The studies related to the phosphate removal potential of a laboratory scale produced adsorbent, the tamarind nut shell activated carbon (TNSAC), have been partly reported in two papers by us which described the production of the various TNSAC forms, and the effects of the process variables (unrinsed and rinsed TNSAC forms and the impregnation ratio) and of the operational variables (adsorbent particle size, initial adsorbate concentration, adsorbent dose and contact time duration) on the performance evaluation of the TNSAC in adsorbing phosphates. The performance predictive models were also presented. This paper discusses the adsorption isotherm relationships and models. The applicability of the system to real field situations, and the reuse possibility and removal of the phosphate loaded adsorbent particles, have also been brought out in this paper. The isotherm plots show higher phosphate adsorptive capacity for greater equilibrium concentration values. The highest adsorptive capacity results at an impregnation ratio of 1.0 for both forms of the TNSAC and at all adsorbent doses. The adsorptive capacity is higher for the unrinsed TNSAC in comparison to the rinsed TNSAC, corresponding to the same equilibrium concentration. The rinsed TNSAC yields an isotherm model similar to the Freudlich isotherm. The unrinsed TNSAC does not yield any conventional isotherm forms, yet they have been modelled. Generalized models for predicting the adsorptive capacity have been evolved, and such models also incorporate the impregnation ratio, and the term for the adsorbent particle size. The models have indicated high correlation coefficients. When present in wastewaters, the adsorption of phosphates on the TNSAC is seen to reduce due to the preferential adsorption of other pollutants present in the wastewaters. The spent and phosphate loaded TNSAC particles are seen to be removable. Physical adsorption is seen to be the predominantly likely mechanism of phosphate adsorption on the TNSAC, which suggests the reuse possibility of the TNSAC.
