Aging of exogenous arsenic in flooded paddy soils: Characteristics and predictive models

Understanding the arsenic (As) aging process is important for predicting the environmental behavior of exogenous As in paddy soils. In this work, samples of sixteen paddy soils with various soil properties were spiked with two concentrations (30 and 100 mg kg^-1) of arsenate and subjected to a 360 day-long incubation under continuous flooding condition. Soil available As extracted by 0.05 M NH₄H₂PO₄ was monitored through the aging process. Results showed that the available As%, the percentage of remaining available As in aged soils to added total As, fell from 44.2% to 41.9% on the 1st day to 22.0% and 23.0% on the 115th day for the low and high As spiked soils, respectively, then it remained basically unchanged after the 115th day. The pseudo-second order equation could adequately describe the aging kinetics of exogenous As in paddy soils. There was no significant difference in As aging parameters between the two spiked concentrations. Contents of soil free Al and Mn oxides, clay and cation exchange capacity strongly affected the aging rate of exogenous As. An empirical model, incorporating soil pH, cation exchange capacity, Olsen-P and flooding time, was developed to predict well the change of soil available As% during aging process (R² = 0.711). The model could be potentially utilized to manage As-contaminated paddy fields and normalize ecotoxicity and bioaccumulation datasets in attempt to derive more widely applicable soil environmental quality criteria for As.