Microalga species are potential scavengers of heavy metals. The active functional groups of a number of biomaterials are capable to detoxify Cr(VI) to Cr(III). A 2nd order kinetic model was developed in terms of concentration of protonated acidic groups of Spirulina sp. biomass for the reduction of Cr(VI) into Cr(III). Cr(VI) reduction reaction grounded on the concentration of functional groups was validated over a broad range of pH, temperature and anionic strength. Lower pH favoured Cr(VI) reduction reaction and the experimental results well fitted to the kinetic model. The overall rate constant, ktkt, decreased logarithmically from 22.7 to 2.8 mM−1 s−1 with rise of pH from 0.5 to 6.0. Whereas, kt increased nearly by 23% with elevation temperature from 25 to 45 °C. Higher concentration (>0.235 mM) of background anions (Cl−, SO42- and NO3-) was resulted in decreases in ktkt values. The rate constant expression developed can be employed to quantify Cr(VI) reduction into Cr(III) using Spirulina biomass.