5. Conclusions
The test results indicate that the Pb2+ deposition concentration rapidly decreased with increasing migration distance. On the whole, with increasing SP particle size, the SP deposition amount will significantly increase, while the Pb2+ deposition amount will conversely decrease. Actually, the presence of SPs may promote or inhibit Pb2+ migration, which seems to be closely related to the concentration of injected Pb2+, the particle size and concentration of injected SPs, the seepage velocity, and the change in absolute zeta potential in the surface charge.
Pb2+ deposition increases with increasing SP injection concentration; in addition, more Pb2+ will be adsorbed onto the surface of SPs and migrate or be deposited as coupled Pb2+ and SPs with the flowing water. The heavy metal ions adsorbed onto the SPs will change the dielectric properties of the SPs, resulting in positively charged surfaces. The theoretical predicted results are in good agreement with the test results, indicating that the deposition-release model presented in this paper can suitably reflect the transport process of a single suspension by seepage.
The microstructure photos reveal that larger SPs are first deposited within a relatively short distance from the injection end of the sand column, while SP deposition gradually decreases with increasing migration distance. Clearly, the median particle size of the deposited SPs near the injection end will be larger than that of the injected SPs and then gradually decrease with increasing migration distance, which represents a clear particle-separation characteristic due to the flowing water.