A NEW HYBRID INORGANIC SORBENT FOR HEAVY-METALS REMOVAL

Iron oxyhydroxides, commonly known as ferrihydrites, are unable to remove dissolved heavy metals at acidic pH, especially below 5.0, due to fierce competition from hydrogen ions. A new hybrid iron-rich inorganic sorbent has been identified and extensively studied in relation to heavy metals removals in fixed-bed processes for influent pH as low as 3.5. Every single particle of this new hybrid sorbent essentially contains ferrihydrite along with a crystalline silicate phase, akermanite, in close proximity (in the order of 100 Angstrom) to one another. Akermanite has a unique ability to produce hydroxyl ions through incongruent hydrolysis reactions without being washed out from the fixed bed. The simultaneous presence of akermanite and ferrihydrite in a single particle has a synergistic effect on the sorption process: while akermanite helps neutralize aqueous-phase hydrogen ions (thus enhancing sorption capacity of ferrihydrites), neighboring sorption sites in ferrihydrites quickly remove dissolved heavy metals, thus avoiding precipitation. Equally important, the hybrid sorbent can be regenerated with any amine/ammoniacal solution and reused for multiple number of cycles. Some precipitations may occur within the column at relatively high influent concentrations of heavy metals (around 50 mg/l) or due to chromatographic effect. Such precipitates are, however, amenable to removals by conventional backwashing.