Copper removal from synthetic wastewater using the bacterium Achromobacter xylosoxidans : biosorption equilibrium, kinetic, thermodynamic and optimizing parameters by Box-Behnken design approach

Background Heavy metals such as copper are known water pollutants that can be extremely harmful to the environment and pose serious risks to human health and aquatic organisms. The present study aimed to determine the efficiency of Achromobacter xylosoxidans isolated from waste soil as a cost-effective biosorbent for the removal of copper (II) (Cu 2+ ) ions from synthetic and real wastewater. Methods The rate of copper biosorption by the bacterial strain was examined using batch experiments in 100 ml of aqueous solution, in which separate biosorption tests were performed for temperature (20–45 °C), initial pH (4–9), copper concentration (10–240 g L −1 ), contact time (0-150 min), and biosorbent dose (0.25-5.0 g L −1 ). Using the Box-Behnken design and response surface methodology, the biosorption of Cu 2+ ions by A. xylosoxidans biomass was also mathematically modeled to assess the interactive impacts of the independent factors of temperature, bacterial dosage, and initial metal ion concentration. Results The analysis of variance (ANOVA) results revealed that the actual data were in line with the predicted data. Under the optimal circumstances of initial copper concentration of 112.35 mg L −1 , a biosorbent dose of 1.29 g L −1 , and a temperature of 34.8 °C, the copper biosorption efficiency of A. xylosoxidans was estimated to be 80.15%. The chemisorption of copper ions was controlled by second-order kinetic models, while the Langmuir model indicated monolayer biosorption with a maximum biosorption capacity of 106.38 mg g −1 . Thermodynamic characteristics revealed the spontaneous and endothermic nature of Cu 2+ biosorption. Desorption and regeneration tests showed that A. xylosoxidans could be desorbed with hydrochloric acid (HCl) to recover Cu 2+ after three successive biosorption-desorption cycles. High efficiency of the bacteria strain (88.55%) was also shown when applied to a real wastewater sample, indicating effective biosorbent performance in complex environments. Based on Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy-energy-dispersive X-ray (SEM-EDX) data, it was suggested that the biosorption mechanism of copper ions involved a combination of complexation, electrostatic interactions, and ion-exchange. The efficiency of the strain in adsorbing Cu 2+ from contaminated synthetic and real wastewater strongly demonstrated the potential of A. xylosoxidans as a promising biosorbent in practical applications.