Heavy Metal Contents of Vegetables Grown in Soil, Irrigated with Mixtures of Wastewater and Sewage Sludge in Pakistan, using Ultrasonic‐Assisted Pseudo‐digestion

Abstract

AbstractThe success of risk assessment of metal‐contaminated soils depends on how precisely one can predict the bio‐availability of trace and toxic metals in soil and transfer to the human food chain. A field experiment was carried out from 2004 to 2005 to evaluate the long‐term effect of sewage sludge application in agricultural lands where mostly vegetables are grown. The aim of this study was to predict the uptake of Cd, Cu, Cr, Ni, Pb and Zn by different vegetables grown on agricultural soil irrigated for long period with untreated domestic waste water and sewage sludge (SIDWS) as test samples and for comparative purposes, same vegetables grown on agricultural soil irrigated with fresh canal water (SIFW), collected as control samples. A sample preparation method based on ultrasound‐assisted pseudo‐digestion (UASD) of soil, sewage sludge and vegetable samples in ultrasonic bath was developed. A conventional wet acid digestion method was used for comparison purposes. The EDTA‐extractable metal in SIFW and SIDWS was also studied. The extracts and digests were analysed by flame atomic absorption spectrometer/electrothermal atomic absorption spectrometer (FAAS/ETAAS). Analytical results for six metals and metalloid by UASD in bath and conventional wet digestion methods showed a good agreement, thus indicating the possibility of using low mass and minimum quantity of digesting acid mixture in less time when compared with the conventional digestion method (CDM). The validation of the proposed UASD method was assessed by using certified reference materials BCR 483 and (whole meal flour) BCR 189. Recoveries ranging from 95.5 % to 102.3 % for understudied metals were obtained for different samples (soils and vegetables). The average relative standard deviation of UASD method varied between 2.3 % and 7.9 % for N = 12, depending on the analyte. The vegetables grown in SIDWS showed high level of heavy metals when compared with control samples. This investigation highlights the increased danger of growing vegetables in the agricultural land, continuously irrigated and dressed with waste water and solid sewage sludge.