Insight into the Adsorption Behavior of Carbon Nanoparticles Derived from Coffee Skin Waste for Remediating Water Contaminated with Pharmaceutical Ingredients

Abstract

Coffee skins, a cheap, agricultural waste, were carbonized in a tubular furnace under a nitrogen stream and then ball milled to fabricate coffee skin-carbon-nanoparticles (CCNPs). SEM showed 35.6–41.6 nm particle size. The 26.64 and 43.16 peaks in the XRD indicated a cubic graphite lattice. The FT-IR broadband revealed a 2500–3500 cm−1 peak, suggesting an acidic O-H group. CCNPs possessed a type-H3-loop in the N2-adsorption-desorption analysis, with a surface of 105.638 m2 g−1. Thereafter, CCNPs were tested for ciprofloxacin (CPXN) adsorption, which reached equilibrium in 90 min. CCNPs captured 142.6 mg g−1 from 100 mg L−1 CPXN, and the 5:12 sorbent mass-to-solution volume ratio was suitable for treating up to 75 mg L−1 contamination. The qt dropped from 142.6 to 114.3 and 79.2 mg g−1 as the temperature rose from 20 °C to 35 °C and 50 °C, respectively, indicating exothermic adsorption. CPXN removal efficiency decreased below pH 5.0 and above pH 8.0. Kinetically, CPXN adsorption fits the second-order model and is controlled by the liquid-film mechanism, indicating its preference for the CCNPs’ surface. The adsorption agreement with the liquid-film and Freundlich models implied the ease of CPXN penetration into the CCNP inner shells and the multilayered accumulation of CPXN on the CCNPs’ surface. The negative ∆H° and ∆G° revealed the exothermic nature and spontaneity of CPXN adsorption onto the CCNP. The CCNPs showed an efficiency of 95.8% during four consecutive regeneration-reuse cycles with a relative standard deviation (RSD) of 3.1%, and the lowest efficiency in the fourth cycle was 92.8%.