Effects of non-essential heavy metal bioremediation on enzyme in soils irrigated with treated wastewater

Abstract

Treated wastewater (TWW) can serve as a substitute option for irrigation to decrease the water scarcity pressures in semi-arid regions of Limpopo Province, South Africa. However, prolonged use of TWW can degrade soil quality and enzyme activities due to heavy metal contamination. Bioremediation has been proposed as an efficient method to eliminate the contaminants from the soil using indigenous microbes. Thus, the objectives of this study were to evaluate the effectiveness of bioremediation of non-essential metals in soils irrigated with TWW and further assess the response of enzyme activities post-bioremediation of non-essential heavy metals. The TWW was collected from the University of Limpopo Experimental Farm (ULEF) at the night dam's exit point and analysed for pH, EC, and non-essential heavy metals (As, Cd, Cr (VI), and Pb). The quantified water parameters were compared against the Republic of South Africa (RSA) and Food and Agriculture Organization (FAO) thresholds, and the parameters were above the permissible levels except for pH, EC, and Pb. Soil samples for contamination were collected from ULEF and analysed for physicochemical properties, non-essential heavy metals, and enzyme activities. The selected isolated soil microbes were identified as B. cereus, A. fumigatus, and E. cloacae. The bioremediation study was arranged in a completely randomized design with four treatments which were control (without inoculation), and soils inoculated with three selected microbes for 12 weeks. The results revealed that irrigation with TWW led to higher levels of non-essential heavy metals, pH, and EC in the soil and inhibited enzyme activities. The study proved that selected microbes decreased the concentration of non-essential metals, pH, and EC levels (P ≤ 0.05) post-contamination with TWW. Inoculation of B. cereus decreased the concentration of As and Pb by 20% and 56% respectively, while E. cloacae decreased Cr and Cd by 50% and 15% respectively. Additionally, inoculation of B. cereus increased phosphatase and arylsulfatase activities by 636% and 319% respectively. The control sample without inoculations, proved to bioremediate the soil and increase the urease activity by 1308%. Since the microbial inoculation had an insignificant effect on β-glucosidases, it was suggested that future research should be done to optimize bioremediation strategies by selecting suitable microbes that will stimulate β-glucosidases activity