Enhancement of petroleum hydrocarbon adsorption performance by attapulgite modified biochar: Performance and mechanism analysis

Two biochar-based adsorbents, namely original corn cob biochar (BC) and attapulgite (ATTP)-biochar composite (BA), were prepared via two-step pyrolysis at 400℃ and 700℃ under oxygen deficiency for petroleum hydrocarbons removal from water. Experimental results revealed that attapulgite modified the structure of biochar, increased the quantity of surface functional groups, and thereby significantly enhancing its adsorption capacity. Petroleum hydrocarbon adsorption experiments showed that adsorption kinetics was more accurately characterized by the pseudo-second-order model, and isothermal adsorption by the Freundlich model, supported by R² and error analysis. This finding suggested that chemisorption through multi-molecular layers was the predominant mechanism of adsorption. Regarding the effect of pH, BC exhibited the maximum adsorption capacity under weakly acidic conditions (pH=5.0), while BA achieved optimal adsorption performance in neutral to weakly alkaline environments (pH=7.0-9.0), and BA exhibited an adsorption rate 41.8% higher than that of BC. In terms of salinity, it exerted a notable influence on the adsorption capacity of biochar; however, BA demonstrated superior adaptability over a wider salinity range (0.5% to 8.0%), and a 65.14% increase in overall adsorption efficiency compared to BC. Gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared spectroscopy (FTIR) indicated that the adsorption mechanism primarily encompassed surface adsorption, interfacial adsorption, micropore filling, hydrogen bonding, π-π bond interactions, and chelation effects. Additionally, specific redox reactions might have occurred alongside the adsorption process. In conclusion, this low-cost, environmentally friendly, and highly efficient carbon material held considerable promise for the removal of oil pollutants in saline-alkali environments.