Suitability analysis, bioavailability, and mobility of heavy metals in the sludge produced from polluted river water, Central Ethiopia

Abstract

Little Akaki River is one of the major rivers flowing through Addis Ababa City and receives industrial, domestic, and municipal wastewaters constituting organic, nutrients, and trace metals. A sedimentation tank was established nearby the river to produce sludge from the polluted river water and use it as organic fertilizer. The objectives of the study were to investigate the mobility and bioavailability of heavy metals in the sludge and to evaluate the environmental risk and suitability of the sludge for agricultural land application. The concentration of trace metals in sludge was determined using inductively coupled plasma optical emission spectrophotometry and metals fractionations were done following modified community bureau of reference procedures. The results showed that the organic matter content of the sludge ranged from 43 to 57%. This study revealed that 39.17% of Zn; 19.22% of Cr; 30.23% of Cd, and 31.25% of Pb were found in readily bioavailable forms. The average mobility factor of all trace metals was 67%, indicating horizontal migration of trace metals in soil. The risk assessment code revealed that all trace metals fall in the medium to high environmental risk classes. The concentrations of trace metals in the analyzed sludge samples were below the standard limits of sewage sludge, indicating the suitability of the sludge for organic fertilizer. However, due to the high mobility, bioavailability, and environmental risks of trace metals, using the sludge as organic fertilizer on agricultural croplands may further escalating soil pollution, adversely affecting the environment and public health.