Physical and hydraulic properties of bioretention substrate using hexadecyl trimethyl ammonium bromide (HDTMA) modified zeolite

This study using hexadecyl trimethyl ammonium bromide (HDTMA) modified zeolite as a component of bioretention substrate, to investigate the effect of HDTMA modification on the basic physical and hydraulic properties of substrate layer. Two different levels of HDTMA modified zeolite (ZHD10 and ZHD50) were mixed with a mixture consists of peat soil, river sand and compost (fixed volumetric proportion at 5:4:1) with varying volumetric percentage (25%, 50%, and 75%) to form substrate media. The modification only changes the physical properties of zeolite and media with zeolite slightly, while significant changes in surface hydrophobicity and hydraulic properties were observed. A distinct decline of saturated hydraulic conductivity (Ks) values of zeolite can be observed after the modification, Ks values drop 36.5% for ZHD10 and 55.1% for ZHD50. In contrast, Ks values of substrate media using zeolite increase after the modification at the same volumetric ratio of zeolite. When 50% of zeolite (v/v%) was used in substrate, Ks for natural zeolite, ZHD10 and ZHD50 was 0.024, 0.038 and 0.075 cm/s, respectively. Such alterations in Ks are associated with the changes of surface hydrophobicity after the modification and ion exchange between modified zeolite and other materials after soaking into water. Changes in water retention characteristics (WRC) curves were in good accord with the variations in Ks, and can be interpreted by the changed Ks of tested materials. The orientations of HDTMA molecules loaded on zeolite surface were suggested to play crucial roles in altering the hydraulic properties of zeolite added substrate.