Investigation of the Influential Factors on the Adsorption of Amoxicillin from Aqueous Solutions Using surfactant-Modified Bentonite

The presence of pharmaceutical residues, particularly antibiotics, in aquatic environments has become a serious environmental concern because these compounds are not completely removed by conventional wastewater treatment processes. Their persistence in water can contribute to the development and spread of antibiotic resistance and may pose risks to aquatic ecosystems and human health. Among these contaminants, amoxicillin is widely used and may enter water resources through domestic, hospital, and pharmaceutical wastewater. Therefore, the development of efficient, economical, and practical methods for its removal is necessary. In this study, the removal of amoxicillin (AMX) from aqueous solutions using organoclay (OC) was investigated. The OC adsorbent was prepared by modifying raw bentonite with the cationic surfactant cetyltrimethylammonium bromide (CTAB). This modification was carried out through cation exchange and surface treatment to improve the interaction between the clay surface and the antibiotic molecules and to increase the suitability of bentonite for the removal of organic contaminants. The effects of four operating variables, including solution pH, initial AMX concentration, adsorbent dosage, and contact time, were evaluated. The experiments were designed and analyzed using response surface methodology (RSM) and a central composite design (CCD) in Design-Expert software. The investigated ranges included an initial AMX concentration of 5–205 mg/L, pH of 1–13, adsorbent dosage of 0.5–10.5 g/L, and contact time of 10–290 min. This design made it possible to evaluate the individual effects of the variables and their combined influence on the removal efficiency. The results showed that the adsorbent dosage and contact time had positive effects on AMX removal. Increasing the adsorbent dosage increased the number of available active sites, while extending the contact time allowed the antibiotic molecules to interact more effectively with the OC surface. In contrast, increasing the initial AMX concentration reduced the removal efficiency, which can be attributed to the limited number of available adsorption sites and their progressive saturation at higher pollutant concentrations. Among the studied variables, pH was identified as the most influential factor. The highest removal efficiency was obtained under alkaline conditions, particularly within the pH range of 8–10. This behavior was related to changes in the chemical species and ionization state of AMX in the solution, which enhanced its electrostatic interaction with the positively charged regions of the CTAB-modified bentonite surface. The response surface model predicted optimum conditions at a pH of 7.41, an initial AMX concentration of 59.77 mg/L, an OC dosage of 5.06 g/L, and a contact time of 166.73 min. Under these conditions, the predicted removal efficiency was 89.34%. Model validation was performed under near-optimum conditions, including a pH of 7, an initial AMX concentration of 60 mg/L, an OC dosage of 5 g/L, and a contact time of 165 min. The experimental removal efficiency was 86.06%, demonstrating acceptable agreement with the predicted value and confirming the applicability of the developed model for process optimization. Under similar conditions, raw bentonite achieved only 12.53% AMX removal. Therefore, modification with CTAB substantially improved the adsorption performance of bentonite and enhanced its potential for treating antibiotic-contaminated water. Overall, the results indicate that CTAB-modified organoclay is an effective and economical adsorbent for removing amoxicillin from aqueous media and pharmaceutical wastewater.