The capacity of microorganisms to remove heavy metal from wastewater has been a subject of diverse interest. Whereas some heavy metals are essential for effective microbial activity, some heavy metals could be toxic to the microorganisms at concentrations higher than their minimal inhibitory limit. The kinetics of Zn2+ removal from aqueous solution was evaluated in terms of substrate removal rate for two identical suspended and immobilized bioreactors. The suspended growth bioreactor was used as a control system (CS) and contains only biomass. The immobilized bioreactor (IB) contains both biomass and microwave incinerated rice husk ash (MIRHA). The bioreactors were operated at a fixed HRT of 29.1 hours, whereas Zn2+ influent concentration was varied in the range of 0.5, 1, 2, 5, 10 and 15 mg/L. At steady state conditions, the results show that Zn2+ removal was in the range of 72, 75, 72.5, 68.2, 70.3 and 58.7 for CS, whereas it was in the range of 88, 90, 83, 88.6, 86.2 and 83.7 for IB. The substrate removal rate was found as 1.1856 g/L.d for CS and 4.2693 g/L.d for IB. The results clearly show that Zn2+ removal was more favorable in IB, indicating that the performance of the bioreactor was enhanced by the addition of MIRHA. © Published under licence by IOP Publishing Ltd.