Adsorption and removal of arsenic (V) using crystalline manganese (II,III) oxide: Kinetics, equilibrium, effect of pH and ionic strength

Abstract

Manganese (II,III) oxide (Mn₃O₄) crystalline powder was evaluated as a potential sorbent for removal of arsenic (V) from water. Adsorption isotherm experiments were carried out to determine the adsorption capacity using de-ionized (DI) water, a synthetic solution containing bicarbonate alkalinity, and two natual groundwater samples. Adsorption isotherm data followed the Langmuir and Freundlich equations, indicating favorable adsorption of arsenic (V) onto Mn₃O₄, while results from the Dubinin–Radushkevich equation were suggestive of chemisorption of arsenic (V). When normalized to the sorbent surface area, the maximum adsorption capacity of Mn₃O₄ for arsenic (V) was 101 μg m⁻², comparable to that of activated alumina. Arsenic (V) adsorption onto Mn₃O₄ followed pseudo–second-order kinetics. Adsorption of arsenic (V) was greatest at pH 2, while adsorption at pH 7–9 was within 91% of maximum adsorption, whereas adsorption decreased to 32% of maximum adsorption at pH 10. Surface charge analysis confirmed the adsorption of arsenic (V) onto the acidic surface of the Mn₃O₄ sorbent with a pH_PZC of 7.32. The presence of coexisting ions bicarbonate and phosphate resulted in a decrease in arsenic (V) uptake. Comparable adsorption capacities were obtained for the synthetic solution and both groundwater samples. Overall, crystalline Mn₃O₄ was an effective and viable sorbent for removal of arsenic (V) from natural water, removing greater than 95% of arsenic (V) from a 1 mg L⁻¹ solution within 60 min of contact time.

Keywords:

• Manganese oxide
• arsenic (V)
• arsenate
• adsorption
• pH
• ionic strength