Phosphate removal from wastewater using scallop and whelk shells

Low-cost shells (scallop and whelk) were used as adsorbents for the removal of phosphate from synthetic and industrial wastewater. In synthetic wastewater samples, batch experimental conditions were optimised using a Central Composite Design (CCD). Column studies were carried out for the treatment of wastewater from a shellfish processing industry. The CCD results showed that scallop and whelk shells could adsorb phosphate over a broad pH value range (4.5–10.0) despite having a point of zero charge (pH_pzc) of 9.4. However, maximum adsorption capacity was obtained at a pH value of 4.5, initial concentration of 90 mg/L PO₄³⁻ and adsorbent dose of 10 g/L. The pseudo-second-order kinetic model fitted the batch experimental data for both scallop and whelk shells (R² ≥ 0.99) while the isotherm data was best fitted to Elovich (R² = 0.967) and Freundlich (R² = 0.9781) models respectively. The maximum adsorption capacity obtained experimentally for scallop and whelk shells was approximately 9.49 mg/g and 8.80 mg/g, respectively. Evidence of phosphate sorption onto the shells was shown via SEM-EDX, FTIR and XRD characterization studies. Scallop shells removed 46.68–85.48 % of phosphate from industrial wastewater in a fixed bed column without being exhausted after 7 h of operation. The desorption study results indicated the low regeneration potential of the spent adsorbents (<3.0 %), thus, suggesting a complex chemisorption mechanism. The results of this study indicate that unmodified waste shells are best employed in wastewaters having low phosphate levels and the spent adsorbents could be re-used as a slow-release fertilizer and as a conditioner in acidic soils.