WASTEWATER-INDUCED HEAVY METAL ACCUMULATION IN OKRA (ABELMOSCHUS ESCULENTUS L.) A COMPREHENSIVE REVIEW OF UPTAKE DYNAMICS AND HEALTH RISKS

Abstract

Scarcity of fresh water around the world has motivated peri urban farmers to use untreated or partially treated waste water in irrigating vegetable crops such as okra. Nevertheless, toxic heavy metals (cadmium, lead, and chromium) are present in wastewater and accumulate in plant tissues and pose severe threats to human health. This review synthesizes the peer reviewed literature on the dynamics of uptake, distribution pattern of tissue, translocation mechanism and associated health risks of heavy metals in wastewater irrigated okra. The major findings show that roots do accumulate 60 to 85 percent of the total metal load, and fruits do contain lower but often unsafe levels of 0.5 to 3.0 milligrams per kilogram dry weight of cadmium and 1.0 to 8.0 milligrams per kilogram of lead. Cadmium is highly mobile with translocation factors ranging between 1.2 and 2.5, whilst lead is mostly immobile with translocation factors less than 0.5. Health risk assessment indicates that for children, it is more than 75 percent of studies on cadmium and 60 percent on lead, the health risk index is above safe values. The level of cadmium carcinogenic risk commonly surpasses the accepted level of 10-4 in okra in the peri urban areas. Some of the effective mitigation strategies involve waste water pretreatment using constructed wetlands that remove 35 to 60 percent of the metals, soil amendments such as biochar and lime that reduce the metals in fruit by 25 to 60 percent, and adoption of low accumulating cultivar. Boiling causes a reduction of 15-35 percent in the quantity of metals but does not eradicate danger. This review concludes that wastewater irrigated okra often fails to comply with the food safety regulations and presents a significant non carcinogenic and carcinogenic risk especially to children.