Enhanced removal of C.I. direct black 80 by phosphoric acid activated plant biomass supported by DFT insights.

The world is facing shortage in drinking water due to dryness and pollution. Among the most worrying pollutants are azo dyes, such as C.I. Direct Black 80, used extensively in the textile industry. Their resistance to natural degradation, their toxic potential and their visibility even at low concentrations pose a major environmental challenge. The development of high-performance, long-lasting adsorbent materials derived from unused biomass represents a promising approach to the treatment of polluted water. In this work, two activated carbons (AC1 and AC2) were prepared from discarded Zygophyllum gaetulum stems by chemical activation with phosphoric acid (H₃PO₄). Materials were characterized by scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), thermogravimetric derivative (TGD), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The effects of different operating parameters in batch tests (pH, adsorbent mass, contact time, temperature, initial concentration) were evaluated. Density Functional Theory (DFT) calculations were also performed to elucidate the electronic properties of the dye and identify the key interaction sites involved in the adsorption process. The point of zero charge (pHpzc) was defined as 5.95. Kinetic, isothermal and thermodynamic studies were carried out, as well as adsorption/desorption cycles to assess reusability. The activated carbons presented an amorphous, thermally stable structure enriched with oxygenated functional groups. Optimum conditions were achieved at pH = 3, with a dose of 1 g/L and a contact time of 1 h, enabling removal rates above 96% with corresponding maximum adsorption capacities of 146.95 mg·g⁻¹ for AC1 and 155.95 mg·g⁻¹ for AC2. The pseudo-first-order model and Langmuir isotherm showed the best fits, suggesting a mechanism dominated by physisorption at homogeneous sites. Thermodynamic parameters confirm a spontaneous, exothermic process based on physical interactions. Regeneration tests revealed yields of more 80% after five cycles. The results obtained demonstrate the high potential of activated carbons derived from Zygophyllum gaetulum as effective, regenerable and economically viable biosorbents for the treatment of dye-contaminated water, while at the same time contributing to the valorization of a so far neglected plant biomass.