Effect Of Low-Molecular-Weight Organic Acids On The Mobilization Of Cadmium In Different Forms

Low-molecular-weight organic acids are widely present in all types of soil at micromolar concentrations due to plant root secretion,synthesis by microbial and decomposition of animal or plant residues.Effect of low-molecular-weight organic acids on the mobilization of cadmium in soil has been intensively investigated.But the effect of low-molecular-weight organic acids at rhizophere concentrations on desorption and speciation of heavy metals in soil has been rarely reported.In addition,stabilizing is the often applied to remediate heavy-metal contaminated soil.Research on effect of low-molecular-weight organic acids on Cd bound to goethite,montmorillonite and biochar can be used to evaluate the performance of these three kinds of stabilizers and provide technical assessment on their feasibility for soil remediation.The effect of low-molecular-weight organic acids on Cd and Pb desorption and morphological changes of a garden soil was studied in detail.In addition,goethite,montmorillonite and biochar are prepared to adsorb Cd;then three materials contained stabilizing bound cadmium are subject to MgCl₂ elution to removal mobile Cd on the surface of the materials.XPS was conducted to find the way Cd bound in goethite.Finally,six different low-molecular-weight organic acids are selected to study the effect of Cd desorption from the three kinds materials.The results showed that:?1?The impact of low-molecular-weight organic acids toward Cd and Pb desorption wasminor and the desorption concentrations are all in?g/kg level;but portion of carbonate bound Cd and Pb was converted to exchangeable fraction.?2?Goethite,montmorillonite and biochar havegood adsorption capability toward Cd.The adsorption capacity can reach 3.58,3.64,4.07 mg/g for these sorbents respectively.There are 2.52,0.93and1.04 mg/g Cd in goethite after elution by MgCl2.The XPS results show that Cd can be adsorbed on goethite thorough formation of Cd-O-Fe.?3?Six kinds of low-molecular-weight organic acids all have the ability to monilize Cd on goethite,montmorillonite and biochar and show the same effecttoward Cd mobilization sorbed on the three studied sorbents.For each organic acid,the ability to mobilize Cd sorbed on the sorbents follows the order,follow the order:citric acid>oxalic acid>malic acid?tartaric acid?maleic acid?succinic acid.The maximum activation ratio and unit molar ratio of citric to goethite,montmorillonite and biochar were 4.2%,11.0%5.6 and 4.3,4.1,2.3 mg/mmol,respectively.In conclusion,risk of the heavy metals in soil can be enhanced by low-molecular-weight organic acids after remediation by the amendments,which heights the inportant role of low-molecular-weight organic acids in controlling heavy metal mobilization.In addition,it may be useful to add organic acid to soil to mobilize heavy metals in order to enhance heavy metal remvoal in phytoremediation process.