The impact of particle size on the cumulative process of street dust migration

JIANG QIAN

【Abstract】Street dust is a carrier of various pollutants widely distributed in the urban road environment， and has obvious particle size effect on environmental pollution. In this paper， the effects of particle size distribution on dust accumulation， heavy metal pollution and cleaning conditions are mainly discussed. Based on the particle size effect of street dust， the future research and control of non-point source pollution are given a breakthrough.

【Key words】Street dust； Particulate matter； Particle size； Heavy metal； Migration； Accumulation

中圖分类号： O572.3 文献标识码： A 文章编号： 2095-2457（2018）05-0166-002

1 Particle size on the impact of street dust dispersion

Street dust is a kind of environmental medium with complex composition and origin.It accumulates a large amount of heavy metal pollutants and is the"source"and "sink" carrier of heavy metal pollution in the environment [1，2].Particulate matter in street dust can migrate and transform in the atmosphere，water，soil and other environments to form a heavy metal pollution disaster chain.

Sediment particles have the potential to transport to the atmosphere under the action of human activities and winds.Particles with different particle sizes are transported to the atmosphere under the action of wind by different diffusion modes such as hanging， jumping and creeping （pushing） Pollutants[3]. When the particulate is suspended in the atmospheric environment for a long time after the start of the particulate matter without settling， it is referred to as the suspension when the particulate is moved forward at the same speed as the wind speed. Suspended particles are typically particles smaller than 100 ìm or even smaller than 50 ìm. Due to its small size and light weight， the free settling velocity in the air is very small. It is not prone to sedimentation after it is raised by the wind and can cause long-distance handling and its potential harm to the atmosphere is greater. Particulate matter with a diameter of 100ìm ～ 150ìm leaves the surface into the atmosphere under the action of wind， and moves with the air flow. However， because the density of the particles is much larger than the density of the air and the resistance is small when the air is moving， the ground can still be quite large Kinetic energy. Therefore， the falling particles not only rebound themselves but also continue to jump， and the impact of its impact on the part of the particles around the landing point to produce a continuous jump movement. Particles within this particle size range are most likely to transport contaminants to the atmosphere in a leaner manner. Larger particle size particles on the surface rolling or sliding called creep， in a certain unit of time creep mass movement can be intermittent， the creeping movement of particles can account for 20% of the total particulate matter～About 25%. Sediment particles in the particle size distribution of 0.5mm～2.0mm particle size prone to creep movement， its potential transport to the atmosphere is weak [5-6] At present， the particle size of dust particles in the street <1mm In the range of particle size， it is relatively easy to produce "lift-sedimentation-rise" cycle reciprocating phenomenon under the action of external forces such as wind and human activities，and the potential transport capacity to the atmospheric environment is strong， which is easy to affect the urban environmental quality and human health Adversely affected [7-9].

2 Particle size of dust accumulation - erosion process

The accumulation of particulate matter and heavy metals in street dust in the environment is a complex kinetic process[10]. Saheen believes that precipitation events are an important factor affecting the accumulation of pollutants in street dust and proposed a conceptual model of "accumulation-erosion" of surface pollutants in 1975， which has been widely recognized and applied [11-12]. The accumulation process occurred mainly during non-rainfall periods. The settlement of street dust was larger than that of wind and manual cleaning， which resulted in a certain amount of pollution load. Moreover， the scouring of street dust is mainly caused by precipitation， and a large amount of sediment is taken off by runoff[13]. The effect of particle size on the accumulation of street dust mainly lies in its adsorption of heavy metal pollutants[14]. The effect of particle size on adsorption is mainly influenced by the specific surface area of the adsorbate in the sediment. Generally speaking， the smaller the particle size is， the larger the relative surface area is， and the stronger is the adsorption capacity of pollutants （not considering the microscopic Voids structure）. As small particles are easily influenced by the wind and human activities， they are easy to enter the atmosphere during the sunny day. Therefore， the average particle size of street dust tends to increase with the accumulation of time. The particle size of sediment particles plays a decisive role in the migration of heavy metal pollutants in runoff[15]. Particle size larger particles difficult to wash away by precipitation runoff， the particle size range of <10ìm particles easily migrate with precipitation runoff. Among them， the <250ìm particle size level of particulate matter on precipitation runoff in the contribution of heavy metal pollution contribution of up to 80%， <44ìm particle size of suspended particulate matter precipitation runoff in total suspended particulate matter more than 70% of the urban water environment More harmful[16]. In rainfall runoff， because of the strong migration ability of small particle size particles， the smaller the particle size and the higher the erosion rate during the scouring process， the more heavy metal pollutants will enter the water body with runoff.

3 Outlook

The surface runoff of urban dust formed by dust particles in street dust is an important way for heavy metals to enter the surface water， contributing 35%-75% of the total heavy metal pollution load on urban road runoff [17-18]. With continuous research and development， it is found that proper assessment of road sediment and its loaded pollutants is an important theoretical basis for controlling surface runoff pollution in urban areas[19-20]. However， at present， there is a lack of particulate thresholds that can be used to characterize pollutants such as heavy metals and polycyclic aromatic hydrocarbons （PAHs） in road sediment， similar to PM10 for PM10. There are many factors affecting the amount of pollutants adsorbed by road sediment， but most of the current focus on the underlying surface texture， seasonal changes， cleaning efficiency and traffic intensity and so on. There are few studies on microscopic void structures that affect the adsorption of sediment particles. Therefore， it is impossible to fully understand the substantive reasons for the amount of heavy metals attached to fine particles. In addition， the existing research also found that the main cleaning methods at present are not only an important means of urban road cleaning， but the bigger part is the "builders" of dust pollution in the urban atmosphere[21]. Therefore， we can focus on the principle of adsorption of fine particle size particles to enhance their understanding of the microstructure， make a breakthrough research on the heavy metal pollution threshold of road sediment， and improve the existing cleaning methods to reduce the environmental pollution caused by fine particles.

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