Arsenic Removal From Groundwater By Modified Granular Natural Siderite: Performance And Application

Arsenic removal from high-As groundwater is of serious concern and hasbecome an important global problem. It is an urgent and challenging issue to developeconomic and efficient materials for As removal from As-contaminated groundwater.This study was carried out to investigate the possibility of using the fine size ofnatural siderite (NS) as the major components in fabricating modified granular naturalsiderite (MGNS) for As removal, which were synthesized by addition of alumina sol,extrusion granulation, and calcinations. The calcinated strips were manually cut intoparticles with length of1.0~2.0cm. The modification greatly improved the adsorptioncapacity of the material. Results of As removal by different modified granularadsorbents showed that the maximum removal efficiency was reached with anadhesive dosage of10mg Al/g NS and calcination temperature of350℃for90minutes. Results of XRD, FTIR, and Fe K-edge XANES analysis showed that sideritewas partially transformed into hematite in the process of modification, and MGNSwas considered as an Fe(II)/(III) hybrid system.Modified granular natural siderite was evaluated for adsorption characteristics bymeans of batch tests. Results showed that MGNS had higher adsorption rate andcapacity for As(III)/As(V) in comparison with natural siderite. Arsenic(III) and As(V)adsorption achieved equilibrium at24h and36h, respectively, with adsorptioncapacity estimated from Langmuir isotherm at25℃of2.66mg/g and2.19mg/g,respectively. Thermodynamic studies indicated that As(III)/As(V) adsorption onMGNS was spontaneous, favorable, and endothermic. In addition, MGNS adsorbedAs(III)/As(V) efficiently in a relatively wide pH. Effects of dissolved organic matter,Ca/Mg cations, background electrolyte, and coexisting anions were not significantwithin the concentration ranges observed in high As groundwater.Column study of As adsorption on MGNS demonstrated that MGNS-packedcolumn reactor removed As(III) more efficiently than As(V). A mass balancecalculation indicated that total As load in any As(III)-treated columns was around1.2mg/g before the Chinese drinking water standard in rural areas of50μg/L As was broken through. Field pilot test indicated that removal efficiency on MGNS from realhigh-As groundwater was relatively lower compared to As(III)-spiked DI water, eventhough As(III) was the dominant As species in the natural groundwater. The TCLP(toxicity characteristic leaching procedure) results suggest that the spent adsorbentsgenerated in the column filters were inert and could be landfilled.Results of studies on As removal mechanism indicated that adsorption andheterogeneous oxidation of As(III) were believed to be the main mechanism of As(III)removal by MGNS, and As(V) should be predominantly adsorbed on MGNS viainner-sphere bidendate binuclear surface complexes, as a result of the ligand exchangereactions between surface OH or CO3groups and solution arsenate anions.