Mobility Of Arsenic And Heavy Metals In Typical Sewage-irrigated Soils In The North China Plain

Arsenic and heavy metals pose a serious threat to ecology,environment and human health,due to their high toxicity and accumulation in living organisms.Arsenic and heavy metal pollution in soils caused by anthropogenic activities,such as farming and mining,has been frequently reported worldwide and has become one of the major environmental issues which the international communities have to face.The national survey shows that the build-up of heavy metals/metalloids in agriculture soils introduced by sewage irrigation is a widespread phenomenon in China,which is endangering the securities of cultivated land and food.The bioavailability of arsenic and heavy metals is influenced by elemental forms and mobilities,and therefore discovering the main mechanisms that controls arsenic and heavy metal mobilization has become hotspots of the research on soil and underground environment.This study revealed the forms and mobility of arsenic and heavy metal mobilization in soils from a typical sewage-irrigated area in the North China Plain via field investigation,laboratory experiment and numerical simulation.The main findings are shown as follows.(1)The enrichment degrees of arsenic and heavy metals in the surface layer of sewage-irrigated soils(SIS)were significantly higher than those in control soils,and the contents of soil Zn,Cu,Pb and As exceeded the risk control standard for soil contamination of agricultural land in China.Moreover,the higher relative contents of arsenic and heavy metals associated with ionically bound,strongly adsorbed and very amorphous fractions were found in SIS,indicating that sewage irrigation exerted great influence on forms of soil arsenic and heavy metals and increased the risks on soil and groundwater contamination.According to the monitoring data of shallow groundwater in the study area,the detection rate of arsenic was much higher than heavy metals,with the concentration ranged from 0.37-9.65 μg/L.(2)Results of column experiments demonstrated that acid rain infiltration was better at releasing soil Cr and Pb than near-neutral p H influents(rainfall and compost leachate),and proton-promoted dissolution of very amorphous Fe oxides was the major cause for mobilized Cr and Pb in porewater.Arsenic and Cu mobility in soils were enhanced by compost leachate infiltration,which resulted from reductive dissolution,complexation and desorption.(3)Concentrations of porewater Cu,Pb,Fe and Mn in the column infiltrated by compost leachate exhibited a visible increase after elevating the water level,while no marked variations in concentrations of porewater arsenic and heavy metals were monitored under leaching of acid rain.Exogenous dissolved organic matters(DOM)introduced by compost leachate acted as electron donors and shuttles to accelerate the metabolism of indigenous iron reducing bacteria like Geobacter in the anoxic condition,causing dissimilatory reduction of Fe oxides and therefore release of arsenic and heavy metals into porewater.Due to the lack of bioactive DOM,the reductive dissolution of Fe oxides was not triggered in the column infiltrated by acid rain.(4)The adsorption isotherms of studied soils for As(V)fitted well with nonlinear models,and maximum As(V)adsorption capacities,calculated by Langmuir model,ranged between 0.335 and 0.691 μmol/g.Arsenic(V)adsorption capacities decreased with the increase in p H values.Phosphate strongly competed for adsorption sites with As(V),while bicarbonate showed a relatively weak performance in competing for surface sites.Reactive transport model calculations demonstrated that desorption was the main controlling mechanism for As enrichment in porewater at the early period.As the infiltration continued,the relative contribution of desorption to As enrichment in porewater was gradually decreased,whereas the relative contribution of reductive dissolution and complexation became bigger and bigger.