Abstract
The contamination of surface and groundwater with phosphate originating from industrial and household wastewater remains a serious environmental issue in low-income countries. Herein, phosphate removal from aqueous solutions was studied using low-cost volcanic rocks such as pumice (VPum) and scoria (VSco), obtained from the Ethiopian Great Rift Valley. Batch adsorption experiments were conducted using phosphate solutions with concentrations of 0.5 to 25 mg·L−1 to examine the adsorption kinetic as well as equilibrium conditions. The experimental adsorption data were tested by employing various equilibrium adsorption models, and the Freundlich and Dubinin-Radushkevich (D-R) isotherms best depicted the observations. The maximum phosphate adsorption capacities of VPum and VSco were calculated and found to be 294 mg·kg−1 and 169 mg·kg−1, respectively. A pseudo-second-order kinetic model best described the experimental data with a coefficient of correlation of R2 > 0.99 for both VPum and VSco; however, VPum showed a slightly better selectivity for phosphate removal than VSco. The presence of competitive anions markedly reduced the removal efficiency of phosphate from the aqueous solution. The adsorptive removal of phosphate was affected by competitive anions in the order: HCO3− >F− > SO4−2 > NO3− > Cl− for VPum and HCO3− > F− > Cl− > SO4−2 > NO3− for VSco. The results indicate that the readily available volcanic rocks have a good adsorptive capacity for phosphate and shall be considered in future studies as test materials for phosphate removal from water in technical-scale experiments.
Highlights
Phosphate, a molecule consisting of the elements phosphorus (P) and oxygen [1], is an essential macronutrient for all life [2]
The scanning electron microscope (SEM) analysis revealed that the surface morphology of the volcanic rocks such as pumice (VPum) was rough and wrinkled, which potentially provides more adsorption sites for phosphates (Figure 1a) than VSco, where a relatively smooth and uniform surface structure was observed (Figure 1c)
The measured energy dispersive X-ray (EDX) image indicates that the oxides of Al, Si, Fe, and K were the main constituents of the adsorbent materials, while the rest of the elements, such as Mn and Pb, were detected at lower values (
Summary
A molecule consisting of the elements phosphorus (P) and oxygen [1], is an essential macronutrient for all life [2]. Phosphorus is the primary source of productivity and is fundamental for freshwater ecosystems’ sustainability [3]. A high phosphate concentration above 0.1 mg·L−1 [4] in water bodies has serious environmental repercussions, such as eutrophication, which is associated with massive algal growth, the excessive growth of microorganisms, and the depletion of dissolved oxygen (DO) in the water bodies, and, in turn, it harmfully effects life in the aquatic ecosystems [5,6,7,8,9]. Industrialization, modern agriculture, and human activities are the primary sources that can pose a serious threat to freshwater ecosystems due to the pollutants released from these into the environment [12]
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