Sorption enhancement of heavy metals onto a modified iron-rich material

Selective removal of trace concentrations (mg/L or {dollar}mu{dollar}g/L) of dissolved heavy metals from contaminated water and wastewater is a challenging and a commonly encountered separation problem facing various industries and publicly owned treatment works (POTW). The use of beads or granules of chemically stable polymeric chelating exchangers in fixed-bed processes have, to a great extent, resolved the problem technically but they are often found too expensive to justify their application for heavy metals removal from water and wastewater.; Both amorphous and crystalline forms of iron oxyhydroxides, commonly referred to as ferrihydrite, have long been known to exhibit high sorption affinity towards dissolved heavy metal cations at alkaline pH. Ferrihydrite would be an attractive sorbent in fixed-bed columns for metal-removal processes because they are likely to be much less expensive compared to polymeric chelating exchangers often used for the same purpose. However, the commercial use of ferrihydrite in fixed-bed column has been practically non-existent due to its extremely small particle size leading to excessive pressure drop in a column, and its ineffectiveness at acidic pH resulting from the fierce competition of hydrogen ions.; In this study, a new hybrid iron-rich inorganic sorbent (HISORB) has been characterized and extensively studied in relation to heavy metals removal in fixed-bed processes for influent pH as low as 3.5. Every single HISORB particle essentially contains ferrihydrite along with a crystalline silicate phase, akermanite, in close proximity (in the order of 100 A) to one another. Equally important, HISORB can be regenerated with any amine/ammoniacal solution and reused for multiple numbers of cycles.; From an application viewpoint, the following observations are noteworthy: (1) HISORB after loading with heavy metals was amenable to regeneration by using ammonia/amine ligand-type solutions and thus, can be reused for multiple numbers of cycles. (2) Comparative column run studies indicate that HISORB can be as effective as a commercial synthetic weak-acid and chelating ion-exchange resins in removing dissolved heavy metals. (3) The performance of HISORB in removing heavy metals was found to be independent of solution pH between 3.5 and higher, while the heavy-metal removal capacity of ion-exchange resin declined dramatically as the influent solution pH decreased from 5.0 to 3.5. (4) On-line detection of exact breakthrough time for heavy metals from the column is a major problem with chelating ion exchangers. With HISORB, a modified pH meter at the exit of the column could detect such breakthroughs fairly accurately.; The underlying mechanism of heavy metals sorption onto HISORB i.e., enhanced sorption of heavy metal onto ferrihydrite with the synergistic effect by akermanite, was utilized to develop other new heavy-metal sorbents, namely, Synthetic Inorganic Sorbent (SISORB) and Fused Oxides Sorbent (FOSORB), by employing a heat fusion method on a laboratory scale. These newly synthesized inorganic sorbents have been demonstrated to be more effective than HISORB and therefore, they offer new opportunities in treating heavy-metal-laden wastewaters with a significant economic advantage. (Abstract shortened by UMI.)...