Impact And Control Mechanisms Of The Geochemical Environment On The Ageing Of Arsenic In Vadose Zone

Aging of arsenic in vadose zone is a process that after the entrance of exogenous arsenic into vadose zone, mobile fractions such as water soluble fraction and exchangeable fraction transform into relatively immobilized fractions with increasing time which leads to the decrease of activity and bioavailability of arsenic in vadose zone. The current ecological and groundwater contamination risk assessments of soil in the world are based on the ecotoxicological data on the experimental conditions of newly added arsenic, the long-term effects of aging process for arsenic in vadose zone is neglected, and the ecological and groundwater contamination risk of arsenic in vadose zone is often overestimated. Therefore, the recognition and prediction of aging process for arsenic in vadose zone is very important for the standards establishment of ecological and groundwater contamination risk assessments. In addition, understand the aging process of arsenic in vadose zone is also beneficial to improving the current chemical immobilization remediation technology about arsenic contamination in vadose zone, reducing the remediation and treatment cost, and maintaining the original physical and chemical properties of vadose zone.The improved sequential extraction technique and indoor cultivation experiment were applied in this study, and the main objectives of this study were to investigate:1) the exogenous arsenic aging process in vadose zone and the effects of added iron powder and humic acid to arsenic aging in vadose zone; 2) the effects of particle components and contamination load to exogenous arsenic aging process in vadose zone; 3) the effects of temperature and redox conditions to exogenous arsenic aging process in vadose zone. The major objective was to reveal the influence of related geochemical environmental factors to arsenic aging in vadose zone and its control mechanism. The main results are as follows:1) Since the entrance of exogenous arsenite and arsenate into vadose zone, the aging effect continued throughout the whole experiment process. The arsenic aging process can be divided into three phases. There was no significant effect of added humic acid to exogenous arsenite and arsenate aging process in vadose zone, while the added iron powder could significantly shortened the exogenous arsenite and arsenate aging process in vadose zone.2) The added humic acid hardly affected the exogenous arsenite and arsenate aging degree in vadose zone, while the added iron powder sharply increased the exogenous arsenite and arsenate aging degree. The finer particle size, the higher temperature and the lower contamination burden are accompanied by the higher aging degree of exogenous arsenite in vadose zone. The more oxidation the environment trended to, the higher the aging degree of exogenous arsenite and arsenatc became.3) The aging effect decreased the groundwater contamination and ecological risk of exogenous arsenite and arsenate under kinds of conditions. Humic acid slightly decreased the groundwater contamination risk of exogenous arsenate in vadose zone, while there was basically no effect on exogenous arsenite. The added iron powder sharply decreased the groundwater contamination risk of exogenous arsenite and arsenate in vadose zone.4) The finer particle size, the higher temperature and the lower contamination burden are accompanied by the lower groundwater contamination risk of exogenous arsenite in vadose zone. The more oxidation the environment trended to, the lower the groundwater contamination risk of exogenous arsenite and arsenate in vadose zone became.5) In terms of the changes of ecological risk over time, the lower contamination burden and the higher temperature cause the shorter aging process of exogenous arsenite in vadose zone. The more oxidation the environment trended to, the longer the aging process of exogenous arsenate in vadose zone became.