Physicochemical Characterization of Lomo Black Shale and Application as Low Cost Material for Phosphate Adsorption in Aqueous Solution

Abstract

The characterization of shale powder fraction from Lomo Nord (Ivory Coast) was performed in order to find potential applications for this material as phosphate low-cost adsorbents. The crushed shale (ø ˂ 80µm) consisted of quartz, albite, microcline, chlorite, dolomite and kaolinite as associated minerals. The net effective charge of shale particle, determined using microelectrophoresis, is shown to be negative for all pH values. Batch experiments were performed with phosphate synthetic water to study the adsorption equilibrium and influence of contact time, adsorbent dosage, initial concentration, temperature and pH on the phosphate adsorption onto the shale. Kinetic data revealed that more than 0.046 mg/g of phosphate was adsorbed onto shale within 10 hours. The maximum phosphate adsorption capacity of the shale powder was 0.32 mg/g. The dependency of the phosphate sorption capacity with the pH is clearly evidenced. Shale achieved the maximum removal of phosphate (70 %) over the pH range 4 – 10. Thermodynamic tests indicate that phosphate sorption to shale increased with temperature (from 20 to 40°C), indicating the spontaneous and endothermic nature of sorption process (∆H° = 8.22 kj/mol, ∆S° = 0.015 kj/mol, ∆G° = 3.52 - 3.82 kj/mol). ∆G° values shows dominance of physisorption process. But, the results indicate that the pseudo second order model was most suitable for describing the kinetic data, revealing that some chemical interaction also occur. The Freundlich, Langmuir and Dubinin-Radushkevich models were used to simulate the sorption equilibrium. The Freundlich model showed more applicability than others. These finding demonstrate that Lomo shale’s could be used as an efficient low-coast adsorbent to remove phosphate from water.

Keywords: Phosphate, adsorption, isotherms, kinetics, shale, thermodynamics.