Natural based composites of hydroxyapatite/Gum Arabic designed for removal of toxic metal arsenic (III) from waste water were synthesized and evaluated. Several composites with various compositions were prepared by the wet chemical method and analyzed using various spectroscopic and analytical methods such as: Fourier transform infrared spectroscopy, total organic carbon production, XRD analysis and scanning electron microscope. The rates of weight loss and water absorption of the HAp/GA composites as a function of time were evaluated in phosphate-buffered saline solution at 37 °C and a pH of 7.4. The effects of several variables on adsorption of arsenic (III) by HAp/GA composites were evaluated. The variables include arsenic (III) concentration, contact time (t) and complex surface nature of HAp/GA composite. Three surface complexation models were used to study the mechanisms controlled the adsorption. The models were Langmuir, Freundlich and Dubinin Radushkevich. The adsorption kinetic of arsenic (III) on the composite surface was described by three modes: pseudo first order, pseudo second order and the intra particle diffusion. The results revealed that, the rate of adsorption of arsenic (III) by HAp/GA composites was controlled by two main factors: the initial concentration of arsenic (III) and the contact time. The kinetic studies also showed that, the rate of adsorption is a second order. The results indicate that, composite offered in this study could be a valuable tool for removing toxic metals for contaminated water by adsorption.Graphical abstract.

Abstract:The adsorption of chromate on octacalcium phosphate (OCP) was investigated as a function of contact time, surface coverage, and solution pH. The ion exchange method was adapted to establish the interaction mechanism. Stoichiometry exchange of H + /OH − was evaluated at a pH range of 3-10, and obtained values ranged between 0.0 and 1.0. The surface complexes formed between chromate and OCP were found to be > S(HCrO 4 ) and > S(CrO 4 ). The logarithmic stability constant logK 1-1 , and the logK 10 values of the complexes, were 6.0 in acidic medium and 0.1 in alkaline medium, respectively. At low pH and low surface coverage, the bidentate species > S(HCrO 4 ) 2 with logK 10.5 of about 2.9, was favored at a hydration time of less than 150 min. The contribution of an electrostatic effect to the chromium uptake by the OCP sorbent, was also evaluated. The results indicate that the adsorption of chromate on OCP is of an electrostatic nature at a pH ≤ 5.6, and of a chemical nature at a pH > 5.6.

a b s t r a c tHydroxyapatite (HAp) is a biomaterial with a large number of useful applications. In this work, a method for the preparation of HAp at nanoscale was developed (nano-HAp). The prepared nano-HAp was used in the extraction of methyl orange (MO) from water. The special chemical composition, the crystal structure of HAp and molecular structure of MO made extraction occurs at a good efficiency. Structure of the prepared HAp was confirmed by powder X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscope. Batch experiments were used to evaluate the tendency of the prepared nano-HAp for extracting the MO from water. The effect of temperature, time, pH and the amount of adsorbent on the rate of adsorption were evaluated and found to have a noticeable effect on the adsorption efficiency. The optimum conditions for extraction of MO by nano-HAp were determined. In all runs, in the first 5-15 min of contact time at room temperature the nano-HAp showed the highest rate of adsorption. Then, the rate leveled off after about 20 min. The temperature showed almost directly proportional with the rate of adsorption. The thermodynamic studies revealed a spontaneous nature of adsorption at different temperature.