Study On The Adsorption Of Organic Matter And The Competitive Adsorption And Transport Of Heavy Metals By Aluminum Sludge Layered Packing In Rainwater Bioretention Ponds

This study adopted aluminmun water treatment residuls(Al-WTRs)as the layered filling media of stormwater bioretention with an aim to effectively removing organic pollutants and heavy metals in stormwater runoff.The objectives of this study were to investigate the morphology of Al-WTRs,the adsorption performances of organic pollutants and heavy metals under different adsorption conditions,and the transport of those pollutants by numerical simulation to solve related model parameters with Hydrus-1D and to ultimately evaluate the feasibility of Al-WTRs recycled into stormwater bioretention.In this paper,two organic pollutants,phenol and pyridine,were selected,and their adsorption behaviors onto Al-WTRs were studied by static adsorption tests.By batch tests,it was concluded that the removal rates of phenol and pyridine was positively correlated to adsorption reaction time,aluminum sludge dosage and temperature,and negatively correlated to the initial concentration of pollutants.With the increase of p H,phenol and pyridine showed opposite removal effects by Al-WTRs.Under alkaline conditions,the removal of phenol by Al-WTRs was better than the other while the removal of pyridine was better under acidic conditions.The isotherm studies showed that maximum adsorption capacity of phenol and pyridine by Al-WTRs was 10.5 mg phenol/g Al-WTRs and 3.065 mg pyridine/g Al-WTRs,respectively.It displayed that Al-WTRs had good adsorption removal ability for phenol and pyridine.However,compared with phosphorus and heavy metals,the time of adsorption equilibrium of phenol and pyridine was relatively longer,and the initial stage required longer adaptation periods.The thermodynamic analysis showed that the adsorption of phenol and pyridine was a spontaneous,endothermic,and entropy-increase process.Due to the diversity of heavy metals in stormwater runoff in actual stormwater events,this study investigated competitive adsorption under various conditions,and explored competitive adsorption characteristics of heavy metals at different p H,different initial concentration,and different temperature.The experimental results showed that Al-WTRs particles exhibited preferential selectivity to Cu²⁺under the conditions of the coexistence of heavy metal ions while Pb²⁺,Cd²⁺,and Zn²⁺adsorptions were inhibited to some extent.The preference of adsorption order was affected by the following factors including higher atomic mass,larger ionic radius,smaller hydration radius,and higher electronegativity.Because the micro-surfaces of Al-WTRs have complex physicochemical characteristics,heavy metal ions might be affected by electrostatic attraction,hydroxyl substitution,surface complexation and others.In order to further study the adsorption and transport of pollutants in stormwater bioretention systems,soil column break-through tests of heavy metals were carried out on the basis of static adsorption tests.The one-dimention transport models of heavy metals in soil columns in the single heavy metal systems and multiple heavy metals systems were established by using theconvection-dispersion model(CDE)and the chemical non-equilibrium two-zone model(TSM)with first-order kinetics to curve-fit the break-through curves,respectively.The results showed that TSM model better explained the migration behaviors of heavy metals in soil columns than CDE model,and the fitting results of the two models were statistically different.Numerically solving the f and?of the TSM model parameters showed that the f increased significantly in the ion coexistence state.It meant that the instantaneous adsorption points were affected by the gradient differences of solid-liquid concentrations,which caused the conversion of adsorption from non-equilibrium state to equilibrium state.The?became larger under competitive conditions indicating that the coexistence of other heavy metals affected the adsorption rates.