Effects And Mechanisms Of Organic Carbon And Inorganic Carbon On Arsenic Removal By Modified Granular Natural Siderite

Groundwater arsenic?As?contamination is a hot issue of concern worldwide.High arsenic groundwater is usually characterized by high concentrations of dissolved organic carbon and inorganic carbon.Modified granular natural siderite?MGNS?,a special hybrid Fe?II?/Fe?III?system,had higher adsorption capacity for As?III?than As?V?.Therefore,it is important to study effects and mechanisms of organic carbon and inorganic carbon on arsenic removal by MGNS.?NOM-MGNS?-As and?NOM-As?V??-MGNS were used to investigate effects of adsorbed and dissolved NOM on As removal by MGNS,respectively.Results showed that no significant effects of both adsorbed and dissolved HA/FA on As?III?adsorption.However,As?V?removal was inhibited.The inhibition order of As?V?adsorption is?HA-MGNS?-As?V?>?FA-MGNS?-As?V?>?FA-As?V??-MGNS >?HA-As?V??)-MGNS.The adsorbed NOM had more inhibitory effect on As?V?adsorption than dissolved NOM.Steric Exclusion Chromatography-HPLC?SEC-HPLC?and High-Performance Size Exclusion Chromatography-UV-Inductively Coupled Plasma Mass Spectrometry?HPSEC-UV-ICP-MS?were used to reveal the mechanism of the inhinitory effect of NOM on As?V?removal by MGNS.In the?NOM-As?V??-MGNS system,the inhibitory effect of dissolved FA and HA on As?V?was mainly through competitive adsorption,although the formation of macromolecular complex lowed the adsorption kinetic rate.Because the electrostatic repulsion between dissolved FA and As?V?was stronger,the inhibitory effect of dissolved FA on As?V?adsorption was greater than that of dissolved HA.In the?NOM-MGNS?-As?V?system,HA with greater molecular weight occupies more adsorption sites in relative to FA.Thus,adsorbed HA inhibited As?V?adsorption more strongly than adsorbed FA.Secondly,effects of pH,HCO₃^-and intermittent treatment on As?III?removal were studied by column tests.Results showed that the adsorption capacity of As?III?decreased with the increase of influent pH.The inhibition of HCO₃^-on As?III?removal was stronger than pH at the same conditions.Intermittent treatment enhanced As?III?removal.As species,iron mineral species,SEM and XRD patterns of As-loading MGNS showed that the important process contributing to high As?III?removal was the mineral transformation from siderite to goethite in the column filters.In addition,only As?V?was found in the effluent solution,and As?V?was found to be the main As species on used MGNS.More As?III?was oxidized to As?V?in the intermittent treatment,indicating that oxidation of As?III?is also an important factor affecting its removal.Moreover,the amount of hydroxyl radicals?·OH?produced by MGNS was greater than that of natural siderite.Effects of Cl^-,NO₃^-and SO₄^2-on the ·OH produced by MGNS were very weak.However,HCO₃^-greatly inhibited the generation of ·OH by MGNS.When HCO₃^-concentrations in solutions were greater than 50 mM,the amount of ·OH produced by MGNS was less than the detection limit.In addition,SiO₃^2-and PO₄^3-inhinited ·OH production,which were weaker than HCO₃^-.The presence of Ca²⁺/Mg²⁺ had a slight increase in the amount of ·OH produced by MGNS.Therefore,in the process of adsorption of As?III?by MGNS,the inhibition of HCO₃^-on As?III?removal was connected to inhibition of siderite dissolution,As?III?oxidation and mineral transformation.