ABSTRACT
Background: The objective of this study was to examine the adsorption-de- sorption
phenomena of heavy metals in an agricultural Ferralsol treated with sewage
solid waste at rates usually applied for soil fertility management. Methods: The study was carried out under laboratory conditions, using a Ferralsol
sourced from Makerere University Agricultural Research Institute, Kabanyolo
(MUARIK). Soil and sewage solid waste were analysed for pH, organic matter,
cation exchange capacity and heavy metals (copper, zinc, chromium and lead).
Soil was treated with sewage solid waste at input rates of 0, 50, 100, and 150
g per pot (equivalent to 0, 2.5, 5.0 and 7.5 metric tones·ha﹣1 respectively); and supplemented with phosphorus. The phosphorus was applied at
rates of 0, 0.795, 1.591 and 2.385 g per pot (equivalent to 0, 25, 50 and 75
kg·ha﹣1, respectively). Batch adsorption was used to study the
sorption-desorption of heavy metals on the treated soil and the Langmuir and
Freundlich models were used to analyse the data. Results: Adsorption and
desorption isotherms fitted better to Freundlich equation than Langmuir model.
Chromium was the most sorbed and retained metal; while lead was the least
retained overall. The desorption process was virtually irreversible,
considering the low amounts of the metals desorbed. Chromium fitted relatively
better to both models than the copper, zinc and lead. All the four metals were
less desorbed at high metal concentrations. Conclusion: The four metals
would not be available at high metal concentrations especially when the
application rate used is ≥ 5.0 tones·ha﹣1 of the sewage solid waste. Thus, the metals
would not be available for plant uptake and the chance to contaminate
groundwater is very limited especially for chromium.