Adsorptive removal of sodium dodecyl sulfate using activated coconut shell based adsorbent: kinetic and thermodynamic study

Adsorptive separation of sodium dodecyl sulfate (SDS) was investigated in the present work using modified adsorbent prepared by thermo-chemical activation of coconut shell based on Phosphoric acid as an activation agent (PAACS-I). The effect of different operating conditions on extent of SDS removal has been studied. Between the investigated pH range of 2.0 to 8.0, the removal of SDS was found to be independent of pH whereas definite variation for all other parameters was established over the investigated range with existence of optimum conditions. Extent of SDS removal as 100%was obtained at natural pH of solution in 120 min under optimized conditions of 80 mg/L as the initial concentration, 2 g/L as the adsorbent loading, 150 rpm as the speed of rotation and 303 K as the operating temperature. The maximum adsorption capacity under optimized condition was determined as 111.1 mg/g. The kinetic data fitted well into the pseudo second order kinetic model. Multi-linear nature of Webber-Morris curve fitting indicated that the intra-Particle diffusion was not the sole controlling mechanism. Isotherm study was performed over the concentration and temperature range as 80–240 mg/L and 303–323 K respectively. Obtained data was analyzed by Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models and it was clearly demonstrated that the Langmuir model provided better fit indicating monolayer adsorption. Thermodynamic analysis established that adsorption was spontaneous and exothermic. The obtained values of ΔG° and ΔH° from thermodynamic analysis were found to be –36.28 kJ/mol and –9.98 kJ/mol respectively. Desorption of SDS was attempted using ethanol, acidic and alkaline water as the regeneration solvents. Ethanol gave maximum desorption as 95% in 1 h. Overall the work clearly demonstrated the promise of modified adsorbent, PAACS-I for the adsorptive separation of SDS, with good possibility of desorption and reuse.