The mechanisms and monitoring of zeolite remediating chemical oxygen demand, NH4+, and Pb2+

Zeolite is utilized as an effective filling material in a permeable reactive barrier (PRB) at contaminated sites. Evaluation of the long-term performance of such barrier demands thorough understanding of the remediation mechanisms and real-time monitoring technique. In this study, the processes of zeolite remediation of three typical major contaminants from municipal solid waste sites (Chemical Oxygen Demand (COD), NH ₄ ⁺, and Pb ²⁺, respectively) were simulated with column test, analyzed with microscopic methods (MIP, BET, SEM, XPS, FTIR, and XRD) and monitored with the spectral induced polarization (SIP) technique. Zeolite remediates COD simulant, KC ₈H ₅O ₄, through cation exchange of K ⁺ and surface complexation of C ₈H ₅O ₄ ⁻. Patchy distribution of K ⁺ with size range of 16–60 μm was both observed and calculated from the peak frequency of imaginary conductivity of SIP responses, which was attributed to the preferential complexation of C ₈H ₅O ₄ ⁻ at low velocity regions (dents) on zeolite surfaces. Zeolite remediates NH ₄ ⁺ mainly through ion exchange due to the higher affinity of NH ₄ ⁺ to zeolite surfaces than that of Na ⁺. Zeolite remediates Pb ²⁺ mainly through surface complexation, with clusters of complexed Pb (II) of wide size range (a few μm to 25 μm) residing on zeolite particle surfaces. The real conductivity of SIP responses for zeolite under contaminant flow-through is primarily sensitive to the ionic strength of influx fluid. A mean size of near 270 μm was calculated from SIP responses in all three cases, which was attributed to the pore throat of zeolite. Spectral induced polarization demonstrated its capability of monitoring three processes of zeolite remediating COD, NH ₄ ⁺, and Pb ²⁺, respectively, and held promise of non-invasively monitoring of long-term performance of contaminant containing barriers.