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.