Enhanced phosphate removal and potential recovery from wastewater by thermo-chemically calcinated shell adsorbents

Shell from the seafood processing industry is an under-utilised waste resource worldwide. Calcite, the major component of shell is commonly used in wastewater treatment for the removal of phosphorus (P). Here, mussel and oyster shell-based adsorbents (MSB and OSB) were used for removal of P as phosphate (PO43-) from aqueous solution and secondary wastewater, following preparation through chemical calcination at 700 degrees C. Batch adsorption experiments were carried out to identify the effects of various operating parameters (e.g., pH, dosage, contact time, initial concentration of P ions, co-existing ions), while a desorption study helped to understand the availability of the bonded P. The optimal contact time for PO43- removal was 120 min using both adsorbents with the dose at 200 mg. Characterisation of the adsorbent was performed using SEM-EDX, pH(pzc), BET, FTIR and XRD. The XRD analysis showed that both calcite and lime were present on the surface of the shell particles. P was adsorbed effectively through inner-sphere complexation and surface microprecipitation mechanisms, while an enhanced maximum P adsorption capacity of 12.44 mg/g for MSB and 8.25 mg/g for OSB was reached. The Redlich-Peterson isotherm model fitted well with the equilibrium isotherm data (R-2 >= 0.97) which also suggested a heterogenic surface. The desorption study (on the saturated adsorbent) found that similar to 97% of bonded P could be plant available in soil. These results suggest that a shell-based adsorbent can serve as a promising material for P removal from real wastewater effluent and subsequently could be used as a soil conditioner.