Effect of background electrolytes on the adsorption of phosphorus (P) onto southern Tunisia natural clays

The purpose of the present research is to investigate the effect of background electrolytes on the phosphorus (P) sorption by four different natural clay samples (NCS): Jebel Tejera-Esghira (JTE) in Medenine area, Douiret (DR) in Tataouine area, Jebel Aidoudi (JA) and Zemlet El Beida (ZB) in Gabes area. The mineralogical and chemical characterization of the NCS showed that they are essentially composed of illite, kaolinite, smectite, with small fractions of quartz and calcite. The P sorption was investigated in batch mode in the presence of three different background electrolytes (CaCl₂, NaCl and KCl with 99% of purity) under different experimental conditions of contact times (3 h), initial concentrations (25–150 mg.L⁻¹), ionic strength (0–1 M), temperature (295–325 K) and pH (3–11). Experimental results indicated that P sorption in the presence of electrolytes is a rapid process since about 95% of P was observed within a contact time of 180mn. The highest P adsorption capacity was determined to 7.39 ± 0.65, 7.96 ± 0.82, 8.07 ± 0.55 and 8.83 ± 1.05 mg g¹ for JTE, JA, ZB and DR. At equilibrium, the P adsorbed amount by the raw NCS was assessed to 7.75 ± 0.55, 8.34 ± 0.75, 8.8 ± 0.3 and 8.85 ± 0.75 mg g⁻¹ for JTE, JA, ZB and DR. This amount increased by about 0.11, 0.08, 0.05%; 0.14, 0.06, 0.04%; 0.17, 0.1, 0.09 and 0.13, 0.11, 0.01% when adding the CaCl₂, NaCl and KCl respectively. Moreover, the NCS capacity in removing increased with the increase of the initial P concentrations and the decrease of the liquid medium pH values. Also, P adsorption decreases with the increase of temperature. The P kinetic and isothermal removal experimental data were well fitted with the Pseudo-second order and Langmuir models respectively indicating that P adsorption onto NCS particles is mainly governed by chemical processes and occurs heterogeneously on multi-layers surfaces.