Effectiveness and mechanisms of ammonium adsorption on biochars derived from biogas residues

The solid by-product of anaerobic digestion derived biochar has been used as an adsorbent for the liquid by-product and is a viable and environmental friendly way to control waste. In this study, two biochars (BC-PM, BC-ST) from pig manure biogas residue (BR-PM) and straw biogas residue (BR-ST) were assessed for ammonium adsorption. The ammonium adsorption on both BC-PM and BC-ST followed the Elovich kinetic model and fit well with the Langmuir isotherm, whereas BC-PM was better than BC-ST at absorbing ammonium. In addition, the adsorption mechanism was elucidated by analysing the physicochemical properties of the biochars. The Brunauer-Emmett-Teller (BET) surface area, pore structure and micromorphology, which are mainly related to the carbon in the biochars, had no direct correlation with the ammonium adsorption capacity. However, the ash in the biochars played an important role in ammonium adsorption. The metal elements in biochar ash significantly decreased after adsorption, especially potassium, which nearly disappeared as a result of ion exchange with ammonium. However, the quartz and carbonate mineral in biochar ash also functioned as ammonium adsorption sites according to Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) analysis. This study revealed the irreplaceable role of ash in biochars for ammonium adsorption, which will guide the preparation of biochars from different types of feedstocks for ammonium adsorption.