Influence of Na-bentonite colloids on the transport of heavy metals in porous media

Abstract

In this work, the influence of Na-bentonite colloids on the transport of Cu, Pb and Zn in porous media was investigated. For the transport experiments a "short pulse" laboratory column system was used. Quartz sand served as column packing material. The metal solutions were injected into the column in the presence and absence of colloids. The quantification of metals at the column outlet was carried out by coupling the column system with an inductively coupled plasma mass spectrometer (ICP-MS). The determination of Na-bentonite colloids was done online by means of a UV detector and the ICP-MS system. Characterisation of the colloid-metal interactions was based on sorption experiments and modelling calculations carried out at pH values of 5 and 7. Furthermore the stability of Na-bentonite colloids was determined in titration experiments. The titration experiments showed that in the range 4 ≤ pH ≤ 12 Nabentonite colloids remained stable. At lower pH values (2 < pH < 4), bentonite particles aggregated rapidly. In the sorption experiments a small pH-dependence of the copper and lead adsorption onto bentonite could be detected. For zinc no significant influence of pH value on the sorption behaviour was found. The results of modelling calculations showed a very high amount of adsorbed heavy metals and were therewith in good agreement with the experimental data. In the transport experiments, Na-bentonite colloids showed high mobility at pH value of 7. At pH 5, the deposition of Na-bentonite particles was observed, which was different in the presence of different heavy metals and increased in the following order: Pb < Zn < Cu. In the absence of Na-bentonite colloids no significant transport of heavy metals was found. In the presence of Na-bentonite a col loid facilitated transport for all heavy metals was observed. The amount of transported heavy metals increased in the following order: Pb < Cu < Zn. The colloid facilitated transport of heavy metals was influenced by the particle deposition and competitive sorption processes between Na-bentonite and column packing material.