Alginate-Bentonite Clay Composite Porous Sorbents for Cu(II) and Zn(II) Removal from Water
2022
Hood, Christine | Pensini, Erica
High concentrations of heavy metals in groundwater are harmful to humans and ecological receptors. This study uses natural alginate-based sorbents for the removal of heavy metals (e.g., copper, zinc, iron, and nickel) from water. The effectiveness of alginate-based sorbents was enhanced by adding calcium bentonite clay and by tuning the porosity of the sorbents. Controlled porosity was obtained by an acid base reaction, using sodium carbonate and acetic acid. The maximum sorption capacity of alginate-based sorbents was 127.9 ± 0.6 mg/g and 148.1 ± 0.2 mg/g for Cu(II) and Zn(II), respectively. The sorption of Zn(II) onto the sorbents followed pseudo first-order kinetics (k₁ = 9.71 × 10⁻³), indicating that the rate limiting step was the diffusion of Zn(II) into the sorbents. In contrast, the sorption of Cu(II) onto the sorbents followed pseudo second-order kinetics (k₂ = 5.80 × 10⁻⁵), indicating that the rate limiting step was chemisorption of Cu(II) into the sorbents. Optical microscopy images of the sorbent cross-section showed pore shrinking following sorption of either Zn(II) or Cu(II), due to crosslinking of alginate by these metal ions. Cu(II) diffusion into the sorbents was further demonstrated by blue discoloration (as shown by images of their cross sections) and by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). Intraparticle diffusion plots of both Zn(II) and Cu(II) showed that the sorption process begins with surface adsorption and is followed by the rate controlled step of intraparticle diffusion. Alginate-based sorbents could also be used to effectively remove other divalent ions (e.g., Ni(II)), whereas removal of trivalent ions (e.g., Fe(III)) was less effective.
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