Application of Biochar in Reducing the Levels of Pesticide Residues in Contaminated Soils and Food Crops. A Strategy to Minimize the Health Risk

Abstract

Organochloride pesticides (OCPs) also known as ―endocrine disrupting‖ chemicals are one of the persistent group of contaminants in soil that has gained worldwide concerns over the past few decades. They can transferred into the vegetables grown on the contaminated soils and eventually entered the food chain. The associated risk to human health makes the remediation of pesticide-contaminated soil a crucial undertaking. This research work is divided into different parts and several experiments were conducted to achieve the proposed objectives. The first study was carried out to evaluate the concentrations of OCPs in soils and vegetables grown in selected sites of Khyber Pakhtunkhwa (KP), Pakistan. The soil and selected vegetable species including Spinacia oleracea, Raphanus sativus, Brassica rapa, Lactuca sativa, Allium cepa and Allium sativum were collected from six districts (Peshawar, Charsadda, Nowshera, Mardan, Swabi and Swat), and were analysed for selected OCPs. Results showed that the concentrations of the selected OCPs exceeded their respective maximum residue limits (MRLs) in most of the soil samples and showed great variation from site to site. In case of the studied vegetables, the most contaminated observed was Lactuca sativa (28.9 μg kg-1), followed by Raphanus sativus (26.6 μg kg-1), Spinacia oleracea (25.8 μg kg-1), Allium cepa (16.3 μg kg-1), Brassica rapa (15.6 μg kg-1), and Allium sativum (14.7 μg kg-1). Analysis of health risk revealed that incremental lifetime cancer risks (ILCR) associated with dietary exposure of Σ30OCP through vegetables consumption by adults and children were below the acceptable risk level (10-6), indicating no cancer risk to the consumers. Moreover, exposure to the endocrine disruptor and probable carcinogen heptachlor epoxide (in children only) contributes toward non-cancer risk (HQ>1) from ingestion of selected vegetables. The presence of the banned OCPs in soils and vegetables in xvi the study area indicates that these legacy chemicals are still being used illegally for agricultural purposes which may be a cause for concern. Similarly, the second study was focused on the remediation of OCP-contaminated soils, as most of the OCPs are known to be possible/suspected endocrine disruptors and carcinogenic in nature and may severely affect humans or other living biota. Therefore, a 65-d incubation experiment was conducted to investigate the efficacy of biochars on the accessibility of OCPs, and also to evaluate their influence on soil microbial community. Results indicated that despite greater persistence of OCPs in soil, the application of selected biochars significantly (p<0.01) reduced the accessibility of OCPs in contaminated soil as compared to the control soil during 65-days incubation period. Moreover, the findings from total phospholipid acid (PLFA) and Illumina next-generation sequencing revealed that the incorporation of biochar have altered the soil microbial community structure over time. Higher abundances of Proteobacteria, Firmicutes, Gemmatimonadetes, and Actinobacteria, were found in the biochar amendments. This recommends that biochar amendment (3%) could be advantageous for sustaining soil microbial communities and may thus likely influence the eco-toxicological processes and potential accumulation of OCPs. The third study was conducted to evaluate the effects of biochar (BC), biochar-based organic fertilizer (OF) and their combination (BCOF) on the bioaccessibility and accumulation of endocrine disruptive pesticides (EDPs) into leafy vegetables (Lactuca sativa and Spinacia oleracea) grown in a contaminated soil. The results revealed that application of BCOF and OF significantly (p<0.05) decreased bioaccessible concentrations of EDPs in leafy vegetables by 23 and 44%, respectively, BC was particularly effective and decreased EDPs bio-accessible concentrations by 60%, as compared to control. Biomass production, however, xvii increased the most for the BCOF amendments, (with a 70% increase in crop production compared to the control soil). Moreover, the incremental lifetime cancer risk (ILCR) values for selected EDPs were significantly (p<0.01) reduced in the amended soil as compared to control. These findings suggest that OF, BCOF and BC could be useful soil amendments to minimize the EDPs bioaccessibility in a degraded soil and their subsequent exposure to humans through vegetable consumption. Likewise, to investigate the effects of different biochars on the bioaccessible fractions of OCPs in a contaminated soil and their subsequent bioaccumulation into vegetable crops a fourth study was conducted. Four different biochars, derived from sewage sludge, soybean straw, rice straw and peanut shells were applied to a pesticide-contaminated soil at 2%, and the resulting impacts were measured. The results indicated that following biochar additions, the bioaccessible fractions of OCPs in soil and their accumulation in vegetable crops significantly (p<0.01) reduced, as compared to control. Risk assessment showed that biochar amendments markedly (p<0.01) decreased the hazard quotient (HQ) indices and the incremental lifetime cancer (ILCR) values for OCPs associated with the consumption of vegetable crops. From the results of the present studies it is concluded that the application of biochar could play a promising role in enhancing microbial abundance, reducing accessibility, restricting bioaccumulation of organochlorine pesticides in vegetables and decreasing their associated human health risk.