Coagulation and sedimentation performances of aluminum sulphate and ferric chloride were comparatively investigated in presence of Grewia spp. biopolymer for the treatment of concentrated laterite suspensions. Jar tests experiments were carried out at different laterite suspension concentrations (10, 20, and 30 g/L) and pH values (5 and 7). The performances of these coagulants were assessed in terms of interfacial settling velocities and sediment concentration factors. Results showed that after addition of Grewia spp. biopolymer, sedimentation velocities were greater when ferric chloride and aluminum sulphate were used alone. When hydrolyzing salts were used alone, the highest settling velocities were obtained with 10 g/L of laterite suspension at pH 5 and settling speeds were 0.22 and 0.28 cm/min for aluminum sulphate and ferric chloride, correspondingly. Addition of Grewia spp. biopolymer led to an increase of settling velocities to 0.56 and 0.57 cm/min, respectively. The sediment concentration factor was also found to be high when Grewia spp. was added. With 30 g/L of laterite suspension, sediment concentration factors at pH 5 were 1.47 and 2.12 for aluminum sulphate and ferric chloride separately. Addition of Grewia spp. biopolymer with aluminum sulphate and ferric chloride produced more compact sludge with sediment concentration factors of 4 and 3.13, respectively. Flocs structures could successfully explain the obtained results.

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Diffusion coefficients of biopolymers extracted from Grewia spp. barks in water were studied under various physicochemical conditions. Extractions were carried out on barks of 3 cm in length at different pH (5-9), temperature (25-50°C) and ionic strength (10-3-10-2 M), using CaCl 2 and NaCl as background electrolytes. Factorial experimental plan design was used and diffusion coefficients were determined using second law of Fick equation, with linear and logarithmic approximation. Obtained results show that diffusion coefficients of Grewia spp. biopolymers are in the range of 9.78458E-06±1.6462E-08 cm 2 /min, and are not significantly influenced by the pH, the ionic strength and the temperature. Maximum diffusion was obtained with CaCl 2 at high ionic strength. Both linear and logarithmic approximations were found to be suitable for diffusion coefficients and were dependent on the background electrolyte used.