Modified Oyster Waste Shells as a Value-Added Sorbent for Lead Removal from Water.

Used oyster shells are considered a waste product of the seafood industry. Herein, we hypothesize that oyster shells can be used as a raw ingredient to synthesize effective sorbents. This study investigated the lead sorption ability of modified oyster shells [phosphoric-acid treated oyster shell (POS) and oyster shells with phosphoric acid (OS + P)] from water. XRD confirmed the presence of calcium phosphate. Lead removal efficiency as a function of sorbent type was as follows: OS + P > POS > OS. Sorption was quantified effectively using Freundlich isotherms with correlation coefficients ranging from 0.98 to 0.99. The Freundlich parameter values of Kf and n indicate that POS and OS + P exhibit stronger lead sorption than OS. These results indicate that modified oyster shells with OS + P and POS could be effective sorbents for lead removal from water. Kinetic models were well described by a pseudo-second-order rate constant for all the sorbents.

Modified Crushed Oyster Shells for Fluoride Removal from Water.

Elevated concentrations of fluoride ions (F-) in natural groundwater are a worldwide problem. Discarded oyster shells were ground to ≤100 µm particle size to produce oyster shell powder (OS). A subset of the OS was heated to produce calcined oyster shell (COS). A subset of the COS was further treated with 1 M phosphoric acid to produce phosphoric-acid-treated oyster shell (POS). OS and COS were combined with phosphoric acid (1.6 mM and 3.2 mM) to produce OS + P (oyster shell with phosphoric acid) and COS + P (calcined oyster shell with phosphoric acid). OS and COS removed 46% and 50% (10 g/L of sorbent dose) but POS, OS + P and COS + P removed 96%, 100% and 76% (1 g/L of sorbent dose) when the initial concentration of fluoride was 10 mg/L. The sorption kinetics of POS, OS + P and COS + P followed second-order reaction rates, and sorption isotherms of all sorbents were well-described by the Freundlich sorption isotherm. These results indicate that oyster shells can be an effective sorbent for fluoride removal, with the added benefit of re-use of a waste product.