Adsorption of Orange I by modified dolomite: performance and mechanism

Synthetic azo dyes such as Orange I represent a significant cause of environmental contamination. Their discharge into the aquatic medium creates tremendous concerns for the environment and living organisms as most of them are recalcitrant, toxic, and carcinogenic. In this context, Algerian dolomite treated at 900 °C (D900), characterized by thermal analysis, laser granulometry, and zetametry, was applied in the adsorption of Orange I from aqueous solutions. Several parameters influencing adsorption were investigated. The mechanism of Orange I-D900 interaction was also explored by infrared spectroscopy. Thermal analysis showed that 900 °C is a sufficient temperature to decompose dolomite [CaMg(CO3)2] into CaO and MgO. An initial concentration of 200 mg L−1, pH of 5, solid/solution concentration of 3 g L−1, and temperature of 55 °C, were found to be the optimum conditions, for a maximum amount adsorbed by D900 of 36.8 versus 11.5 mg g−1 for dolomite. The equilibrium data were suitably fitted by the Redlich–Peterson model, with values of coefficient of determination and average relative error ≥ 0.98 and < 10.0%, respectively. Thermodynamic parameters show that the adsorption process switches from non-spontaneous for dolomite to spontaneous for D900. The Orange I-D900 interaction mechanism involves two phenomena: an electrostatic attraction between the negative charge of Orange I embodied by the sulfonate group and the positive charge of D900 represented by MgOH+, and a hydrogen bonding between Ca(OH)2 and Mg(OH)2, evidenced by infrared spectroscopy, and the hydroxyl group of the azo-molecule. The mechanistic approach is essential to expand the application of dolomite in wastewater remediation.