Heavy Metal Removal Mechanism And Risk Assessment Through Bioretention Technology

With the rapid development of urbanization, increasing urban impervious areas interruptstormwater infiltration channels and result in a sharp increase of stormwater runoff volumeand peak flow. In addition, due to human activities, atmospheric deposition and other factors,the impervious surface accumulated a large number of pollutants, which discharged into themunicipal storm sewer by the stormwater runoff flushing and ultimately into the cityreceiving water bodies. With the increasing perfection of the city point pollution control,non-point pollution caused by stormwater runoff has become an important pollutants sourceof the watershed. Heavy metal is different from organic pollutants for its characteristics ofpollutant persistency, area widely and management difficult, therefore, it is difficult to bedegraded in environment and easy to be accumulated in the human body through food chainand other ways. It will produce toxic action to the human body when reaches a certainconcentration, which has become one of hot researches.Laboratory and field test results about urban stormwater runoff heavy metal removal inbioretention showed that90%of the heavy metal can be removed. Based on foreign researchmethods and contents, carried on the bioretention improved experiment of heavy metalremoval and provided the basis for the application; studied the chemical speciation,accumulation and migration characteristics of heavy metal in bioretention media, whichprovides the basis for bioretention media life; carried out the risk assessment for applicationand reuse in different ways and analyzed its harm or loss, which provides the basis for theoperation and management of bioretention measures. The main contents and conclusions areas follows:(1) Sand, zeolite, sandy soil and quartz sand were selected as bioretention media;synthetic runoff with different pollutant loadings (COD, nitrogen, phosphorus and heavymetals) were periodically applied. The removal of different bioretention media on pollutantsfrom stormwater runoff and its influence factors and effect of pH on the stability of heavymetals from accumulate internal bioretention media were studied. Experimental resultsshowed that removal efficiency of phosphorus was better, there were some removal of CODand nitrogen but less stable. Four kinds of media had high removal efficiency on heavy metals,the average removal rate over than90%and the removal rate of Pb was close to100%. Mediatype had a certain selectivity for heavy metals removal. Effluent concentrations of heavymetals decreased with medium depth increases, increased with influent concentrationincreases and decreased with increasing rainfall interval, however, they had little effect on theremoval efficiency of heavy metals and the average removal rate still over than90%. COD inthe stormwater runoff had significantly inhibited effects on removal of Zn and Cd, theinfluence of different media on the Cu was different and the inhibition wasn’t obvious, andpromoted the removal of Pb. Under acidic conditions, there were obvious heavy metalsdissolution process in four kinds of media; pH value was lower, the more the dissolution ofheavy metals, and change process of dissolution concentration of different heavy metal elements in the same medium was similar.(2) Continuous monitoring of four bioretention columns were conducted, the chemicalspeciation, accumulation and migration characteristics of heavy metals in different mediawere studied. The results showed that heavy metals within the media exist mainly in theresidual fraction. The Zn and Cd were largely associated with soluble-exchangeable fraction,followed by sorbed-carbonate fraction; Cu was largely associated with sorbed-carbonate andsorbed-organic fraction; Pb was largely associated with sorbed-carbonate and oxidizablefraction, which showed that heavy metals in the media were more stable. Four kinds of heavymetals showed a strong accumulation characteristic in the surface layer. Accumulation of Znand Cd in planting soil have reached severe accumulation, Cu and Pb were moderate. The Cucontent decreased with increasing depth, Zn and Cd did not exist obvious stratification rule,but still higher in the surface layer, Pb content was increasing. With the increase of mediumdepth, pollution degree of heavy metal accumulation reduced. Most of the heavy metals intobioretention cell were removed in0~20cm soil layer, the heavy metals concentrationsincreased with medium depth increase. With the continuous operation of the device, the heavymetals concentrations had increasing trend in-40and60cm depth. The migration of heavymetal ion in the medium increased with the increase of time, the range of migration andconcentration distribution of different heavy metal ions in the same medium was different.(3) Construction wastes were selected as the adsorbents, static and dynamic adsorptionexperiments were carried out to investigate the adsorption of heavy metals to constructionwastes with different particle size in simulated stormwater runoff system. The experimentalresults have shown that the pseudo-second-order kinetics model could better characterize theadsorption processes of heavy meatlas than the pseudo-first-order kinetics model. TheFreundlich isotherm model was successfully used to fit the adsorption equilibrium data. Theconstruction wastes with the four tested sizes could greatly remove heavy metals fromstormwater runoff and their average removal rate over than90%. The particle size gradationof construction wastes greatly influenced the equilibrium time, rate and the capacity of heavymetals adsorption.(4) When stormwater is used to recharge ground water, water quality is better, theconcentration of heavy metals meet the Environmental Quality Standards for Surface WaterGrade Ⅲ; when used to recharge groundwater, risk grade of COD was high, Cd wasintermediate; when used in landscape water, city miscellaneous and industry, the risk gradewas lower and meet Recycling Water Quality Standards.