# Environmental Behavior and Remediation Methods of Roxarsone
## 3 Transformation of ROX
### Microbial Degradation  
It has been found in many studies that microbes mainly control ROX degradation in livestock waste and environmental media. Sterilizing soil markedly decreases the degree to which ROX is degraded, and ROX degradation processes are strongly affected by environmental factors, such as the temperature, humidity, and organic matter content [36]. ROX is stable in fresh and dried animal feces, but composting manure with a moisture content of 50% at 40 \({}^{\circ}\)C causes ROX to be rapidly converted into inorganic arsenic within 30 d through mainly biological processes [4]. ROX can be degraded by both anaerobic and aerobic microorganisms, as shown in Table 1. ROX is more readily and more efficiently degraded in an anaerobic environment than an aerobic environment [37]. ROX-degrading bacteria have been isolated from animal manure, soil, and sediment. Organic-rich manure and soil provide the necessary conditions for microorganisms to transform ROX into more toxic inorganic arsenic.  
The metabolic pathway involved in microbial degradation of ROX begins with nitro-reduction, which produces HAPA. Glucose, hydrogen, and lactate can be used as electron donors to accelerate the reduction of nitrogen substituents. As(III) and As(V) are produced through the cleavage of As-C bonds and aromatic rings in HAPA [38]. HAPA can also be acetylated to give \(N\)-acetyl-4-hydroxy-\(m\)-arsanilic acid [6]. A novel sulfur-containing arsenic species (AsC\({}_{9}\)H\({}_{13}\)N\({}_{2}\)O\({}_{6}\)S) has also been detected during the biotransformation of ROX by _Enterobacter_ sp. CZ-1 [39]. Various proteins and genes are involved in ROX degradation, including 816 proteins involved in ROX degradation by _Alkaliphilus oremlandii_ OhILAs [40]. Two _nhoA_ genes (_nhoA_1 and _nhoA_2), encoding \(N\)-hydroxyarlamine \(O\)-acetyltransferases, were found to be responsible for HAPA acetylation by _Enterobacter_ sp. strain CZ-1 [41]. Exoelectrogenic bacteria can effectively use ROX as an electron acceptor for anaerobic respiration. MtrC and UndA are the key cytochromes involved in extracellular reduction in ROX by _Shewanella putrefaciens_ CN32, but extracellular and intracellular reduction occur simultaneously and As(III) is the main inorganic arsenic species that is produced [42]. ROX degradation is also associated with proteins that detoxify arsenic (_ars_ operon) and proteins that allow respiration using arsenic (_arr_ operon). The _arsl_ gene encodes Arsl C-As lyase [43]. The _arsM_ gene abundances in samples of anaerobic granular sludge were found to positively correlate with the arsenic volatilization rates for ROX-loaded digesters [44]. _arsEFG_ genes cloned from _Shewanella putrefaciens_ 200 were found to be responsible for the production and efflux of HAPA(III) and to contribute to HAPA mobilization and toxicity [45]. ArsK is a novel efflux protein for As(III), trivalent ROX (ROX(III)), and methylarsenite (MAs(III)) [46].  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Type** & **Phylum** & **Species** & **Isolation Environment** & \multicolumn{2}{c}{**Degradation Efficiency**} & \multicolumn{1}{c}{**Catalogues**} & \multicolumn{1}{c}{**Reference**} \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\ \hline \multirow{6}{*}{Anarebot microorganisms} & Firmicutes & \multicolumn{2}{c}{_Classtitution_ sp. strain OhILAs} & \multicolumn{1}{c}{Manure} & \multicolumn{1}{c}{84 h, 100\%} & HAPA and inorganic arsenic & [47] \\  & Firmicutes & _Al