Oxyanions are polyatomic ions usually consisting of at least one oxygen atom boned with a metal or non-metal element. Arsenate (AsO₄^3-), dichromate (Cr₂O₇^2-), and phosphate (PO₄^3-), examples of a few oxyanions, are dangerous as they pose critical health adversity and pollute the water environment and ecosystem. Arsenate and dichromate are toxic to humans, even at low concentrations. Phosphate causes harmful algal blooms on surface water bodies, which release toxins that threaten humans, animals, birds, and aquatic species. So, removing these oxyanions from wastewater or drained water from many industrial and non-industrial sources is paramount for protecting our planet’s water systems. Layered zinc hydroxides are effective materials (sorbents) that can capture these harmful oxyanions from water. Many studies are reported in the literature regarding layered zinc hydroxides for the uptake of the oxyanions – arsenate, dichromate, and phosphate. However, the uptake chemistry of these oxyanions with the layered zinc hydroxides (LZHs) is often misunderstood, as many studies presented the interaction of oxyanions with LZHs as just adsorption or do not explore uptake long enough to investigate the anion-exchange chemistry after the initial hours. Also, the studies in the literature that expressed the interaction of oxyanions with LZHs as anion exchange often do not provide experimental evidence to support it. Herein, this work will present a simple experimental exploration showing that arsenate, dichromate, and phosphate uptake onto zinc acetate hydroxide (ZAH), a layered zinc hydroxide, involves adsorption followed by anion-exchange process. The work will showcase pH investigations vs. time during the uptake of the oxyanions using the ZAH as experimental evidence to establish the anion exchange process during the uptake. This study performed the pH investigations long enough (several days to weeks, depending on uptake equilibrium time) to explore the uptake chemistry beyond the initial hours (6-12 hours). A proposed mechanism for an oxyanion interaction with the ZAH will be presented by corroborating the pH change with time. Additionally, an uptake experiment of an oxyanion under low-temperature hydrothermal conditions was conducted, which will further confirm the anion exchange of the oxyanion with the ZAH, leading to a surpassing of its uptake capacity at room temperature.

Monica Percino

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