Regulating the exposed crystal facets of α-FeO to promote FeO-modified biochar performance in heavy metals adsorption.

α-FeO modified biochar (FeO/BC) was prepared to remove Cu(II), Pb(II) and As(V). By adjusting the calcination temperature, the morphology and exposed crystal facets of α-FeO on the biochar were changed which further affected the adsorption performance. The kinetics and isotherms were investigated systematically to reveal adsorption effect of the adsorbent on Cu(II), Pb(II) and As(V). The results indicated that chemisorption process was the dominant adsorption mechanism. FeO/BC-350 exhibited superior adsorption capacity for Cu(II) (258.22 mg/g) and Pb(II) (390.60 mg/g), and FeO/BC-250 showed relatively good adsorption capacity for As(V) (5.78 mg/g). By adsorption mechanism analysis, electrostatic adsorption, ion exchange, precipitation and complexation were coexisted in the process of removing metal ions by FeO/BC. The repeatability test and the effect of ion strength exhibited the strong stability of FeO/BC. Meanwhile, density functional theory (DFT) calculations manifested that the (202) facet of α-FeO on FeO/BC-350 possessed the lowest adsorption energies of Cu(II) and Pb(II). While for As(V), it was the (104) facet of α-FeO on FeO/BC-250 that exhibited the lowest adsorption energy. DFT results revealed that different FeO/BC had different adsorption affinities to various heavy metals. In general, this work not only prepared a promising adsorbent via a simple procedure, but also served as a reference for researchers in designing absorbents with specific active facet for efficient heavy metals remediation.