Jie-wen Ma,Ya-qing Wu,Chen-Lu Xu,Zhuan-xi Luo,Rui-lian Yu,Gong-ren Hu,Yu Yan
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.
Magnetic hollow Fe 3 O 4 -based Mg/Al layered double hydroxide (Fe 3 O 4 @SiO 2 @LDH) was successfully synthesized and subsequently the laboratory scale tests were conducted to systematically investigate the sequestration of roxarsone (ROX) and arsenate (As(V)). Owing to its abundant surface metal hydroxyl (M-OH) groups and exchangeable CO 3 2− groups in the interlayer of MgAl-LDH nanosheets , the obtained Fe 3 O 4 @SiO 2 @LDH exhibited remarkable behavior for As(V) and ROX removal. Both As(V) and ROX could be effectively immobilized at a relatively wide pH range (3–8), and the maximum adsorption capacities of As(V) and ROX were 96.47 and 71.67 mg/g, respectively. Meanwhile, the negligible mutual effect demonstrated the Fe 3 O 4 @SiO 2 @LDH can simultaneously remove inorganic and organic arsenic compounds . The competitive cations/anions (except for PO 4 3− ions) and ionic strength were negligible on As(V) and ROX removal, whereas the inhibitory effect stem from humic acid (HA) seemed to be more seriously for As(V) than that of ROX. Adsorption mechanism of As(V)/ROX were attributed to electrostatic attraction and ion/ligand exchange process, whereas hydrogen bonding and π-π interaction may also involve in the ROX removal. Furthermore, the excellent performance in fix-bed column tests and prominent reusability indicated this magnetic Fe 3 O 4 @SiO 2 @LDH was a promising candidate adsorbent and of great potential application in organic/inorganic As-contamination wastewater treatment .
High-risk roxarsone (ROX) can be converted to highly toxic arsenite (As(III)) through biological and abiotic processes, which can cause severe arsenic contamination of water and crops. Traditional oxidation processes can only treat organic arsenic compounds instead of releasing inorganic arsenic (iAs). Herein, an Fe(II)/resorcinol formaldehyde resin (RF) solar light-driven in-situ Fenton system was applied to remove ROX and its released iAs. Under solar light illumination, RF can reduce oxygen to produce H 2 O 2 through a two-electron reduction pathway. Fe(II) simultaneously catalyzed the decomposition of H 2 O 2 to generate ·OH, participating in the oxidation of ROX. The C-As bond of ROX was preferentially attacked by ·OH to form 2-nitrohydroquinone and As(III). Moreover, As(III) was further converted to low-toxicity arsenate (As(V)) through further oxidation with oxygen bubbling. Finally, the released As(III) (1.9%) and As(V) (98.1%) were removed by adsorption onto the surface of ferric hydroxide. As a result, more than 99.9% of ROX was degraded by the Fe(II)/RF in-situ Fenton system during the pre-oxidation process. The total organic carbon (TOC) removal efficiency of ROX and total arsenic removal efficiency of the Fe(II)/RF-O 2 in-situ Fenton system reached 75% and more than 97%, respectively. Furthermore, the residual arsenic in this Fenton system was less than 12.5 µg/L, which met the recommended limit for arsenic (100 µg/L) in reclaimed water. In addition to the removal of ROX, the Fe(II)/RF in-situ Fenton system showed excellent general applicability in the removal of emerging organic contaminants (EOCs). The TOC removal rate of EOCs was approximately 50%–64%. Thus, the Fe(II)/RF-O 2 solar light-driven in-situ Fenton system provides a novel strategy for the high-efficiency removal of organoarsenic compounds and EOCs.
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