Evaluating cadmium bioavailability in contaminated rice paddy soils and assessing potential for contaminant immobilisation with biochar

Abstract

Cadmium (Cd) contaminated soils from the Mae Sot district in northwest Thailand, a region in which rice Cd concentrations often exceed health limits (0.4 mg/kg) set by the World Health Organisation, were examined for isotopically exchangeable Cd (Cd E values using a 111 Cd spike) to determine how this rates as a predictor of rice grain Cd in comparison with soil total Cd and solution extractable Cd (using the commonly applied BCR scheme and, in an attempt to distinguish carbonate bound forms, the Tessier soil sequential extraction scheme reagents). Step 1 of the BCR scheme (0.11 M CH3COOH) and step 1 of the Tessier scheme (1M MgCl2) showed the highest R 2 values in regressions with rice Cd (91% and 90%, respectively), but all predictors were strongly linked to rice Cd (p<0.001) and could be used for prediction purposes. One soil, of the six tested, was an exception to this, where all predictors overestimated grain Cd by a factor of 2.5 to 5.7, suggesting that rice grain Cd had been restricted here by the differing flooding regime and subsequent changes to redox conditions. E values and Tessier step 1 extractions were closely related, indicating that these measurements access similar pools of soil Cd. Separately, the isotopic exchangeability (representing bioavailability) of Cd was also assessed in two soils amended with rice husk and miscanthus biochars (0, 1, 5, 10, 15 and 20% w/w) in order to assess the utility of the biochars as a soil amendment for immobilising Cd in-situ. One soil showed significant reductions in Cd E value at 5% rice husk biochar addition and at 15% miscanthus biochar addition however, based on the E value-rice grain Cd regression relationship previously established, the E values in the amended soils still predicted for a rice Cd concentration above the health limit. In the second soil, neither of the biochars successfully reduced the Cd E value. This indicates that further work is needed to customise biochar properties to suit specific soil and contaminant situations if they are to be used successfully for remediation of metal contaminated soils.

Influence of Silicon and Selenium and Contribution of the Node to Cadmium Allocation and Toxicity in Rice

“…Rice feeds more than 50% of the world’s population . Cadmium (Cd) contamination of soil is a serious environmental and agricultural issue and widely occurs in paddy soil around the world. − Cd contamination of soil is widespread and usually caused by anthropogenic sources, including industrial smelting and mining, sewage sludge, application of Cd-containing pesticide and fertilizer, waste disposal, and vehicle exhaust . Soil Cd can readily accumulate and be transported in a soil–crop system because Cd is more mobile than other heavy metals and then potentially accumulate throughout the food chain, posing a public health risk .…”

Physical modification of biochar to expose the inner pores and their functional groups to enhance lead adsorption

“…Biochar has been used as an option to treat heavy metal contamination in soil and water. 1 Biochar is a carbon-rich product made by charring feedstocks, mainly from the biological by-products in the absence of air. The use of biochar as a biosorbent for the treatment of wastewater and soil contaminated with heavy metals is a better alternative to conventional high cost sorbents such as activated carbon.…”

Ecotoxicological assessments of biochar additions to soil employing earthworm species Eisenia fetida and Lumbricus terrestris

“…Nevertheless, the effects on chloride concentrations are important because it is well known that chloride ions, when in sufficient abundance, can increase the mobility of toxic elements such as Cd that may be present in the soil (Smolders and McLaughlin 1996 ). This will be a particularly important consideration to take into account for scenarios in which biochar is used to remediate contaminated soils (Beesley et al 2011 ; Kosolsaksakul et al 2018 ). The effect on salinity (and electrical conductivity) associated with increased chloride concentrations is also important to consider because this can impact the suitability of the soil environment for microbes, plants and soil fauna.…”

“…Before placement in the vessels, the soils had been moistened to 60% WHC. The treatments imposed on each soil were 5%, 10%, 15% and 20% biochar ( w / w ; n = 3), intended to reflect heavy biochar addition rates tested elsewhere (Li et al 2011 ; Major et al 2010 ), and that may be implemented in soil remediation and restoration efforts (Kosolsaksakul et al 2018 ).…”