Air-drying and liming effects on exchangeable cadmium mobilization in contaminated soils: A repeated batch extraction study

Air-drying of soil samples is known to increase the exchangeable fraction of cadmium (Cd) in soil. Liming, on the other hand, reduces bioavailability of heavy metals in soil by metal hydrolysis reactions and/or coprecipitation with carbonate. However, due attention has not been paid to desorption and mobilization of Cd from air-dried and limed soils at realistic ionic strengths. We compared Cd desorption from moist and air-dried samples of two texture-contrasting soils which had been spiked with 3 or 30 mg-Cd kg− 1 after liming with 25 mmol kg− 1 of Ca(OH)2, or without prior liming. Repeated batch extraction with water and 1–10 mM CaCl2 solutions was carried out, and chemical speciation of dissolved Cd performed using Visual MINTEQ. Desorption of Cd from the exchangeable pool in the soils was not exhaustive, with smaller fractions of Cd remaining after five extractions in the coarse-textured Chiba soil having lower CEC than the fine-textured Takada soil. Air-drying enhanced Cd desorption from the both soils particularly in the first extraction, with the cumulative Cd desorption after five extractions 1.5 times, on average, as large as that from the unlimed moist soils. Liming suppressed the Cd release, resulting in cumulative desorptions 0.14–0.19 times those from the unlimed counterparts. The air-drying and liming effects on Cd desorption were reflected in the apparent Cd2+/Ca2+ exchange selectivity coefficient, which was lowest in the first extraction upon air-drying, and significantly increased upon liming. Alternatively, assuming that constant Cd2+/Ca2+ exchange selectivities obtained from the moist unlimed soils apply, exchangeable Cd pools in the air-dried and limed soils were evaluated. The exchangeable pool was found to increase, on average, by 26% and 30% upon air-drying, and decrease by 79% and 86% upon liming in the Takada and Chiba soils, respectively. These results demonstrate that the size of exchangeable Cd in contaminated soils is prone to change upon drying and liming, and that the potentially desorbable Cd is only slowly mobilized under ionic strength conditions prevailing in the field.

Research Highlights
► A new method was developed for evaluating exchangeable Cd pool in soil.
► Air-drying enhanced Cd desorption from soils, particularly in the first extraction.
► The size of exchangeable Cd pool is prone to change upon drying and liming.
► Potentially desorbable Cd is only slowly mobilized.