Effects Of Parent Material And Dissolved Organic Matter On Distribution And Transformation Of Soluble Cadmium And Lead In Agricultural Soils In The Lower Pearl River Catchment

The bioavailable fraction of a heavy metal in soil can be easily accumulated in plants.At present,understanding on the evolutionary process and the mechanism of regional soil environmental quality based on the survey of heavy metal are still limited.Therefore,soil properties and compositions,such as pH,organic matter,and iron oxides influencing heavy metal activity are of interest to researchers.In this study,a total of 142 typical contaminated agricultural soil samples representing 5 parent materials?limestone,sand shale,diluvium,alluvium,and granite?in the low Pearl River System,China were collected to investigate the factors influencing the speciation and distribution of labile Cd and Pb in the soils.Then the contaminated soils that can be easily affected by dissolved organic matter?DOM?were selected.Low-molecular-weight DOC?i.e.,Gly?was added into soil to study the effect of Gly degradation on the nitrogen cycle through simulation experiments.The role of Gly degradation and transformation processes in the change of heavy metals bioavailability in soil was tentatively elucidated.Last but not least,an alkaline carbonate–based adsorbent(Fe³⁺-modified argillaceous limestone,FAL)were developed.The feasibility of applying FAL on the competitive adsorption of heavy metals(i.e.,Cd²⁺,Pb²⁺and Ni²⁺)was evaluated.The interaction between FAL and heavy metals in the presence of a high-molecular-weight DOC?i.e.,HA?and a low-molecular-weight DOC?i.e.,Gly?was explored based on XPS analysis together with XRD and EDS analysis.The current results may broaden the way of using argillaceous limestone,a naturally abundant but not yet fully explored material,as an cost-effective option of heavy metal adsorbent in the remediation practice.The main results and conclusions are as follows:?1?The soil developed from typical parent materials in the low Pearl River System was investigated.The results showed that the ratio of dissolved Cd ions to total Cd is higher than that of dissolved Pb ions to total Pb,and the labile fractions were linearly correlated to their corresponding total contents.The step regression analyses suggested that the key factors impacting labile Cd and Pb varied in different parent material soils.Pb activity was highly sensitive to pH in alkaline limestone soils.The quartz sand remained in granite-produced soils could enhance Cd activity.And DOM compositions considerably influenced Cd and Pb activities in sand shale,diluvium,and alluvium soils.Land use impacts heavy metal activities.The content of labile Cd and Pb in paddy soils were higher than those in non-paddy soils,although total metals in the soils were comparable.It could be ascribed to the long-term equilibrium of metals between the solution and solid phases of the paddy soils.Therefore,pH and dissolved organic matter are the main factors affecting the bioavailability of two heavy metals.The present results would provide a theoretical basis for preliminary prediction of heavy metal activity and provide a technical support for heavy metal activity management and pollution control based on soil parent materials.?2?Based on the results of the above study,the contaminated soil developed by diluvium was selected as the tested soil.The effect of glycine?Gly?degradation on the bioavailability of soil Cd/Pb was studied by the addition of glycine.The effects of its degradation process on soil nitrogen cycling and Cd/Pb bioavailability were investigated.The results showed that soil microbial communities may also utilize free Gly.The rapid cycling of low molecular weight DOM?i.e.,Gly?promoted faster conversion of Gly to ammonium nitrogen,while the conversion of ammonium nitrogen pools to nitrate nitrogen pools was slower.Gly formed complexes with Cd²⁺in the soil and stimulated CO₂ emissions from the soil.Gly can rapidly remove CaCl₂-Cd/Pb and inhibit the migration of Cd/Pb in the soil.These results indicate that the complexation process and degradation process between soluble low molecular weight organic ligands and heavy metal ions can significantly affect the migration and activity of heavy metals.?3?The feasibility of applying FAL on the competitive adsorption of heavy metals was evaluated.The current results revealed an efficient adsorption on Cd²⁺,Pb²⁺and Ni²⁺in mono-metal system.Further experiments demonstrated a high selectivity of Pb²⁺during the competitive adsorption of Cd²⁺,Pb²⁺and Ni²⁺.The adsorption selectivity of the metal ions followed the order of Pb>>Cd>Ni.XPS analysis together with XRD and EDS analysis revealed that the metal adsorptions were mainly regulated via the synergistic mechanisms of ion exchange by Na⁺,Ca²⁺,and Al³⁺,precipitation to form CdCO₃ and Pb₂?OH?₂?CO₃?₂,as well as complexes of FAL-OPb and FAL-ONi by hydroxyl groups on the surface of FAL.In addition,it is interestingly that various DOMs?i.e.,humic acid?HA?and glycine?Gly??exerted different effects on the adsorption behaviors,probably due to the different affinities for Pb²⁺,Cd²⁺,and Ni²⁺and the redistribution of newly-formed metal-DOM complexes.The application of FAL would be a promising option in leading to an efficient heavy metal removal.