title: Effects of kaolinite colloids on Cd²⁺ transport through saturated sand under varying ionic strength conditions: Column experiments and modeling approaches.
abstract: Column experiments were performed under various ionic strengths (0.0-0.9 mM) using 10 mg L(-1) of Cd(2+) without kaolinite colloids and 10 mg L(-1) Cd(2+) mixed with 100 mg L(-1) kaolinite colloids. The nonequilibrium two-site model (TSM) described the behavior of both Cd(2+) transport and Cd(2+) co-transported with kaolinite colloids better than the equilibrium model (CD(eq)) (R(2)=0.978-0.996). The results showed that an increase in ionic strength negatively impacted the retardation factors (R) of both Cd(2+) and Cd(2+) mixed with kaolinite colloids. The presence of kaolinite colloids increased the retardation factors of Cd(2+) from 7.23 to 7.89, 6.76 to 6.61 and 3.79 to 6.99 for ionic strengths of 0.225, 0.45 and 0.9 mM, respectively. On the other hand, the presence of kaolinite colloids decreased the retardation factor of Cd(2+) from 8.13 to 7.83 for ionic strength of 0.0 mM. The fraction of instantaneous sorption sites (f) parameters, kinetic constant for sorption sites (α) and Freundlich constant (K(f)) were estimated from HYDRUS-1D of TSM for Cd(2+) transport. The fraction of instantaneous sorption sites was found to increase for an increase in ionic strength. K(f) values of Cd(2+) transport without kaolinite colloids for 0.0, 0.225 and 0.45 mM were found to be higher than those of Cd(2+) transport with kaolinite colloids, except for ionic strength of 0.9 mM. Hence, the presence of kaolinite colloids probably retarded the mobility of Cd(2+) in porous media for higher ionic strengths. Furthermore, retardation factors and K(f) values of both Cd(2+) transport and Cd(2+) co-transport were shown to decrease when ionic strength increased. Interestingly, according to TSM, the fraction of instantaneous sorption sites tends to increase for an increase in ionic strength, which imply that the mechanism of Cd(2+) sorption onto quartz sand can be better described using equilibrium sorption rather than nonequilibrium sorption for an increase in ionic strength.
authors.affiliation: International Postgraduate Programs in Environmental Management, Graduate School, Chulalongkorn University, Bangkok, Thailand; Center of Excellence on Hazardous Substance Management (HSM), Chulalongkorn University, Bangkok, Thailand.
authors.forename: Rakkreat
authors.initials: R
authors.lastname: Wikiniyadhanee
authors.affiliation: Center of Excellence on Hazardous Substance Management (HSM), Chulalongkorn University, Bangkok, Thailand; Department of Geology, Faculty of Science, Chulalongkorn University, Phayathai Road, Pathumwan, Bangkok 10330, Thailand. Electronic address: csrilert@gmail.com.
authors.forename: Srilert
authors.initials: S
authors.lastname: Chotpantarat
authors.affiliation: Department of Civil, Construction and Environmental Engineering, Iowa State University, Ames, IA, USA.
authors.forename: Say Kee
authors.initials: SK
authors.lastname: Ong
mesh_terms: D002104:Cadmium / Q000032:analysis / Q000737:chemistry; D003102:Colloids / Q000737:chemistry*; D007616:Kaolin / Q000737:chemistry*; D008956:Models, Chemical; D008962:Models, Theoretical; D009994:Osmolar Concentration; D016062:Porosity; D011791:Quartz; D012822:Silicon Dioxide; D014874:Water Pollutants, Chemical / Q000032:analysis / Q000737:chemistry
chemical_list: D003102:Colloids; D014874:Water Pollutants, Chemical; D002104:Cadmium; D011791:Quartz; D007616:Kaolin; D012822:Silicon Dioxide
keywords: Cd(2+) transport; Colloids; Hydrus-1D; Ionic strength; Kaolinite