| Summary: | The biosorption behaviour of arsenic(V) and cadmium(II) ions by unmodified and five types of chemically modified <i>Chlorella vulgaris</i> and <i>Spirulina platensis</i> was investigated. The biosorption rates of As(V) and Cd(II) in binary metal solutions were lower than those in sole metal systems, which exhibited a competition between As(V) and Cd(II) ions to occupy the active sites of the adsorbent. Among the five chemical reagents, NaCl and ZnCl<sub>2</sub> were the most suitable modifiers for improving the biosorption performance of <i>C. vulgaris</i> and <i>S. platensis</i>, respectively. The maximum biosorption capacities of As(V) and Cd(II) were: (a) 20.9 and 1.2 mg/g, respectively, for <i>C. vulgaris</i> modified with NaCl; (b) 24.8 and 29.4 mg/g, respectively, for <i>S. platensis</i> modified with ZnCl<sub>2</sub>, which were much higher than those using other chemically modifying methods. The pseudo-second-order kinetic model fitted well with all the biosorption processes. The SEM analysis revealed that the modification changed the surface morphologies and enhanced the porosity of the algae biomass. The FTIR analysis established the presence of diverse groups of compounds that were largely hydroxyl, carboxylate, amino, and amide groups on the adsorbents that contributed significantly to the upregulated biosorption. This work showed the potential application of chemically modified <i>C. vulgaris</i> and <i>S. platensis</i> biomasses to effectively remove both from water.
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