Understanding the effect of particle size of waste concrete powder on phosphorus removal efficiency

Abstract

Reusing of fine particles of waste concrete (FPWC) is a disturbing problem for waste concrete recycling process. In this study, the FPWC developed from waste concrete was used as an innovative absorbent to remove phosphorus (P) from wastewater. The results showed, with the change of particle size of the FPWC, the maximum phosphorus binding capacity of the FPWC varies in the range of 1.07–4.96 mg P/g. The P-removal ability of FPWC increases as its particle size decreases, because of the FPWC with smaller particle size has more Ca-rich and porous hardened cement paste (HCP) powder. A “components separation” mechanism was proposed to explain the change of HCP powder content in FPWC with different particle size distribution. Both the Thomas and Yoon-Nelson model can be used to describe the P-removal behaviors of FPWC. The phosphorus removal mechanism is that the Ca2+ and OH− can release from HCP powder and forms a local alkaline condition with high Ca2+ concentration. The condition was beneficial to the formation of Ca5(PO4)3(OH) which could be attached on the surface of the FPWC. Based on the Fick’s law, the Ca2+ release behavior form FPWC can be separated into two stages, and it could be described by pseudo-second order model. Totally, FPWC developed from waste concrete blocks exhibited sufficient potential in phosphate removal.