Inhibition of radiocesium adsorption on 2:1 clay minerals under acidic soil environment: Effect of organic matter vs. hydroxy aluminum polymer

Abstract

Radiocesium (RCs) is selectively adsorbed on interlayer sites of weathered micaceous minerals, which primarily controls the mobility of RCs in the soil environment. Pedogenesis under acidic condition (under wetter climate regime) implies that RCs adsorption sites may be covered with soil organic matter (OM) and/or intercalated with hydroxy-Al polymer (Al(OH)x). While these interactions well-known control RCs adsorption, the relative importance of the two processes is virtually unstudied in field environment. We hypothesized that Al(OH)x intercalation plays dominant role in the adsorption inhibition in both surface and subsurface soils because Al(OH)x has more direct effect on RCs access to frayed edge site (FES), the sites intermediate between an expansible and non-expansible interlayers. Soils from two forest soil profiles under temperate and tropical climate (Ono, Japan and Kinabalu, Malaysia) were sampled every 3cm from 0 to 30cm and sieved to isolate soil particles of ≤20μm diameters for the analysis of radiocesium interception potential (RIP) after a series of pretreatment. One subset was treated with H2O2 to remove OM. Another subset was further treated with hot sodium citrate to remove Al(OH)x after the H2O2 treatment. The changes in specific surface area (SSA) by the N2-BET method before and after the OM removal were determined to assess OM coverage on soil mineral surfaces. The contributions of OM removal to the total increase in RIP were surprisingly high (74.5–93.8%) in the uppermost soil layers (0–3cm), implying that (i) OM reduced the access of RCs to FES, and (ii) OM binding with Al (organo‑aluminum complexation) reduced Al(OH)x formation in the interlayers. Towards deeper layers, OM contribution progressively declined to 2.4–13.9% whereas the Al(OH)x effect increased up to 86.1–97.6% in both profiles. The highest OM coverage on mineral surface (83.7%) was observed in the uppermost soil layer of Ono series. The stepwise OM removal of this sample by a weaker oxidizing reagent (NaOCl) for 6, 12, 24, and 30h led to a gradual decline in OC content from 145gkg−1 to 67.2gkg−1. The OC losses were accompanied by a slight but progressive liberation in SSA from 8.3 to 12.2m2kg−1, indicating that a portion of the soil mineral surfaces became exposed. However, RIP did not largely increase after the NaOCl treatment up to 30h but remained quite low relative to the RIP value after the more complete OM removal by H2O2. These results indicate that the fraction of OM relatively resistant to chemical oxidation (presumably strongly bound to reactive portions of mineral surface) appeared to contribute to the inhibition of RCs adsorption on 2:1 interlayer sites. In conclusion, the ability of weathered micaceous minerals to retain RCs was largely reduced by two different mechanisms: OM coverage on the mineral surface (esp. at surface) and Al(OH)x interlayering (esp. at subsurface soil layers). Our results imply that long-term RCs dynamics may be strongly controlled by soil carbon level and thus ecosystem carbon balance (e.g., forest and grassland vs. bare and cropland) as well as the abundance and/or weathering degree of micaceous minerals.