Multi-mode soil chemical passivation and crop protection of severe cadmium and arsenic polluted soils with engineered silica

Abstract

Large tracts of agricultural land are enriched in toxic trace elements (TTE), particularly cadmium and arsenic. Functionalized mesoporous silica (FMS) is used extensively as an advanced process and waste-stream management tool for TTE removal in the chemical industries. Their adoption in agriculture though is extremely limited, encompassing only a narrow selection of FMS materials and crop/soil types. Understanding the function of FMS in diverse and relevant agri-settings is a priority. Not only in terms of their ability to immobilize TTE, but also in relation to the uncharacterized risks they pose to the soil's supply of essential nutrients, concurrent plant ionome responses and crop performance. Here, a series of plant mesocosm experiments were conducted on seriously degraded soils from zinc smelting operations. Two different crops, Oryza sativa and Brassica rapa with opposing redox managements were studied to understand the mechanisms of the FMS–soil–plant interactions. Companion FMS-incubations were undertaken on urban-industry impacted and mining-accident/“cancer-village” soils, to test performance across a wider range of contamination scenarios. In addition to the multi-functionality of FMS for targeted TTE immobilization along with the chemical inertia for needed plant nutrients, its abilities as a vector for plant growth/soil remediation stimulants was also investigated. Growth/protection enhancers were preloaded on FMS and then trials were performed to characterize their release. The plant mesocosm experiment demonstrated FMS can effectively immobilize ∼36% of the total Cd and ∼37% of the bioavailable Cd in soil into a highly recalcitrant/plant unavailable fraction. This significantly reduced in planta Cd accumulation by >80% across contrasting soil redox scenarios. Bioavailability of Cd and As decreased simultaneously by 98% and 57%, in companion soil incubations. Finally, FMS successfully accumulated and released dosed agri-chemicals in solution-based experiments. These findings establish FMS as a multi-functional soil amendment, offering a novel and integrated solution for complex agricultural soil issues.