Abstract
Sand column tests were conducted to investigate the seepage transport of silicon powders (SPs) with two wide particle size ranges (30-2000 nm and 2-70 μm), including the cotransport of SPs and copper ions. The results show that the graded large-scale SP has an obvious inhibiting influence on the transport of copper ions. In contrast, in the presence of the graded small-scale SP, the concentration of copper ions in the effluent tends to increase; i.e., there appears to be a promoting effect. However, after a long transport distance, the presence of SPs, regardless of particle size, has an overall retarding effect on heavy metal pollutants (e.g., copper ions). The promoting effect of the increase in seepage velocity on the concentration of copper ions in the effluent is greater with the graded large-scale SPs than with the graded small-scale SPs. In terms of the microstructural characteristics by metallographic microscopy, the average particle size of the deposited graded small-scale SPs is almost constant at different transport distances, while that of the deposited graded large-scale SPs tend to decrease significantly with increasing transport distance; i.e., notable bed filtration is exhibited in the latter case. This physical mechanism also determines the sequence and rate of the retarding effect of SPs on heavy metal ions under seepage flow.
Highlights
IntroductionThe transport processes, retardation mechanisms, and control measures of heavy metal pollutants in permeable soils remain an important topic [1, 2] in the fields of environmental geotechnical engineering, ocean engineering, hydrology, engineering geology, municipal engineering, and even agricultural sciences [3], covering the seepage control of municipal solid waste landfills and industrial tailings ponds [4,5,6], treatment of contaminated groundwater [7,8,9], mining processes of metallic minerals [10, 11], environmental damage caused by the use of pesticides and fertilizers [12], and treatment of municipal wastewater [13, 14]
Many researchers have studied the influences of different types of suspended matters (e.g., montmorillonite colloid, humic acid, and silicon powder (SP)) on the seepage migration of Pb and Cd, concluding that suspended particles can promote the transport of these heavy metal ions under certain conditions [19, 20], with specific occurrence conditions given in some literature [21, 22]
To investigate the effects of pore size distribution on particle size selection and particle recovery, Ikni et al [48] studied the seepage transport process and the bed filtration of fine sand by using step-input injections; porous media with different pore sizes were utilized in the experiments, and the results showed that the mixture medium had a wider pore size distribution and a higher particle retention, with the coarser particles being the first to be recovered
Summary
The transport processes, retardation mechanisms, and control measures of heavy metal pollutants in permeable soils remain an important topic [1, 2] in the fields of environmental geotechnical engineering, ocean engineering, hydrology, engineering geology, municipal engineering, and even agricultural sciences [3], covering the seepage control of municipal solid waste landfills and industrial tailings ponds [4,5,6], treatment of contaminated groundwater [7,8,9], mining processes of metallic minerals [10, 11], environmental damage caused by the use of pesticides and fertilizers [12], and treatment of municipal wastewater [13, 14]. The movement of heavy metal pollutants and tiny suspended particles in soils is a complex process, in which the two either promote or inhibit each other. Many researchers have studied the influences of different types of suspended matters (e.g., montmorillonite colloid, humic acid, and silicon powder (SP)) on the seepage migration of Pb and Cd, concluding that suspended particles can promote the transport of these heavy metal ions under certain conditions [19, 20], with specific occurrence conditions given in some literature [21, 22]. The promoting effect of suspended particles gradually diminishes after their size exceeds a certain threshold, and the presence of heavy metal ions inhibits the transport of suspended particles to a certain extent, suggesting a coupling mechanism between the two Bai et al [23] discussed the influence of suspended particles on the migration of lead ions in porous media under pulse injection, showing that the existence of suspended particles promotes the migration of lead ions and that this promoting effect changes significantly with changes in particle size; they attributed this finding to the socalled “size exclusion effect.” the promoting effect of suspended particles gradually diminishes after their size exceeds a certain threshold, and the presence of heavy metal ions inhibits the transport of suspended particles to a certain extent, suggesting a coupling mechanism between the two
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