Adsorption behaviors and mechanisms of Al-Fe dual-decorated biochar adsorbent for phosphate removal from rural wastewater

The discharge of phosphorus from rural wastewater causes eutrophication, so phosphorus removal is regarded as an important part of water treatment. In this study, corn biochar (BC) was prepared at pyrolysis temperature of 450 °C, and then Al-Fe dual-decorated biochar (AFBC) was obtained through a two-step modification method (AlCl3 immersion before FeCl3 immersion) on the base of BC. The characterization results showed that Fe and Al were decorated on corn biochar in the form of FeOOH, Fe2O3, and AlOOH. Compared with Al/Fe mono-decorated biochar (ABC/FBC), AFBC exhibited superior phosphate removal performance in a wide pH range (pH 4.0–11.0). The adsorption behavior of AFBC was a spontaneous exothermic process, and conformed to the Langmuir and pseudo-second-order kinetic models. The adsorption mechanism was dominated by the electrostatic attraction and coordination between metal (hydrogen) oxide on biochar surface and phosphate. In the dynamic adsorption-desorption experiment, AFBC could reduce total phosphorus (TP) from 5.54 mg/L to 0.5 mg/L within 40 BV of actual rural wastewater. In addition, using phosphate-loaded AFBC as a soil conditioner promoted the growth of ryegrass, and it also provided an effective method to achieve bio-waste resource utilization and phosphorus reuse.