Quantification of the redox properties of microplastics and their effect on arsenite oxidation

Microplastics have attracted global concern.The environmental-weathering processes control their fate,transport,transformation,and toxicity to wildlife and human health,but their impacts on biogeochemical redox processes remain largely unknown.Herein,multiple spectroscopic and electrochemical approaches in concert with wet-chemistry analyses were employed to characterize the redox properties of weathered microplastics.The spectroscopic results indicated that weathering of phenol-formaldehyde resins(PFs)by hydrogen peroxide(H2O2)led to a slight decrease in the content of phenol functional groups,accompanied by an increase in semiquinone radicals,quinone,and carboxylic groups.Electrochemical and wet-chemistry quantifications,coupled with microbial-chemical characterizations,demonstrated that the PFs exhibited appreciable electron-donating capacity(0.264-1.15 mmol e-g^(-1))and electron-accepting capacity(0.120-0.300 mmol e-g^(-1)).Specifically,the phenol groups and semiquinone radicals were responsible for the electron-donating capacity,whereas the quinone groups dominated the electron-accepting capacity.The reversible redox peaks in the cyclic voltammograms and the enhanced electron-donating capacity after accepting electrons from microbial reduction demonstrated the reversibility of the electron-donating and-accepting reactions.More importantly,the electron-donating phenol groups and weathering-induced semiquinone radicals were found to mediate the production of H2O2 from oxygen for arsenite oxidation.In addition to the H2O2-weathered PFs,the ozone-aged PF and polystyrene were also found to have electron-donating and arsenite-oxidation capacity.This study reports important redox properties of microplastics and their effect in mediating contaminant transformation.These findings will help to better understand the fate,transformation,and biogeochemical roles of microplastics on element cycling and contaminant fate.

Bacterial community succession during the enrichment of chemolithoautotrophic arsenite oxidizing bacteria at high arsenic concentrations

To generate cost-effective technologies for the removal of arsenic from water, we developed an enrichment culture of chemolithoau- totrophic arsenite oxidizing bacteria （CAOs） that could effectively oxidize widely ranging concentrations of As（III） to As（V）. In addition, we attempted to elucidate the enrichment process and characterize the microbial composition of the enrichment culture. A CAOs enrichment culture capable of stably oxidizing As（III） to As（V） was successfully constructed through repeated batch cultivation for more than 700 days, during which time the initial As（iiI） concentrations were increased in a stepwise manner from 1 to 10-12 mmol/L. As（III） oxidation activity of the enrichment culture gradually improved, and 10-12 mmol/L As（III） was almost completely oxidized within four days. Terminal restriction fragment length polymorphism analysis showed that the dominant bacteria in the enrichment culture varied drastically during the enrichment process depending on the As（III） concentration. Isolation and characterization of bacteria in the enrichment culture revealed that the presence of multiple CAOs with various As（Ⅲ） oxidation abilities enabled the culture to adapt to a wide range of As（Ⅲ） concentrations. The CAOs enrichment culture constructed here may be useful for pretreatment of water from which arsenic is being removed.

Arsenic redox transformation by Pseudomonas sp.HN-2 isolated from arsenic-contaminated soil in Hunan,China

A mesophilic,Gram-negative,arsenite[As（Ⅲ）]-oxidizing and arsenate[As（V）]-reducing bacterial strain,Pseudomonas sp.HN-2,was isolated from an As-contaminated soil.Phylogenetic analysis based on 16 S r RNA gene sequencing indicated that the strain was closely related to Pseudomonas stutzeri.Under aerobic conditions,this strain oxidized 92.0%（61.4 μmol/L） of arsenite to arsenate within 3 hr of incubation.Reduction of As（V） to As（Ⅲ） occurred in anoxic conditions.Pseudomonas sp.HN-2 is among the first soil bacteria shown to be capable of both aerobic As（Ⅲ） oxidation and anoxic As（V） reduction.The strain,as an efficient As（Ⅲ） oxidizer and As（V） reducer in Pseudomonas,has the potential to impact arsenic mobility in both anoxic and aerobic environments,and has potential application in As remediation processes.

Effects of cobalt doping on the reactivity of hausmannite for As(Ⅲ)oxidation and As(Ⅴ)adsorption

Hausmannite is a common low valence Mn oxide mineral,with a distorted spinel structure,in surficial sediments.Although natural Mn oxides often contain various impurities of transitional metals(TMs),few studies have addressed the effect and related mechanism of TM doping on the reactivity of hausmannite with metal pollutants.Here,the reactivity of cobalt(Co)doped hausmannite with aqueous As(Ⅲ)and As(Ⅴ)was studied.Co doping decreased the point of zero charge of hausmannite and its adsorption capacity for As(Ⅴ).Despite a reduction of the initial As(Ⅲ)oxidation rate,Co-doped hausmannite could effectively oxidize As(Ⅲ)to As(Ⅴ),followed by the adsorption and fixation of a large amount of As(Ⅴ)on the mineral surface.Arsenic K-edge EXAFS analysis of the samples after As(Ⅴ)adsorption and As(Ⅲ)oxidation revealed that only As(Ⅴ)was adsorbed on the mineral surface,with an average As-Mn distance of 3.25–3.30 A,indicating the formation of bidentate binuclear complexes.These results provide new insights into the interaction mechanism between TMs and low valence Mn oxides and their effect on the geochemical behaviors of metal pollutants.