Usability of Rice Straw as Biosorbent for the Removal of Phosphate from Aqueous Solution: Isotherms, Kinetics and Thermodynamics

The present study evaluated the adsorption of phosphate ions from an aqueous solution using rice straw-based adsorbents. The batch study was performed to find the effect of pH, initial ion concentration, a dose of adsorbent, contact time and temperature. The maximum adsorption capacity of raw and modified rice straw is 6.47 mg/g and 16.60 mg/g. Both adsorbents (raw rice straw and modified rice straw) showed good adsorption capacity at pH 8, adsorbent dose of 0.5 g for modified rice straw and 0.6 g for raw rice straw with an initial ion concentration of 20 ppm, contact time of 60 min at 100 rpm. Adsorption isotherm, kinetics and thermodynamic studies were also performed for adsorption data. The monolayer adsorption capacity was measured for raw rice straw at 6.21 mg/g and modified rice straw at 6.36 mg/g. Freundlich isotherm (correlation coefficient 0.91 for raw rice straw and 0.96 for modified rice straw) was best fitted to experimental data compared to Langmuir isotherm. The pseudo-second-order showed a correlation coefficient of 0.99 for both adsorbents and was perfectly fit to adsorption data. A thermodynamic adsorption study showed that the adsorption of phosphate ions is endothermic and non-spontaneous at a lower temperature. The present study revealed that modified rice straw adsorbent has good removal efficiency for phosphate ions.