Co-Transportation of Zn (II)–Ni (II) and Inorganic Soil Colloids in Saturated Porous Media: Effects of Flow Rate, Solution Chemistry, Colloid Concentration and Porous Media

Soil colloids may be a carrier for heavy metal ions co-transporting to groundwater. Therefore, co-transportation of Zn(II)–Ni(II), important mixture contaminants in industrial sites, and inorganic soil colloids (iSC) in the saturated porous sand column were studied under various hydrologic and hydrochemical conditions such as flow rate, solution pH, ion strength (IS), fulvic acid (FA), and different sand media. The transport of iSC and sequent transport of dissolved Zn(II) and Ni(II) solution revealed the retention of dissolved Zn(II) and Ni(II) by retained iSC. Medium flow rate (1.5 mL·min⁻¹) and lower solution pH (4.0) facilitated the transport of dissolved Zn(II)–Ni(II) in co-transport with iSC while FA (10 mg L⁻¹) attenuated their transport with and without iSC. The electrolyte of NaCl (1 mmol L⁻¹) increased the transport of dissolved Zn(II) and Ni(II) while decreasing their co-transport with iSC. The co-transportation of Zn(II)–Ni(II) with iSC decreased the transport of Zn(II)–Ni(II) compared to their transportation without iSC (control group). Al oxide–coated sand obviously increased the transport of Zn(II) and Ni(II) while Fe oxide–coated sand decreased their transport. Furthermore, these data were well-fitted with two-site model and the obtained adsorption partition coefficient (K_d) indicated the variation in the retention of dissolved metals indirectly. Therefore, iSC attenuated the transport of dissolved Zn(II)–Ni(II) in co-transport of Zn(II)–Ni(II) and iSC, but the sequent washing with water facilitated the release of the remained colloid-adsorbed Zn(II)–Ni(II). This reveals that soil colloids extend the transport of colloidal-adsorbed Zn(II)–Ni(II) to the groundwater due to the sequent washing (e.g. rainwater or irrigation).