Water Treatment Residual Coated Wood Mulch for Mitigation of Toxic Metals and Phosphate from Polluted Urban Stormwater Runoff

Polluted urban runoff as a primary non-point pollution source poses a serious threat to urban human and ecological health. Various urban runoff pollutants challenge the exiting stormwater treatment systems. To support the development of a sound and sustainable urban stormwater management, this dissertation targets at transforming a common landscaping practice, i.e. mulching, into a new stormwater treatment technology through coating aluminum water treatment residual (Al-WTR) on the surface of wood mulch chips. WTR is an industrial waste produced from drinking water treatment plants and primarily comprises amorphous aluminum (hydr)oxides. The Al-WTR coating on the modified wood mulch enables the adsorption ability for certain urban runoff pollutants. The primary objective of this dissertation is to evaluate the performance of the Al-WTRcoated wood mulch-based stormwater treatment technologies for alleviation of toxic metals and phosphate in urban stormwater. Three toxic metals (i.e. Cu, Zn, and Pb) and phosphate were selected as representative pollutants due to their abundance in urban runoff and high public and ecological concerns. In order to achieve the primary objective, three tasks were sequentially completed in this dissertation, including: 1) Al-WTR coated wood mulch was synthesized and characterized, followed by treatment trials to examine the treatment ability and leaching potential of the innovative filter media; 2) batch experiments were carried out to determine the adsorption kinetic and isotherm patterns of Al-WTR coated wood mulch for the four selected urban runoff pollutants, to explore the reaction mechanisms behind the adsorption, and to evaluate the effects of key factors affecting the adsorption behaviors; and 3) two-stage column studies (i.e. small column tests and semi-field experiments) were performed to study the short-term and long-term performance of Al-WTR coated wood mulch for capturing the selected urban runoff pollutants under a continuous flow state, in addition to preliminary cost analysis. Results show that Al-WTR coated wood mulch is capable of effectively removing toxic metals and phosphate from urban runoff and substantially alleviating the accumulation of these pollutants in the underlying soil. Because the adsorption is kinetically controlled, the depth of a mulch filtration bed plays an essential role in the effluent concentrations. Adsorption of these cationic metals and phosphate is achieved principally via the formation of inner-sphere surface complexes. The irreversible adsorption leads to the minimal leaching of adsorbed pollutants or aluminum from the modified mulch chips. Furthermore, preliminary cost analysis suggests that the cost of Al-WTR coated mulch is moderately increased relative to the price of raw wood mulch. The dissertation demonstrates the potential of the innovative Al-WTR coated wood mulch for revolutionizing urban stormwater treatment and has multiple positive impacts on stormwater management and water treatment industries, environment, and society