UPTAKE OF CADMIUM, COPPER AND ZINC IN SELECTED PLANTS COMMONLY GROWN IN KENYA FROM POST-METHANATION DISTILLERY EFFLUENT
Pot experiments were carried out on a field plot in University of Eldoret (UoE) to study uptake of selected metal contaminants from distillery effluent in four plants commonly grown in Kenya: Ipomoea batatas, Eucalyptus grandis, Saccharum officinarum and Zea mays. The irrigation water included treated distillery effluent (TDE) from the Agrochemicals and Food Company (ACFC), in Kisumu County and tap water (TW) from Eldoret Water and Sanitation Company Limited (ELDOWAS), a public water supply in Uasin Gishu County. The experiment was a 2 (irrigation water qualities) by 4 (test crops) factorial, with 4 replications. The control treatment involved irrigation of pots that comprised soil only. Variation in plant growth characteristics between the irrigation treatments was studied by physical measurements (leaf count, leaf and stem lengths), over a 120-day period. Samples of the irrigation water, leachates, soil and various plant parts were analyzed through selected tests to evaluate the effect of application of TDE on the soil-plant systems. Analysis of the levels of cadmium (Cd), copper (Cu) and zinc (Zn), and total nitrogen (N) and phosphorous (P), in the soil-plant systems, were determined by flame atomic absorption spectroscopy (FAAS) and ultraviolet-visible (UV-Vis) spectroscopy, respectively. Toxicity stress in soil-plant systems was measured in leachate samples by use of Toxi-Screening KitMicrobiotest ®. Data was stored in Microsoft Excel Spreadsheets and analyzed by use of Statistical Programme for Social Scientists (SPSS version 20.0.0).The alpha level for all statistical tests (Student’s t-test, One-way ANOVA and Spearman’s correlation test) was set at 0.05. The metal levels in TDE irrigation water were below the National Environmental Management Authority (NEMA) standards, for heavy metals in irrigation water and heavy metals in effluent for discharge to the environment. Plants irrigated with TDE gave higher leaf count, leaf and stem length measurements as well as higher concentrations of N, P, Cu and Zn than those irrigated with TW. Deposits of Cd, a toxic metal, were higher in TDE irrigation treatment. Uptake of Cd, Cu and Zn varied between test crops. Spearman’s correlation determined significant positive correlations between Cu and Zn (r s = 0.237, n=72, p= 0.045) and Cd and Zn (r s =0.236, n=72, p = 0.046), N and P (r s =0.605, n=72, p < 0.0001).Cadmium and Cu had a negative correlation (r s = -0.59, n=72, p= 0.623). The inhibition of bioluminescence in Vibrio fischeri by leachates , an indication of toxicity stress in the plants irrigated with TDE , was determined as follows: maize (31.21%) > sweet potato (28.34%) > eucalyptus (16.43%) > sugarcane (15.20). The detection of Cd, a toxic metal, in sweet potato tuber (0.020 mg L-1), sugarcane stem (0.014 mg L-1) and maize grain (0.002 mg L-1) indicates potential risk of Cd being introduced to the human food chain. For that reason, TDE is not recommended for cultivation of food crops. The production of effluent-irrigated eucalyptus plantations can secure additional treatment of distillery effluent and minimize the adverse environmental effects associated with its disposal.
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