Opposite effects of dissolved oxygen on the removal of As(III) and As(V) by carbonate structural Fe(II)

Zeyuan Tian,Yalei Zhang,Binbin Shao,Yong Feng,Deli Wu,Yiyi Guan

doi:10.1038/s41598-017-17108-4

Abstract

Freshly prepared carbonate structural Fe(II) (CSF) was used to immobilize As(III) and As(V) in wastewater under oxic and anoxic conditions. Dissolved oxygen was found to exert opposite effects on these two arsenic species. The sorption density of As(III) was higher under oxic conditions, whereas that of As(V) was higher under anoxic conditions. X-ray diffraction and infrared spectroscopic analyses indicated that crystalline parasymplesite (Fe(II)3(AsO4)2·8H2O) was formed when As(V) was removed under anoxic conditions, while an amorphous Fe-As-containing precipitate was formed when As(III) was removed under oxic conditions. The distribution of arsenic and iron between the solution and sediments suggested that the oxidation of structural Fe(II) promoted coprecipitation process and inhibited surface complexation. X-ray photoelectron spectroscopic analyses revealed that more As(III) was oxidized under oxic condition, which contributed to a higher sorption capacity for As(III). The formation of parasymplesite through surface complexation/precipitation was proposed to be more effective for the removal of As(V) by CSF, while As(III) was more efficiently removed through coprecipitation. Together, the results suggest that CSF may be an effective material for sequestering both As(III) and As(V). In addition, attention should be paid to the dissolved oxygen content when remediating different arsenic species.

Highlights

The conflicting influence of dissolved oxygen (O2) may be due to the impact of O2 on the compositions of the iron and arsenic species
The mineral composition of the adsorbents was determined by X-ray diffraction (XRD) analysis and Fourier transform infrared spectroscopy (FTIR)
The arsenic sorption density revealed that a higher removal efficiency was obtained in the absence of O2 for the removal of As(V) and in the presence of O2 for the removal of As(III) by carbonate structural Fe(II) (CSF)

Summary

Introduction

The conflicting influence of dissolved oxygen (O2) may be due to the impact of O2 on the compositions of the iron and arsenic species. Fe(III) can undergo hydrolysis and transform into different iron oxyhydroxide and oxide species. The products usually consist of ferrihydrite, green rusts, lepidocrocite, magnetite and goethite[8,9,10] Those newly formed oxidative products have been reported to be more active and have higher adsorption capacities than www.nature.com/scientificreports/. The mechanisms of arsenic removal vary among the different iron species. The oxidation of As(III) may lead to an enhanced removal efficiency by adsorbents that have a high affinity for As(V). For those adsorbents that immobilize arsenic by forming solid As(0), such as ZVI, anoxic conditions are beneficial because they preserve the reducing ability of the material[7]. The contribution of the redox transformations of arsenic was evaluated by X-ray photoelectron spectroscopy (XPS) analysis

Objectives

Methods

Results

Conclusion