Effects Of Colloids On Iodine Enrichment In Groundwater Of The Datong Basin,China

Iodine is an essential element for human metabolism.Excessive iodine intake may lead to the risk of thyroid enlargement and hyperthyroidism,increasing the risk of autoimmune thyroid diseases,iodine allergies and poisoning,and intelligence loss.Many studies have been focused on the radioactive isotope of iodine due to large anthropogenic induce of 129 I from nuclear tests.And previous reports generally worked on iodine behavior in the coastal areas because of the major marine source.In this study,logistic regression model and colloids(ultra)filtration technique were conducted to illustrate the iodine behavior and the major factors impacting iodine transport in groundwater system of Datong Basin.The iodine concentration in groundwater at Datong Basin reached up to 1212 ?g/L in this study.Elevated iodine groundwater only existed in the center of the basin.Natural high iodine groundwater(>400?g/L)primarily occurred in shallow aquifer with depths less than 20 m and deep aquifer with depths between 70 and 90 m.Sharp increase in iodine concentration with Ec variation implied that iodine enrichment in shallow groundwater might be controlled by evaporation or vertical infiltration process.SO42-and NO3-could act as the electron acceptors and be used to support native microorganism for the oxidation of organic matter.Organic matter,considered as the source of iodine,could undergo the degradation process caused by microbial activities,resulting in iodine release and enriched in groundwater with high HCO3-concentration.Total iodine contents in the sediments from the borehole variaed from 0.18 to 1.46 mg/kg and elevated iodine concentration was found at the depth of 15.2 and 90 m,which was basically consistent with the iodine distribution in groundwater.The organic matter enriched clay sediments was the dominant iodine sink in groundwater because of the preferential adsorption of iodine onto organic matter,which was inferred by the positive correlations of iodine contents with TOC and LOI.The negative correlation of iodine and pH in the sediments demonstrated the pH-dependent iodine desorption process in the high iodine groundwater.The two end-member model of groundwater ?2H and ?18O values and evidence of Cl/Br molar ratios jointly implied that large-scale irrigation activities in the basin lead to the the vertical infiltration process from the upper surface water to groundwater.The vertical recharging process imported considerable fresh organic matter into shallow groundwater,which was favorable for the enrichment of iodine in groundwater result from degradation of organic matter.The correlations of iodine contents with TOC and LOI in sediments demonstrated that the organic-rich sediments acted as the primary iodine source in the groundwater system due to the preferential adsorption of iodine onto organic matter.The negative correlation of iodine and pH in the sediments demonstrated the pH-dependent iodine desorption process in the high iodine groundwater.The logistic regression model was performed to estimate correlations between hydrogeochemical parameters and measured groundwater iodine content in Datong Basin.10 commonly analyzed parameters during groundwater quality surveys were selected as explanatory,and well depth,pH,Eh and the contents of Cl-,HCO3-,NH4+ were remained as statistically significant variables in the final model.The logistic regression analysis suggested the contribution of Cl-variable on the model,reflecting the effects of vertical irrigation activities on iodine enrichment in groundwater.The evidence of Cl/Br molar ratios and two end-member model of groundwater ?2H and ?18O values jointly indicated the vertical mixing process between the upper surface water and groundwater because of the large-scale irrigation activities in the basin.The vertical recharging process imported considerable fresh organic matter into shallow groundwater,and promoted organic matter degradation process,which was favorable for the enrichment of iodine in groundwater.The correlations of iodine contents with TOC and LOI in sediments demonstrated that the organic-rich sediments acted as the primary iodine source in the groundwater system due to the preferential adsorption of iodine onto organic matter.The variable HCO3-acted as the most important role on the model due to the effects of organic matter biodegradation on iodine release into groundwater of Datong Basin.A good performance was found between groundwater iodine contamination and calculated probabilities.The similar methodology using logistic regression could also be proposed to develop a new model with different hydrogeochemical factors in other study areas under similar conditions.A 400-ml polysulfone cup was used for ultrafiltrations.After successive ultrafiltration through nominal molecular weight,three kinds of fractions could be obtained,i.e.large grain size colloids(fractions between 0.45 ?m and 30 kDa),small grain size colloids(fractions between 30 kDa and 5 kDa)and truly dissolved phases(fractions below 5 kDa).More than 77.8% of total iodine was trapped in the truly dissolved phases.Fe,DOC and IT concentrations decrease distinctly in the fractions with grain size decreasing.Redox conditions played an important role on iodine distribution in both colloids and truly dissolved phases.Under anaerobic conditions,colloidal iodine released into groundwater from sediments due to the reductive dissolution of iron oxyhydroxides and degradation of organic iodine triggered by microbial processes,which was confirmed by the EEMs–PARAFAC model.High pH promoted iodine desorption from colloids because of OH-competitive desorption and surface electrostatic repulsion.NOM primarily controlled the formation of Fe colloids and iodine distribution in colloids.Since iodide was the predominant iodine species in the groundwater and had lower affinity to colloids,the alkaline conditions of the groundwater easily drove most iodine get into the truly dissolved phases.Organic iodine(OI)concentration in the groundwater of Datong Basin ranged from 3.82 ?g/L to 289 ?g/L,with a median value of 68.7 ?g/L.Organic iodine concentration increased from the recharge area to the discharge area.OI,Fe and DOC concentrations decreased in the fractions with grain size decreasing.Redox potential affected organic iodine distribution in small grain size colloids and truly dissolved phases.The prevalent anaerobic conditions facilitated colloidal iodine to release into groundwater from sediments.Elevated pH in groundwater strengthened the sorption capacity of Fe colloids for organic iodine and weaken the affinity of iodine on NOM colloids.Significant correlations of DOC with HCO3-and OI with HCO3-suggested the important roles of microbial activities on organic iodine mobilization in groundwater.The humic substances might be readily dispersible under some reducing conditions and probably release iodine into groundwater in the organic-colloidal form.