Effects of Cocontamination by Toluene on Cr(VI) Adsorption by P-nZVI in a Soil–Water System: A Fixed-Bed Column Study

Abstract

Soil–water cocontamination with heavy metals and nonaqueous phase liquids (NAPLs) raises widespread concerns. Coexisting NAPL pollutants like toluene can antagonize the chemical processes employed for removing target heavy metals. This study investigates the impact of toluene on Cr(VI) adsorption by pumice-supported nano zerovalent iron (P-nZVI). The experiment was performed by passing a 10 mg/L Cr(VI) aqueous solution through a fixed-bed column in two batches, one without and the other with toluene. Results showed that toluene's copresence caused a 71.3% drop (3.14 to 0.9 mg/g) in P-nZVI adsorption capacity for Cr(VI). In toluene's absence, a Cr(VI) concentration in the effluent appeared after 60 h, while in its copresence, a Cr(VI) concentration appeared within 10 h. The average Cr(VI) concentration of the effluent increased from 5.05 mg/L in the first batch to 8.64 mg/L in the second batch. Breakthrough curve modeling demonstrated that the Thomas model best described the adsorption kinetics for the first case (R2 = 0.999), followed by the Yoon and Nelson and the Clark models with R2 = 0.996 and 0.994, respectively. Under toluene cocontamination, both the Thomas and the Yoon and Nelson models showed the best fit (R2 = 0.996). The inverse numerical analysis determined adsorption isotherm parameters (Ks , β, η) and confirmed that the reaction followed Langmuir behavior (β ≃ 1) with slight linearity under individual Cr(VI) presence. The results from this adsorption study, supported by adsorbent characterization before and after the reaction, demonstrated the unfavorable effects of coexisting contaminants on the removal of contaminants of concern.