Assessment of Mobility and Bioavailability of Heavy Metals in Sewage Sludge from Swaziland through Speciation Analysis

Abstract

Speciation analysis of heavy metals in sewage sludge provides a better understanding of the extent of mobility and bioavailability of the different metal fractions and helps in more informed decision making on application of sludge for agricultural purposes. Assessment of bioavailability of metals based on regulations expressed in terms of total metals alone may be conservative and restrictive from the point of view of the use of sludge for agricultural applications. Total metals may also be poor indicators of uptake by plants. Sewage sludge samples generated from seven wastewater treatment plants in Swaziland were analyzed for the four fractions of metals species, namely, exchangeable, reducible, oxidizable and residual fractions. The experimental results indicated that arsenic was predominantly associated with the residual matrix. Chromium was dominantly found in the residual fraction followed by oxidizable fraction. Lead and copper were predominantly associated with oxidizable fraction (bound to organic matter). For more polluted effluents such as Matsapha waste water treatment plant, some metals such as nickel were also found significantly in the mobile fraction. By contrast zinc and to a certain extent nickel were present in significant proportions in the more mobile (exchangeable) fraction. The experimental result indicated that the percentage immobility ranges from 10% to 70% with respect to the residual solid fraction of the metals which should be considered as an additional safety factor in the assessment of the suitability of the sludge for agricultural uses with respect to heavy metals.