Phosphorous Removal from Urban River Using Concrete Blocks Mixed with Hydrotalcite Compounds

Abstract

Concrete blocks with phosphorous adsorption ability have been developed to remove non-point source phosphorous from rivers and canals. The concrete blocks were prepared by mixing cement with a hydrotalcite compound (HT) that has high selectivity and large phosphorous adsorption capacity. The concrete blocks (test pieces) with various the filling ratios of 85, 90 and 95% were prepared using fresh concrete, and the result of the slump test was 0 cm. Also, the compressive strength and phosphorous removal capacity were investigated fundamentally. The results showed that the compressive strength of all the test pieces was above 18N·mm^-2 and they were sufficiently strong for use as laying blocks. For a test piece with a low filling ratio, phosphorous was removed by Ca²⁺ and for a test piece with a high filling ratio, phosphorous was not removed by ion exchange. However, these results suggest that high strength and high phosphorous adsorption ability are compatible with each other, which can be controlled by regulating the mix proportion. Furthermore, field tests were conducted using large concrete blocks containing HT (HT specimens). The results showed that phosphorous was adsorbed at the uppermost layer of the HT specimens. In the tests, the break-through of phosphorous adsorption occurred three months later and the average removal rate for such break-through was 3.69mg P·m^-2·d^-1.