A novel nano-β-FeOOH/Fe₃O₄/biochar composite with enhanced photocatalytic performance and superparamagnetism was successfully fabricated via an environmentally friendly one-step method. The structural properties of the prepared composite were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and a vibrating sample magnetometer. The XPS spectrum of the as-prepared composites confirmed the presence of Fe-O-C bonds between β-FeOOH and biochar, which could be conducive to transfer photo-generated electrons. UV-vis spectroscopy confirmed the existence of an electron–hole connection between β-FeOOH and biochar, which promoted the rapid interface transfer of photogenerated electrons from β-FeOOH to biochar. These novel structures could enhance the response of biochar to accelerate the photoelectrons under visible light for more free radicals. Electron spin resonance analysis and free radical quenching experiments showed that •OH was the primary active species in the photodegradation process of methyl orange by nano-β-FeOOH/Fe₃O₄/biochar. In the synergistic photocatalytic system, β-FeOOH/Fe₃O₄/biochar exhibited excellent catalytic activity for the degradation of azo dye (methyl orange), which is 2.03 times higher than that of the original biochar, while the surface area decreased from 1424.82 to 790.66 m²·g⁻¹. Furthermore, β-FeOOH/Fe₃O₄/biochar maintained a stable structure and at least 98% catalytic activity after reuse, and it was easy to separate due to its superparamagnetism. This work highlights the enhanced photocatalytic performance of β-FeOOH/Fe₃O₄/biochar material, which can be used in azo dye wastewater treatment.

The adsorption performance of KOH modified biochar as adsorbent for phenol in aqueous solution was investigated. The effects of initial pH of the solution, initial concentration of phenol, contact time and temperature on the adsorption capacity of phenol were investigated. Adsorption isotherms, adsorption kinetics and thermodynamic analysis were examined. The results showed that the specific surface area of modified biochar was 487.64m²·g⁻¹, the content of C is 72.06 wt.% and the value of H/C is 0.05, the aromatization degree of modified biochar is very high. The results showed that the maximum adsorption capacity of the modified biochar is 217.06 mg·g⁻¹ at 298 K, which is higher than that of biochar. The adsorption process accords with the Freundlich adsorption isotherm model and pseudo-second-order kinetic equation. Thermodynamic analysis shows that adsorption is spontaneous and exothermic.