclofenac-impregnated mesoporous carbon modified with a cationic surfactant, cetyltrimethylammonium bromide (DF-CMK-3/CTAB), for ultratrace analysis of the arsenic drug roxarsone (ROX). DF-CMK-3 amorphous carbon is a material with a high specific surface area and well-defined, hexagonally ordered, thin mesopores. The functional groups attached to the carbonaceous surface, such as chromene and pyron-like oxygen groups, lactam, and aromatic carbon rings, have the basic character and they can donate electrons. Modification of DF-CMK-3 with a CTAB layer significantly increases the analytical signal due to electrostatic interactions between the cationic surfactant and the anion form of ROX in the acidic medium. The voltammetric procedure at the glassy carbon sensor modified with DF-CMK-3/CTAB exhibited excellent sensitivity (limit of detection of 9.6 × 10-11 M) with a wide range of linearity from 5.0 × 10-10 to 1.0 × 10-4 M. Analysis of real samples (treated municipal wastewater and river water) showed recoveries from 96 to 102% without applying the complicated sample pretreatment step. The sensor demonstrated excellent sensitivity in the analysis of the arsenic drug ROX in the presence of interferences in environmental water samples.

Application of infrared spectroscopy and its theoretical simulation to arsenic adsorption processes

Accurate detection and analysis of arsenic pollutants are an important means to enhance the ability to manage arsenic pollution. Infrared (IR) spectroscopy technology has the advantages of fast analysis speed, high resolution, and high sensitivity and can be monitored by real-time in situ analysis. This paper reviews the application of IR spectroscopy in the qualitative and quantitative analysis of inorganic and organic arsenic acid adsorbed by major minerals such as ferrihydrite (FH), hematite, goethite, and titanium dioxide. The IR spectroscopy technique cannot only identify different arsenic contaminants but also obtain the content and adsorption rate of arsenic contaminants in the solid phase. The reaction equilibrium constants and the degree of reaction conversion can be determined by constructing adsorption isotherms or combining them with modeling techniques. Theoretical calculations of IR spectra of mineral adsorbed arsenic pollutant systems based on density functional theory (DFT) and analysis and comparison of the measured and theoretically calculated characteristic peaks of IR spectra can reveal the microscopic mechanism and surface chemical morphology of the arsenic adsorption process. This paper systematically summarizes the qualitative and quantitative studies and theoretical calculations of IR spectroscopy in inorganic and organic arsenic pollutant adsorption systems, which provides new insights for accurate detection and analysis of arsenic pollutants and arsenic pollution control. PRACTITIONER POINTS: This paper reviews the application of infrared spectroscopy in the qualitative and quantitative analyses of inorganic and organic arsenic acid adsorbed by major minerals such as ferrihydrite, hematite, goethite, and titanium dioxide, which can help identify and evaluate the type and concentration of arsenic pollutants in water bodies. In this paper, theoretical calculations of infrared spectra of mineral adsorbed arsenic pollutant systems based on density functional theory reveal the adsorption mechanism of arsenic pollutants in water at the solid-liquid interface and help to develop targeted arsenic pollution control technologies. This paper provides a new and reliable analytical detection technique for the study of arsenic contaminants in water bodies.

Glassy Carbon Modified with Cationic Surfactant (GCE/CTAB) as Electrode Material for Fast and Simple Analysis of the Arsenic Drug Roxarsone

For the fast and simple sensing of the arsenic drug roxarsone (ROX), the development of a glassy carbon electrode (GCE) modified with cationic surfactant (cetyltrimethylammonium bromide, CTAB) material is critical. The CTAB-modified glassy carbon electrode, in contrast to the unmodified one, showed excellent behavior for electrochemical reduction of ROX using cyclic voltammetry (CV) and square-wave adsorptive stripping voltammetry (SWAdSV) techniques. CV studies reveal an irreversible reduction process of NO₂ to NH-OH in the ROX molecule in NaAc-HAc buffer (pH = 5.6). The electrode material was characterized using CV and electrochemical impedance spectroscopy. The experiments show that the surfactant-modified material has faster electron transfer and a higher active surface area, and permits a diffusion-adsorption-controlled process. After optimization, the SWAdSV procedure with GCE/CTAB has linear ranges of 0.001-0.02 and 0.02-20 µM, and a detection limit of 0.13 nM. Furthermore, the procedure successfully determined roxarsone in river water samples.

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.

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.

Organoarsenic feed additives in biological wastewater treatment processes: Removal, biotransformation, and associated impacts

Aromatic organoarsenicals are widely used in animal feeding operations and cause arsenic contamination on livestock wastewater and manure, thereby raising the risk of surface water pollution. Biological wastewater treatment processes are often used for livestock wastewater treatment. Organoarsenic removal and biotransformation under aerobic and anaerobic conditions, and the associated impacts have received extensive attention due to the potential threat to water security. The removal efficiency and biotransformation of organoarsenicals in biological treatment processes are reviewed. The underlying mechanisms are discussed in terms of functional microorganisms and genes. The impacts associated with organoarsenicals and their degradation products on microbial activity and performance of bioreactors are also documented. Based on the current research advancement, knowledge gaps and potential research in this field are discussed. Overall, this work delivers a comprehensive understanding on organoarsenic behaviors in biological wastewater treatment processes, and provides valuable information on the control of arsenic contamination from the degradation of organoarsenicals in biological wastewater treatment processes.

The Pseudomonas putida NfnB nitroreductase confers resistance to roxarsone

Roxarsone (3-nitro-4-hydroxyphenylarsonic acid, Rox) has been used for decades as an antimicrobial growth promoter for poultry and swine. Roxarsone is excreted in chicken manure unchanged and can be microbially transformed into a variety of arsenic-containing compounds such as 3-amino-4-hydroxyphenylarsonic acid (HAPA(V)) that contaminate the environment and present a potential health hazard. To cope with arsenic toxicity, nearly every prokaryote has an ars (arsenic resistance) operon, some of which confer resistance to roxarsone. Pseudomonas putida KT2440 is a robust environmental isolate capable of metabolizing many aromatic compounds and is used as a model organism for biodegradation of aromatic compounds. Here we report that P. putida KT2440 (ΔΔars) in which the two ars operons had been deleted retains resistance to highly toxic trivalent Rox(III), the likely active form of roxarsone. In this study, a genomic library constructed from P. putida KT2440 (ΔΔars) was used to screen for resistance to Rox(III) in Escherichia coli. One gene, termed, PpnfnB, was identified that encodes a putative 6,7-dihydropteridine reductase. Cells expressing PpnfnB reduce the nitro group of Rox(III), and purified NfnB catalyzes FMN-NADPH-dependent nitroreduction of Rox(III) to less toxic HAPA(III). This identifies a key step in the breakdown of synthetic aromatic arsenicals.

Anaerobic biotransformation of roxarsone regulated by sulfate: Degradation, arsenic accumulation and volatilization

Roxarsone, an extensively used organoarsenical feed additive, is often pooled in livestock wastewater. Sulfate exists ubiquitously in livestock wastewater and is capable for arsenic remediation. However, little is known about impacts of sulfate on roxarsone biotransformation during anaerobic digestion of livestock wastewater. In this study, the biodegradation of 5.0 mg L^-1 roxarsone, and the accumulation and volatilization of the generated arsenical metabolites in a sulfate-spiked upflow anaerobic granular blanket reactor were investigated. Based on the analysis of degradation products, the nitro and arsenate groups of roxarsone were successively reduced to amino and arsenite groups before the C-As bond cleavage. Effluent arsenic concentration was ∼0.75 mg L^-1, of which 82.9-98.5% were organoarsenicals. The maximum arsenic volatilization rate reached 32.6 μg-As kg^-1-VS d^-1. Adding 5.0 mg L^-1 sulfate enabled 66.7% and 45.9% decrease in inorganic arsenic concentration and arsenic volatilization rate, respectively. Arsenic content in the anaerobic granular sludge (AGS) was accumulated to 1250 mg kg^-1 within 420 days. Based on the results of FESEM-EDS and XPS, sulfate addition induced arsenic precipitation in the AGS through the formation of orpiment. Arsenic in the effluent, biogas and AGS accounted for 52.9%, 0.01% and 47.1% of the influent arsenic when the reactor operated stably. The findings from this study suggest that sulfate has effectively regulatory effects on arsenic immobilization and volatilization during anaerobic digestion of organoarsenic-contaminated livestock wastewater.

Superior removal of inorganic and organic arsenic pollutants from water with MIL-88A(Fe) decorated on cotton fibers

Arsenic contamination has attracted worldwide concerns, owing to its toxicity and severe threat to human and environment. It is urgent to develop efficient adsorbents to remove arsenic pollutants. Within this paper, both pristine MIL-88A(Fe) and MIL-88A(Fe) decorated on cotton fibers were successfully fabricated using an eco-friendly method. The pristine MIL-88A(Fe) displayed outstanding adsorption performances towards four selected arsenic pollutants, in which the adsorption capacities toward As(III), As(V), ROX and ASA were 126.5, 164.0, 261.4 and 427.5 mg g^-1, respectively. Additionally, MIL-88A(Fe) exhibited excellent removal efficiencies in a wide pH range and with the presence of different co-existing ions. It was proposed that the coordinative interactions of As-O-Fe between arsenic pollutants and MIL-88A(Fe) contributed to the superior adsorption performances. Furthermore, two MIL-88A(Fe)/cotton fibers composites were synthesized by both post synthesis (MC-1) and in-situ synthesis (MC-2), which demonstrated identically outstanding adsorption activities toward four selected arsenic pollutants. MC-1 and MC-2 enhanced the stability and reusability of MIL-88A(Fe), which was challenging issues of pristine MIL-88A(Fe) powder. Additionally, the fixed-bed column packed by MC-1 or MC-2 can continuously eliminate arsenic pollutants from the water flow. This work provided a new possibility of metal-organic frameworks to accomplish potentially large-scale application to purify the arsenic-contaminated water.

Combined effect of growth promoter roxarsone and copper on the earthworm Eisenia fetida

Roxarsone (ROX) and copper (Cu) are growth promoters in livestock to promote growth and prevent disease. These chemicals and their metabolites are released to the soil through manure application and have a potential adverse effect on soil-dwelling organisms. The objective of this study was to investigate the combined subacute effect of ROX exposure (0, 80, 240, 720 mg kg^-1) and Cu exposure (0, 80, 160 mg kg^-1) in earthworms (Eisenia fetida). Growth, reproduction, spermatogenesis under light microscope, and heavy metal residue were investigated during 56-day exposure period. Results showed that Cu exposure of 80 or 160 mg kg^-1 alleviated the effect of ROX on cocoon production or hatching. The cocoon number exhibited an increase (P < 0.05) at 80 mg kg^-1 ROX on day 28, compared with the 0 mg kg^-1 ROX, in the presence of 80 mg kg^-1 Cu, whereas there was no effect (P > 0.05) in the presence of 160 mg kg^-1 Cu. The hatching success at 80 or 240 mg kg^-1 ROX exhibited a decrease (P < 0.05) on day 28, in the absence of Cu, whereas no effect (P > 0.05) was observed in the presence of 80 or 160 mg kg^-1 Cu. The other reproductive parameters (cocoon weight, juvenile number, and biomass) demonstrated a decrease (P < 0.05) only at 720 mg kg^-1 ROX in the presence or absence of Cu. However, with increasing exposure time, the above reproductive parameters were not affected (P > 0.05) in all groups on day 56. On the other hand, sperm deformity (%) increased (P < 0.05) at 240 or 720 mg kg^-1 ROX on day 28, in the presence or absence of Cu; however, the microstructural alteration in seminal vesicles occurred only at 720 mg kg^-1 ROX, exhibiting disordered distribution and decreased mature sperm bundles. In addition, ROX or Cu residues in earthworms demonstrated an increase with increasing ROX or Cu exposure concentration. Our present results may provide important insight on combined toxicity of chemicals in soils.

Role of ArsEFG in Roxarsone and Nitarsone Detoxification and Resistance

Organoarsenical biotransformations are important components of the global cycling of arsenic. Roxarsone (3-nitro-4-hydroxybenzenearsenate or Rox(V)) and nitarsone (4-nitrobenzene arsenate or Nit(V)) are synthetic aromatic organoarsenicals used in the poultry industry as additives to prevent coccidiosis and improve feed efficiency. Here, we describe a novel pathway of resistance to roxarsone and nitarsone involving biotransformation of their trivalent forms (Rox(III)) and (Nit(III)) to the trivalent organoarsenicals HAPA(III) and pAsA(III), coupled to active extrusion of the aromatic aminobenezylarsenicals from the cells. The arsE, arsF, and arsG were cloned from the arsenic island in the chromosome of Shewanella putrefaciens 200. When expressed in Escherichia coli together, but not alone, arsEFG conferred resistance to Rox(III) and Nit(III) and decreased the accumulation of both. The cells transformed Rox(III) or Nit(III) to HAPA(III) or pAsA(III) by reducing the nitro group to an amine. Everted membrane vesicles from cells expressing arsG accumulated HAPA(III) or pAsA(III). Our data indicate that ArsE and ArsF together reduce Rox(III) or Nit(III) to HAPA(III) or pAsA(III), which are extruded from the cells by the efflux permease ArsG. Identification of the coupled pathway of ArsE, ArsF, and ArsG catalysis is a molecular description of a novel pathway for resistance to roxarsone and nitarsone.

Effect of roxarsone metabolites in chicken manure on soil biological property

Roxarsone (ROX), an organoarsenic feed additive, occurs as itself and its metabolites including As(V), As(III), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in animal manure. Animal manure improves soil biological property, whereas As compounds impact microorganisms. The integral influence of animal manure bearing ROX metabolites on soil biological quality is not clear yet. Herein, the effect of four chicken manures excreted by chickens fed with four diets containing 0, 40, 80 and 120 mg ROX kg^-1, on soil biological attributes. ROX addition in chicken diets increased total As and ROX metabolites in manures, but decreased manure total N, ammonium and nitrate. The elevated ROX metabolites in manures increased soil total As, As species and total N, and increased first and then decreased soil nitrate and nitrite, but did not affect soil ammonium in manure-applied soils. The promoting role of both soil As(III) and ammonium on soil microbial biomass carbon and nitrogen, respiration and saccharase activity, were exceeded or balanced by the inhibiting effect of soil nitrate. The suppression of soil catalase activity by soil As(V) was surpassed by the enhancement caused by soil nitrate and nitrite. Soil urease, acid phosphatase and polyphenol oxidase activities were not suitable bioindicators in the four manure-amended soils. Soil DMA did not affect soil biological properties, and MMA was not detectable in all manure-amended soils. The above highlights the complexity of joint influence of soil As and N on biological attributes. Totally, when ROX is used at allowable dose in chicken diet, soil biological quality would be suppressed in manure-amended soil.

Roxarsone exposure jeopardizes nitrogen removal and regulates bacterial community in biological sequential batch reactors

Roxarsone is widely present in wastewaters of many animal farms in China. However, little is known about how long-term roxarsone exposure influences the nitrogen removal of biological wastewater treatment in agricultural settings. Here we investigated the nitrogen removal performance of a biological sequential batch reactor (SBR) and the changes of bacterial community, upon long-term roxarsone exposure. The long-term roxarsone dosing decreased the SBR nitrogen removal by 52.4%, with an immediate inhibition on denitrification and a delayed inhibition on nitrification. The analyses of bacterial enzymatic activities and 16 S rRNA sequencing revealed that bacterial activities generally decreased, and the nitrogen-cycling bacterial community was changed, particularly by the decrease (Acinetobacter and Methylophilaceae), persistence (Flavobacterium and Methylotenera), and emergence (Aeromonas) of certain bacterial genera. Overall, chronic roxarsone exposure could suppress nitrification and denitrification, which may even have broad implications on the use efficiency and cycling of nitrogen in agroecosystems.

Aromatic organoarsenic compounds (AOCs) occurrence and remediation methods

Many researchers at home and abroad have made a body of researches and have gained great achievements on the environmental occurrence, fate, and toxicity of inorganic arsenic. But there is less research on the use of aromatic organoarsenic compounds (AOCs), which are common feed additives for livestock in the poultry industry. In this review, we outline the current state of knowledge acquired on the occurrence and remediation of AOCs, respectively. We also identify knowledge gaps and research needs, including the elucidation of the environmental fate of AOCs, metabolic pathway, the impact of metabolic modification on toxicity, and advanced analytical or repaired methods that allows for monitoring, identification or removal of the degradation products.

Evidence of anaerobic coupling reactions between reduced intermediates of 4-nitroanisole

Nitroaromatic compounds are widely used in agricultural pesticides, pharmaceuticals, military explosives, and other applications. They enter the environment via manufacturing and municipal wastewater discharges and releases from agricultural and military operations. Because of their ubiquity and toxicity, they are considered an important class of environmental contaminants. Nitroaromatics are known to undergo reductive transformation to aromatic amines, and under aerobic conditions they are susceptible to coupling reactions which may lead to their irreversible incorporation into soil organic matter. However, there is also evidence of coupling reactions in the absence of oxygen between reduced intermediates of the insensitive munitions compound 2,4-dinitroanisole, leading to the formation of azo dimers. The formation of such products is a concern since they may be more toxic than the original nitroaromatic compounds. The objective of this research is to provide evidence of the anaerobic formation of azo coupling products. 4-Nitroanisole was used as a model compound and was spiked into incubations containing anaerobic granular sludge with H₂ as the electron donor. Using liquid chromatography, UV-Vis spectroscopy, and mass spectrometry, the formation of the azo dimer 4,4'-dimethoxyazobenzene was confirmed. However, due to the instability of the azo bond under the reducing conditions of our incubations, the azo dimer did not accumulate. Consequently, 4-aminoanisole was the major product formed in our experiment. Other minor suspected coupling products were also detected in our incubations. The results provide clear evidence for the temporal formation of at least one azo dimer in the anaerobic reduction of a model nitroaromatic compound.

Transformation of roxarsone in the anoxic-oxic process when treating the livestock wastewater

In order to evaluate the influence of roxarsone (ROX) on the livestock wastewater treatment, a lab-scale pilot employing an anoxic-oxic (A-O) process was investigated by adding different concentrations of ROX at different periods. The mass balance of arsenic (As) in the A-O system was established through the analysis of As speciation and As migration in the gas, liquid and solid phases. The results showed that around 80% of total ROX (initial concentration was 50mgROXL^-1) was eliminated in the anoxic reactor (R1) in which at least about 11% of total ROX was transformed to inorganic As^v (iAs^v) due to the direct breaking of the C-As bond of ROX. Inorganic As^III (iAs^III) and arsine (AsH₃) were produced in R1, while the generated iAs^III in the effluent of R1 was almost completely oxidized to iAs^V in the aerobic reactor (R2). However, the concentration of ROX in the effluent of R2 was almost the same as that in the effluent of R1. After 85days operation, iAs^V and residual ROX as the main forms of As were observed after the A-O process. Furthermore, the mass balance of As at steady state revealed that around 0.08%, 3.91% and 96.01% of total As was transformed into gas (biogas), solid (excess sludge) and liquid (effluent). Additionally, the 16S rRNA analysis demonstrated that the existence of ROX in livestock wastewater may play a crucial role in the diversity of bacterial community in the A-O system.

Adsorption of organic arsenic acids from water over functionalized metal-organic frameworks

Organic arsenic acids (OAAs) are regarded as water pollutants because of their toxicity and considerable solubility in water. Adsorption of OAAs such as phenylarsonic acid (PAA) and p-arsanilic acid (ASA) from water was investigated over functionalized (with OH groups) metal-organic framework (MOF, MIL-101), as well as over pristine MIL-101 and commercial activated carbon. The highly porous MIL-101 bearing three hydroxyl groups (MIL-101(OH)₃) exhibited remarkable PAA and ASA adsorption capacities. Based on the effects of pH on PAA and ASA adsorption, hydrogen bonding was suggested as a plausible mechanism of OAA adsorption. Importantly, OAAs and MIL-101(OH)₃ can be viewed as hydrogen-bond acceptors and donors, respectively. Moreover, MIL-101(OH)₃ could be regenerated by acidic ethanol treatment, being a promising adsorbent for the removal of PAA and ASA from water.

Response of soil microbial communities to roxarsone pollution along a concentration gradient

The extensive use of roxarsone (3-nitro-4-hydroxyphenylarsonic acid) as a feed additive in the broiler poultry industry can lead to environmental arsenic contamination. This study was conducted to reveal the response of soil microbial communities to roxarsone pollution along a concentration gradient. To explore the degradation process and degradation kinetics of roxarsone concentration gradients in soil, the concentration shift of roxarsone at initial concentrations of 0, 50, 100, and 200 mg/kg, as well as that of the arsenic derivatives, was detected. The soil microbial community composition and structure accompanying roxarsone degradation were investigated by high-throughput sequencing. The results showed that roxarsone degradation was inhibited by a biological inhibitor, confirming that soil microbes were absolutely essential to its degradation. Moreover, soil microbes had considerable potential to degrade roxarsone, as a high initial concentration of roxarsone resulted in a substantially increased degradation rate. The concentrations of the degradation products HAPA (3-amino-4-hydroxyphenylarsonic acid), AS(III), and AS(V) in soils were significantly positively correlated. The soil microbial community composition and structure changed significantly across the roxarsone contamination gradient, and the addition of roxarsone decreased the microbial diversity. Some bacteria tended to be inhibited by roxarsone, while Bacillus, Paenibacillus, Arthrobacter, Lysobacter, and Alkaliphilus played important roles in roxarsone degradation. Moreover, HAPA, AS(III), and AS(V) were significantly positively correlated with Symbiobacterium, which dominated soils containing roxarsone, and their abundance increased with increasing initial roxarsone concentration. Accordingly, Symbiobacterium could serve as indicator of arsenic derivatives released by roxarsone as well as the initial roxarsone concentration. This is the first investigation of microbes closely related to roxarsone degradation.

Speciation analysis of organoarsenic compounds in livestock feed by microwave-assisted extraction and high performance liquid chromatography coupled to atomic fluorescence spectrometry

The development of a new method to determine the presence of the organoarsenic additives p-arsanilic acid (ASA), roxarsone (ROX) and nitarsone (NIT) in livestock feeds by high performance liquid chromatography coupled to ultraviolet oxidation hydride generation atomic fluorescence spectrometry (HPLC-UV/HG-AFS) after microwave assisted extraction (MAE) was proposed. Chromatographic separation was achieved on a C18 column with 2% acetic acid/methanol (96:4, v/v) as the mobile phase. The limits of detection (LODs) were 0.13, 0.09 and 0.08mgL^-1, and the limits of quantification (LOQs) were 0.44, 0.30 and 0.28mgL^-1. The relative standard deviations (RSDs) for ASA, ROX and NIT determined from five measurements of the mixed calibration standard were 3.3, 5.3, and 5.4%, respectively. MAE extraction of phenylated arsenic compounds using 1.5M H₃PO₄ at 120°C for 45min allowed for maximum recoveries (%) of total arsenic (As) and organoarsenic species, with no degradation of these compounds. The extraction of total As was approximately 97%, and the As species recoveries were between 95.2 and 97.0%. The results of the analysis were validated using mass balance by comparing the sum of extracted As with the total concentration of As in the corresponding samples. The method was successfully applied to determine the presence of these compounds in feed samples. ASA was the only As species detected in chicken feed samples, with a concentration between 0.72 and 12.91mgkg^-1.

Response of microbial communities to roxarsone under different culture conditions

Roxarsone is a feed additive widely used in the broiler and swine industries that has the potential to contaminate the environment, mainly via the use of poultry manure as fertilizer, which results in release of inorganic arsenic to the soil and water. This study was conducted to investigate roxarsone degradation and the response of the microbial community under different culture conditions using high-throughput sequencing technology. Poultry litter was incubated for 288 h in the presence of roxarsone under light aerobic, dark aerobic, or dark anaerobic conditions. The results showed that roxarsone was completely degraded after 48 h of dark anaerobic incubation, while 79.9% and 94.5% of roxarsone was degraded after 288 h of dark aerobic and light aerobic incubation, respectively. Under dark aerobic conditions with microbial inhibitor sodium azide, roxarsone was rarely degraded during the 288 h of incubation, illustrating that microorganisms play an important role in roxarsone degradation. Microbial community structure was significantly different among various culture conditions. Olivibacter, Sphingobacterium, and Proteiniphilum were the top 3 genera in the control samples. Sphingobacterium and Alishewanella dominated the light aerobic samples, while the dominant microflora of the dark aerobic samples were Acinetobacter spp. Pseudomonas and Advenella were the predominant genera of dark anaerobic samples. This study emphasizes the potential importance of microbes in roxarsone degradation and expands our current understanding of microbial ecology during roxarsone degradation under different environmental conditions.

Continuous treatment of the insensitive munitions compound N-methyl-p-nitro aniline (MNA) in an upflow anaerobic sludge blanket (UASB) bioreactor

N-methyl-p-nitroaniline (MNA) is an ingredient of insensitive munitions (IM) compounds that serves as a plasticizer and helps reduce unwanted detonations. As its use becomes widespread, MNA waste streams will be generated, necessitating viable treatment options. We studied MNA biodegradation and its inhibition potential to a representative anaerobic microbial population in wastewater treatment, methanogens. Anaerobic biodegradation and toxicity assays were performed and an up-flow anaerobic sludge blanket reactor (UASB) was operated to test continuous degradation of MNA. MNA was transformed almost stoichiometrically to N-methyl-p-phenylenediamine (MPD). MPD was not mineralized; however, it was readily autoxidized and polymerized extensively upon aeration at pH = 9. In the UASB reactor, MNA was fully degraded up to a loading rate of 297.5 μM MNA d(-1). Regarding toxicity, MNA was very inhibitory to acetoclastic methanogens (IC50 = 103 μM) whereas MPD was much less toxic, causing only 13.9% inhibition at the highest concentration tested (1025 μM). The results taken as a whole indicate that anaerobic sludge can transform MNA to MPD continuously, and that the transformation decreases the cytotoxicity of the parent pollutant. MPD can be removed through extensive polymerization. These insights could help define efficient treatment options for waste streams polluted with MNA.

Defect creation in metal-organic frameworks for rapid and controllable decontamination of roxarsone from aqueous solution

Given the great harm to the human health of organic arsenic compounds (OACs), developing highly efficient adsorbents with both rapid adsorption rate and high saturation capacity is paramount important. Herein, Zr-based metal-organic frameworks (MOFs) of UiO-66 have been successfully exploited for the efficient decontamination of a typical organic arsenic compound of roxarsone (ROX) from aqueous solution. The influences of the most significant parameters such as contact time, adsorbate concentration, pH as well as ionic strength on the adsorption of ROX were investigated. The amount of missing-linker defects in UiO-66 was systematically tuned by changing the concentration of modulator in the reactants. The presence of the defects not only resulted in the dramatically enhanced porosity, but also induced the creation of ZrOH groups which served as the main active adsorption sites for efficient ROX sequestration. As a result, adsorptive capacity of ROX over UiO-66 could be improved to 730 mg/g, which was much higher than those of many reported adsorbents. Meanwhile, the adsorption equilibrium time could be reduced to as short as 30 min. These merits, combined with their excellent stability, prefigure the great potentials of these defect-tunable UiO-66 MOFs as adsorbents for the efficient removal of various OACs from the polluted water.

Study of photodegradation and photooxidation of p-arsanilic acid in water solutions at pH = 7: kinetics and by-products

The paper presents the kinetics and proposed pathways photodegradation and photooxidation of p-arsanilic acid, in a neutral environment by ozone and hydrogen peroxide. The results showed that in a neutral environment, photoozonation process was characterized by the highest decomposition rate constant (k) (k = 31.8 × 10(-3) min(-1)). The rate constants decreased in the order UV/O3 > O3 > UV/H2O2 > H2O2 > UV. It was also found that under pH = 7, decomposition of p-arsanilic acid leads mainly to the formation of aniline, which undergoes secondary reactions. Intermediate products of oxidation and photooxidation by hydrogen peroxide like nitrobenzene, nitrophenol, azobenzenes, and phenylazophenol were identified depending on processes. However, in the photodegradation process, formation of nitrasone as a reaction product of p-arsanilic acid with oxygen in the singlet state was observed. In the case of ozonation and photoozonation, in addition, aniline formation of carboxylic acids was observed.

Comparative cytotoxicity and accumulation of Roxarsone and its photodegradates in freshwater Protozoan Tetrahymenathermophila

Roxarsone (ROX) remains to be as an organoarsenical feed additive used widely in developing countries. However, most of the ROX is excreted unchanged in manure, which could be readily photodegraded into inorganic arsenic derivatives. In this study, the comparative cytotoxicity and arsenic accumulation were evaluated after the exposure of Tetrahymenathermophila (T. thermophila) cell model to ROX and its photodegradates. The cytotoxic effects were estimated according to the relevant cell growth curves, morphologies and MTT assays. The 36 h median effective concentrations for ROX and its photodegradates at various photolysis times (10, 20, and 30 min) are 39.0, 2.08, 1.88, and 1.82 mg (total arsenic) L(-1), respectively. In parallel, the cellular arsenic uptakes were determined by hydride generation-atomic fluorescence spectrometry. Phospholipid layer as basic membrane structure was mimicked to assess the correlation between membrane permeability and cytotoxicity. The biocompatibility of ROX was dependent on its tendency to interact with cell membrane while the cytotoxicity was induced by the trans-membrane of the inorganic arsenic species present in the photodegradates of ROX. Furthermore, the photodegradates of ROX-associated alterations of intracellular protein profiles were analyzed using a proteomic approach. Overall, the significance was clarified that the control of arsenic emission caused by the application of ROX needs to be imposed.

Determination of para-arsanilic acid with improved diazotization reaction using differential pulse cathodic stripping voltammetry in aqueous system

Para-arsanilic acid (p-ASA) has been widely used in the poultry industry to promote growth and prevent dysentery. It is excreted unchanged in the manure and released into non-target sites causing organoarsenic pollution risk to the environment and living system. Therefore, simple and effective analytical strategies are demanded for determining the samples that contain p-ASA. However, direct determination of both p-ASA and ortho-arsanilic acid (o-ASA) using differential pulse cathodic stripping voltammetry (DPCSV) gives the similar voltammograms that directly hamper the analysis used by the DPCSV technique. In this study, a method to determine and differentiate p-ASA from o-ASA via diazotization and coupling reaction of the amine groups followed by the direct DPCSV determination of diazo compounds is presented. The diazotization reaction carried out at pH 1.5 and 0 ± 1°C for 10 min showed two reduction peaks in DPCSV at-70 mV and -440 mV vs. Ag/AgCl (KCl 3 M). However, when the diazotization reaction was performed at pH 12.5 and 0 ± 1°C for 40 min, a coloured azo compound was produced and the DPCSV showed only one reduction peak that appeared at -600 mV vs. Ag/AgCl (3 M of KCl). The results of this study show that only p-ASA compound gave a reduction peak, whereas o-ASA compound did not give any peak. The detection limit of p-ASA was found to be 4 × 10(-8 )M. As a result, the proposed electro-analytical technique might be a good candidate to determine and differentiate the p-ASA present in the poultry and environmental samples.

UV irradiation and UV-H₂O₂ advanced oxidation of the roxarsone and nitarsone organoarsenicals

Roxarsone (ROX) and nitarsone (NIT) are used as additives in animal feeding operations and have been detected in animal manure, agricultural retention ponds, and adjacent surface waters. This work investigates treatment of organoarsenicals using UV-based treatment processes, namely UV irradiation at 253.7 nm and the UV-H2O2 advanced oxidation process. The apparent molar absorptivity was mapped for ROX and NIT across pH and wavelength. For UV irradiation at 253.7 nm, the fluence-based pseudo-first order rate constant (kp(')) and effective quantum yield (Φ) for ROX were 8.10-29.7 × 10(-5) cm(2)/mJ and 2.34-8.37 × 10(-3) mol/E, respectively; the corresponding constants were slightly lower for NIT. The observed rate constants are higher during advanced oxidation (e.g., kp,ROX(')=3.92(±0.19)-217(±48) × 10(-4) cm(2)/mJ). Second order rate constants for organoarsenical transformation by hydroxyl radicals were determined to be 3.40(±0.45) × 10(9) and 8.28(±0.49) × 10(8) M(-1)s(-1) for ROX and NIT, respectively. Solution pH and nitrate concentration did not significantly impact ROX transformation during advanced oxidation; however, bicarbonate and dissolved organic matter from chicken litter reduced ROX transformation through hydroxyl radical scavenging. Inorganic arsenic was the predominant transformation product of ROX during UV-H2O2 treatment.

Organoarsenicals in poultry litter: detection, fate, and toxicity

Arsenic contamination in groundwater has endangered the health and safety of millions of people around the world. One less studied mechanism for arsenic introduction into the environment is the use of organoarsenicals in animal feed. Four organoarsenicals are commonly employed as feed additives: arsanilic acid, carbarsone, nitarsone, and roxarsone. Organoarsenicals are composed of a phenylarsonic acid molecule with substituted functional groups. This review documents the use of organoarsenicals in the poultry industry, reports analytical methods available for quantifying organic arsenic, discusses the fate and transport of organoarsenicals in environmental systems, and identifies toxicological concerns associated with these chemicals. In reviewing the literature on organoarsenicals, several research needs were highlighted: advanced analytical instrumentation that allows for identification and quantification of organoarsenical degradation products; a greater research emphasis on arsanilic acid, carbarsone, and nitarsone; identification of degradation pathways, products, and kinetics; and testing/development of agricultural wastewater and solid treatment technologies for organoarsenical-laden waste.

Biodegradation and speciation of roxarsone in an anaerobic granular sludge system and its impacts

Roxarsone (3-nitro-4-hydroxy benzene arsenic acid) is an organoarsenic feed additive and has been widely used in the poultry industry to prevent coccidiosis and improve feed efficiency. The presence of roxarsone and its degradation products results in the instability of the anaerobic methanogenic process. This study investigated the degradation and speciation of roxarsone in an anaerobic granular sludge (AGS) system and the impacts of roxarsone and its degradation products on the structure of AGS. Roxarsone inhibited methane production, and the added roxarsone was rapidly degraded into 3-amino-4-hydroxyphenylarsonic acid (HAPA). After 240 days of incubation, the distribution of arsenic differed between the aqueous solution and the AGS in the assays of 20 and 350mg/L roxarsone. Species analysis indicated that HAPA was completely degraded in all of the assays with roxarsone addition after 240 days of incubation. Species distribution was affected by the phases and the initial concentration of roxarsone added. The concentration of As(III) was higher than that of As(V) in both the aqueous solution and the AGS in all assays with roxarsone addition. The toxicity of roxarsone and its degradation products resulted in changes in the structure and the microorganism species in the AGS.

Electrochemical stimulation of microbial roxarsone degradation under anaerobic conditions

Roxarsone (4-hydroxy-3-nitrophenylarsonic acid) has been commonly used in animal feed as an organoarsenic additive, most of which is excreted in manure. Roxarsone is easily biodegraded to 4-hydroxy-3-aminophenylarsonic acid (HAPA) under anaerobic conditions, but HAPA persists for long periods in the environment, increasing the risk of arsenic contamination through diffusion. We investigated the electrochemical stimulation of the microbial degradation of roxarsone under anaerobic conditions. After the carbon sources in the substrate were depleted, HAPA was slowly degraded to form arsenite under anaerobic conditions. The degradation rate of HAPA was significantly increased when 0.5 V was applied without adding a carbon source. The two-cell membrane reactor assays reveal that the HAPA was degraded in the anode chambers, confirming that the anode enhanced the electron transfer process by acting as an electron acceptor. The degradation product formed with electrochemical stimulation was arsenate, which facilitates the removal of arsenic from wastewater. Based on the high performance liquid chromatography-ultraviolet-hydride generation-atomic fluorescence spectrometry (HPLC-UV-HG-AFS) and gas chromatography-mass spectrometry (GC-MS) data, the pathway for the biodegradation of roxarsone and the mechanisms for the electrochemically stimulated degradation are proposed. This method provides a potential solution for the removal of arsenic from organoarsenic-contaminated wastewater.

Biological phosphorus removal inhibition by roxarsone in batch culture systems

Roxarsone has been extensively used in the feed of animals, which is usually excreted unchanged in the manure and eventually enter into animal wastewater, challenging the biological phosphorus removal processes. Knowledge of its inhibition effect is key for guiding treatment of roxarsone-contaminated wastewater, and is unfortunately keeping unclear. We study the inhibition of roxarsone on biological phosphorus removal processes for roxarsone-contaminated wastewater treatment, in terms of the removal and rates of chemical oxygen demand (COD), phosphate. Results showed that presence of roxarsone considerably limited the COD removals, especially at roxarsone concentration exceeding 40 mg L(-1). Additionally, roxarsone inhibited both phosphorus release and uptake processes, consistent with the phosphate profiles during the biological phosphorus removal processes; whereas, roxarsone is more toxic to phosphorus uptake process, than release function. The results indicated that it is roxarsone itself, rather than the inorganic arsenics, inhibit biological phosphorus removal processes within both aerobic and anaerobic roxarsone-contaminated wastewater treatment.

Determination of arsenic in chicken feed by hydride generation atomic absorption spectrometry after pre-concentration with polyurethane foam

A pre-concentration procedure with solid-phase extraction was developed for the determination of arsenic (As) in chicken feed using hydride generation atomic absorption spectrometry (HG-AAS). The procedure was based on the sorption of As(III) ions as complexes with ammonium pyrrolidine dithiocarbamate onto a mini-column packed with polyurethane foam. After pre-concentration, the As was removed from the mini-column by acid solution, and the analyte content in the eluate was measured by HG-AAS. The following main experimental conditions were established: adjustment of the As solution pH with 0.05 mol l⁻¹ HCl, 2.88 × 10⁻³ mol l⁻¹ complexing agent concentration and 6.0 mol l⁻¹ eluting hydrochloric acid concentration. The proposed method produced an enrichment factor of 67, with 0.050 and 0.165 µg g⁻¹ limits of detection and quantification, respectively. The procedure was applied to the determination of As content in two types of chicken feed using the proposed procedure and atomic absorption spectrometry with electrothermal atomisation (ETAAS). The t-test indicated that the results were not significantly different at a confidence level of 95%.