Hybrid biochar/layered double oxide for phosphate and humic acid removal from water: dominant role of memory-effect-driven anion intercalation.

A biochar/layered double oxide (LDO) composite was successfully developed for the simultaneous removal of phosphate (P) and humic acid (HA) from aqueous solutions. The composite was synthesised by in situ co-precipitation of Mg2+ and Al3+ salts to form a Mg/Al-layered double hydroxide (Mg/Al-LDH) on palm fibre-derived biochar, followed by calcination at 500 °C. X-ray diffraction confirmed the successful formation of crystalline Mg/Al-LDH with a basal spacing (d 003) of 0.755 nm. The specific surface areas of PF-biochar, biochar/LDH, and biochar/LDO were 465.2, 268.3, and 241.2 m2 g-1, respectively. The Langmuir maximum adsorption capacities for P and HA followed the order: biochar/LDO (28.6 and 152.9 mg g-1) > biochar/LDH (15.0 and 112.7 mg g-1) > PF-biochar (7.40 and 74.1 mg g-1). Thermodynamic analysis showed that the adsorption of P (ΔH° = 15.92 kJ mol-1) and HA (ΔH° = 13.56 kJ mol-1) was endothermic and predominantly governed by physical interactions. XRD analysis of biochar/LDO revealed that the basal spacing of the reconstructed LDH increased from 0.741 nm after rehydration in water to 0.882 nm and 1.147 nm following phosphate and HA adsorption, respectively, demonstrating anion intercalation associated with the memory effect. In addition to the memory effect, phosphate adsorption involved electrostatic attraction and surface complexation, whereas humic acid adsorption additionally involved electrostatic attraction, hydrogen bonding, and π-π interactions. The estimated direct operating cost for preparing the biochar/LDO composite was approximately 1.86 (US$ per kg). These findings demonstrate that the biochar/LDO composite is a promising low-cost and reusable adsorbent for the simultaneous removal of organic and inorganic contaminants from wastewater.