Efficient Anchoring of Cu(II)–Tetracycline Complex in Paper Mill Sludge Biochar-Limited Nanospace

Efficient electron transfer between pollutants and the adsorbent is the key to low-cost purification of complex water. In this work, a paper mill sludge-based porous biochar (PSC) containing many intrinsic metal active sites, carbon defects, and oxygen vacancies can accelerate the electron-transfer process and act as a trap to efficiently remove heavy metals Cu and tetracyclines in complex water bodies. The results show that in a single Cu(II)/tetracycline hydrochloride (TC) system, Fe sites had greater electronegativity and thus were easier to exchange with Cu(II). Furthermore, the presence of Ca species caused lots of carbon defects, which accelerated the surface functional group formation of PSC and thus makes TC be quickly anchored. Notably, in the Cu(II)–TC complex system, the removal efficiency of Cu(II) and TC was significantly increased. XRD confirmed that the increased Ca–O was the primary cause of carbon defects. Therefore, the presence of carbon defects and the fast electron transfer were the essential reasons for the removal of complex Cu(II)–TC by PSC. This study rationally clarified the inherent mechanism and the potential or enhancement orientation for removing Cu(II)–TC pollutants by low-cost paper mill sludge biochar.