Study On Pollution Characteristics Of Colloids And Their Affinity For Heavy Metals In Road Runoff And Co-transport Behavior

Colloidal fines termed as migratory fines with dimension<10?m have been widely identified in surface stormwater runoff.With the small size,high specific surface area and strong sorption capacity,colloidal fines can often carry contaminants such as heavy metals and may persist in aqueous environment over extended periods of time,and then significantly accelerate the transport of heavy metals in infiltration facilities.Therefore,understand of the properties of colloids and their affinity for heavy metals in road runoff are important for environmental behavior prediction and control of heavy metals in urban water body.In this study,the characteristics colloids in road runoff were researched to investigate the current situation of colloids pollution and their affinity for heavy metals,the adsorption behavior and influencing factors of different heavy metals by runoff colloids and the influence of runoff colloids on the migration behavior of heavy metals in infiltration column was investigated.In order to understand the characteristics of colloids which were extracted by centrifugation separation from road runoff,the properties of colloidal fines,such as concentration,particle size and chemical property,and their affinity for heavy metals in urban road runoff with different traffic in Beijing,China were illustrated.The results showed that the concentration of colloids was high and an evident first flush effect was found in the runoff of road with heavy traffic.Most of the colloids were distributed in 1–10?m,and the chemical properties of colloids in road runoff were affected by soil erosion nearby the green belt,while the rainfall characteristics and particle size would not change the chemical properties.The heavy metals?Cu,Zn,Cd,Pb,Fe and Mn?had stronger affinity for the colloidal fines with small size.Cu,Pb,Zn exhibited significant correlations with organic colloids in the road runoff would be supposed to be potentially transport long distance,while Fe,Cd and Mn exhibited significant correlations with inorganic colloids.Zeta potentials and RMV data showed that the stability and the mobility of colloids might be decreased in the road runoff with abundant ions as well as heavy traffic.In order to explore the adsorption behavior of runoff colloid on heavy metals,the freeze-drying was performed to extract colloid powder,and the adsorption capacity of runoff colloid powder which was on Cu²⁺,Pb²⁺and Cd²⁺were investigated through static adsorption experiment,as well as the related influencing factors.The results showed that Pb²⁺,Cd²⁺and Cu²⁺were strongly adsorbed by runoff colloids.Pseudo-second-order kinetic equation and Langmuir model were the suitable ways to describe the process of heavy metal ions adsorption.At 298K,the maximum adsorption capacity of runoff colloidal for Pb²⁺,Cd²⁺and Cu²⁺were 294.27,81.32 and 31.75 mg/g,respectively.The adsorption of Cd²⁺and Cu²⁺by runoff colloids was an endothermic reaction,while the adsorption of Pb²⁺was an exothermic reaction.The adsorption of heavy metal ions by runoff colloids was significantly influenced by the pH values,and the adsorption capacity was best when the pH value is 6.Humic acid could promote the adsorption of heavy metal ions in runoff colloids.Under the coexisting of Cu²⁺,Pb²⁺and Cd²⁺,the adsorption of heavy metals by runoff colloids were inhibited.the adsorption capacity of runoff colloid on heavy metal ions were increased with the decrease of particle size.The adsorption mechanism mainly includes electrostatic attraction,hydroxyl substitution and surface complexation.Colloids in the road runoff were extracted to investigate the variation of migration concentration of Cu²⁺,Pb²⁺and Cd²⁺under coexistence of runoff colloid.Besides,the influence of water flow rate,media size,pH,ionic strength,coexist metal ions,and colloidal particle size on migration behavior of heavy metals carried by colloids were investigated.The results showed that the migration of Cu²⁺,Pb²⁺and Cd²⁺was promoted by runoff colloids.The higher the inlet flow rate,the more significant migration of heavy metals with colloids.When the flow rate increased from 5 mL/min to 10 mL/min,the relative concentrations of Cu²⁺,Pb²⁺and Cd²⁺increased by 13.8%,18.9%and 84.5%,respectively.The smaller the particle size of the medium in the infiltration facility is,the less obvious the heavy metal carrying and migration effect of runoff colloid is.Weak acid?pH=6?was the most favorable condition for the carrying and migration of heavy metals by runoff colloid.The infiltration and migration process of heavy metals carried by the colloid were inhibited due to the existence of Na⁺and various heavy metals in road runoff,and the greater the ionic strength,the more obvious of the inhibition.The colloid particle size could significantly affect the carrying capacity of runoff colloid to heavy metals,in which the infiltration and migration of heavy metals were much promoted by smaller colloids.