3.1 Differences in physical characteristics
In this paper, the morphological characteristics of suspended sand particles are compared in terms of both particle size and shape, and the geometric mean grain size (GD) is used to measure the suspended sand grain size. Fewer materials are deposited in the faster flowing waters of the plain river network, and the water column is dominated by materials with large particles, and the larger particles are more gravitational and less likely to be carried along with the flow of water. Conversely, in the slower flowing waters of the plain river network, it is difficult for the river to carry small particles to flow because of the slower flow and the relatively high amount of sedimentary material. Therefore, on the whole, the AVGGD in the slow flow area (5.15µm) is smaller than the AVGGD in the flat flow area (5.56µm) and smaller than the AVGGD in the strong flow area (5.77µm); the S.DGD of the particle population ranges from 5.1 to 6.1, and the corresponding S.D values are smaller at the points with smaller GD in the lake network, i.e. the particle size is relatively concentrated and uniform. For example, the smallest GD value at the slow-flowing area of the river network is 3.95 µm at the Lu Dian Qiao Bin, and the corresponding S.D value is 4.91.
Roundness (RO) and roughness (FD) were used to measure the shape characteristics of the suspended sand particles. UELRO between 0.15 and 0.21. The FD values of the suspended sand particle population ranged from 1.077 to 1.080, with small differences in FD values within each flow velocity zone, and the AVGFD in the slow flow velocity zone (1.078) was smaller than the AVGFD in the advection velocity zone (1.079) and smaller than the AVGFD in the strong flow velocity zone (1.080), the S.DFD of the particle population ranged from 0.017 to 0.020, and the UELFD between points within each zone ranged from 0.001 ~ 0.003.
The K-S significance tests for GD, RO, FD in each flow zone of the riverine network were all below 0.001, so the geospatial variability of GD, RO, FD was highly significant. According to the statistical analysis of particle shape, in the waterfront river network, the heterogeneity of physical properties within a point is S.DFD < S.DRO < S.DGD; the heterogeneity of physical properties between points is also UELFD < UELRO < UELGD; combined with the results of the inter-regional variability analysis, it is clear that with the refinement of the evaluation of the physical properties of particles from GD, RO to FD, the variation of particle morphological properties The variation between regions, although still different, tends to be the same overall.
3.2 Differences in chemical properties
From SEM-EDS analysis, it can be seen that the suspended sand particles in each sample within the river network of the shoreline contain the four basic elements of C, O, Al and Si, and the proportion of the four basic elements of C, O, Al and Si in each point is higher than 95%. The geographical and spatial differences make the characteristic elements different in different areas of the lakeside river network. The abundance of the slow-flowing and strong-flowing areas of the river network is relatively similar, with some points containing the characteristic elements Mg, Fe and Na, while the abundance of the flat-flowing areas is relatively high, with some points containing the characteristic elements Mg, Fe, Na and K. In general, there is one more element species in flat velocity zone than in slow velocity zone and strong velocity zone. The K-S test for differences in the elemental content of suspended sand particles between the flow velocity zones showed that the elemental differences between the strong, flat and slow flow velocity zones were all less than 0.05, so the elements in the three flow velocity zones were different and highly significant.
There are significant differences in the elemental species within the zones, with the basic elements C, O, Al and Si being present within the different flow rates. In the strong flow zone, both Hongqiao and Liqiao of the Liangxi River contain the special elements Mg, and Qingqi Bridge of the Liangxi River contains the special elements Mg, Fe and Na; in the flat flow zone, Caowangjing Dongjiang Bridge contains only the basic elements at the point, and Liangdong Bridge and Caowangjing Dongjiang Bridge of the Panbu Bridge contain the special elements Mg, Fe, Na and K in addition to the basic elements. In the slow flow zone, the Rao Xiu Bridge site at Lu Dian Qiao Bang contains only basic elements, while the Wenge and Han Cui Bridge sites at Lu Dian Qiao Bang contain Mg, Fe and Na special elements in addition to basic elements. The points containing fewer elements are all located at the connection of lakes and rivers, while the points containing more specific elements are mostly located in the middle of the river network, and these differences in location lead to large differences in element types between points.
The K-S test of elemental content within the regions shows a large variation in elemental content within the three flow velocity regions. The highest abundance and the highest content of characteristic elements were found in the flat velocity zone, and the average percentage content of characteristic elements were Mg (0.25%), Na (0.15%), Fe (0.14%) and K (0.13%). The average percentages of the characteristic elements in the slow-flowing zone are Mg (0.17%), Na (0.07%) and Fe (0.05%) in descending order, while the average percentages of the characteristic elements in the strong-flowing zone are Mg (0.32%), Na (0.08%) and Fe (0.05%) in descending order. The average percentages of the characteristic elements in the different flow velocity zones are in the same order from highest to lowest, with M being the characteristic element with the highest average percentage content in each zone.
Figure 4 Distribution of elemental content of suspended sediment (a) to (c) average content of elemental mass fraction (W%) and elemental percentage (A%) in the three flow regions (d) basic elemental percentage at the point (e) characteristic elemental percentage at the point.
3.3 Reason for variance
In this paper, the reasons for the spatial differences of suspended sand particles and elements were analyzed from hydrology and water quality conditions and characteristics of eroded soil. Hydrological water quality conditions are the main reason for the difference in suspended sand particle morphology. The parameters related to hydrological water quality conditions include total nitrogen (TN), total phosphorus (TP), flow velocity (V) and suspended particle concentration (SSC). The Pearson correlation test was conducted between the parameters related to hydrological and water quality conditions and the morphological parameters of suspended sand particles (GD, FD, RO), and the test results showed that the GD, FD, RO of suspended sand particles were positively correlated with the flow velocity, in which the FD of suspended sand particles was significantly positively correlated with the flow velocity. The GD, FD and RO of suspended sand particles were not correlated with TN, TP, V and SSC. This means that the particle population is larger and more spherical in shape at high flow rates in the waters of the lakeside network.
Particle size is one of the most important factors affecting the content of metallic elements in the particulate state (Vandecasteele et al., 2002), and Al is the most abundant metallic element in the earth's crust. It has been shown that the content of Al in particulate matter increases linearly with decreasing particle size, therefore the content of Al in particulate matter can reflect the size of the particle size (Hill and Aplin, 2001). The influence of particle size effects on the elemental content of metals in the particulate state was investigated in the water system of the Lake Shore network by studying the correlation between the elemental content of Al contained in the particulate matter and the content of the remaining metal elements. The results show that the elemental content of Al shows a strong correlation with the elemental content of Mg and Si.
As soil is an important source of suspended sand in water bodies, the influence of eroded soil properties on the elemental composition of suspended sand particles is considered here (Du et al., 2022). In terms of soil particle size, the lowest gravel content was found in the estuary of the flat-flow velocity zone and the highest in the estuary of the slow-flow velocity zone; the difference in gravel content between the two was four to eight times, which was a significant difference. The gravel content in the study area ranged from 571.1 to 650.5 g/kg, all of which were greater than 500 g/kg, and the gravel content in the estuary of each flow velocity zone was slightly higher than that in the near-shore sites, and the particle size of the soil was larger. The sand and clay content in the estuaries of the low-flow zones are comparable, both being about 110 g/kg, while the sand content in the estuaries of the flat-flow zones is slightly lower than the clay content, and the difference in clay content between the low-flow and flat-flow zones is significant, making the flat-flow zones more abundant than the slow-flow and strong-flow zones, and having special elements K.
3.4 Heavy metal adsorption capacity
Particulate matter is the main carrier of pollutant diffusion in the water column, the reasons for this phenomenon include the strong sorption of heavy metals and nutrients. The sorption capacity of suspended sand particles for pollutants is not only influenced by the physical characteristics of the particles (particle size, etc.) and the chemical environment of the water column (ph, salinity, organic matter, etc.), but the ability of suspended sand particles to sorb Zn and Ni heavy metals also has a significant positive correlation with the particle Al content (R2 > 0.69), i.e. the elemental characteristics of suspended sand particles are related to the sorption capacity of Zn and Ni.
Heavy metals present in the water column as ions are readily adsorbed, complexed or co-precipitated by sediments or particulate matter suspended in the water column. In this paper, the influence of the two-dimensional morphology and elemental properties of suspended sand particles on the enrichment capacity of heavy metals is explored using Ni as an example.
Metal enrichment factors (EFs) are often used to help assess whether metals are enriched in natural concentrations. The calculation of EFs usually involves normalizing metal concentrations to Fe or Al. Al was chosen for geochemical normalization in this study because it has low yield variability, is considered a clastic component of sediments, and is the most frequently used element in coastal and estuarine environments (Kersten and Smedes, 2002; Maanan et al., 2015). Normalization of conserved elements explains the diagenetic and sedimentary inputs of elements of interest, enhancing the prediction of anthropogenic contamination by enrichment factors (Duodu et al., 2016).
Studies have shown that the sorption capacity of heavy metals is related to the particle size, specific surface area and surface roughness of the particles. In previous studies, only the particle size of the particulate matter was considered, and the smaller the particle size, the stronger the heavy metal adsorption capacity. This research approach approximates the particulate matter as a smooth sphere for analysis, ignoring the influence of factors such as the surface roughness of the particulate matter on the adsorption capacity. In this study, the particle size, roundness and roughness are taken into account to fit the shape function of the particles, thus proposing a comprehensive shape function model that affects the Ni enrichment capacity.
SP = F1(GD,RO,FD)=k1RO + k2FD + k3/GD + k4 (3)
In the formula: SP is the shape factor; k1, k2, k3, k4 are the parameters.
In this study, four particle physicochemical characterisation factors, GD, RO, FD and Al content (Al%), were used as analytical variables, and the enrichment factor AE was used to characterise the sorption capacity of suspended sand for Ni in order to remove the interference of soil background values.
Based on the data of particle roundness, roughness and mean particle size, multiple regressions were carried out in combination with the functional model (3) to obtain the integrated shape function for each region. Combining the regional integrated shape functions with the functional model (4), the function AE was obtained for the influence of the physical and chemical properties of suspended sand particles on the sorption of heavy metals as follows:
AE = aF1(GD,RO,FD)+bF2༈Al༉+c ༈4༉
Where: AE is the ability of suspended sand particles to enrich for heavy metals; F1 is the effect of morphological properties; F2 is the effect of elemental properties; a, b and c are parameters. To consider the practical significance, the constraints a > 0 and b > 0.
A comparison of the linear fit of the Ni enrichment factor EF with particle AE, SP, GD and 1/GD in each region shows that the interpretation of particle shape in the riparian network is improved by considering the particle shape factor compared to the effect of particle size only (EF-GD or EF-1/GD). A combination of particle shape factors (RO, FD) and Al content (EF- AE) gives the most accurate interpretation of Ni sorption by particles.
Considering lakeside river network area, the parameters of high, moderate velocity (a, b, c, d, k1, k2) average processing to \(\stackrel{-}{AE}\), this value can better evaluate the suspended sediment particles of Ni concentration skills.
The comparative analysis of EF, AE and GD at each site showed that the AE values at each site in the Lake Shore network fit better with the EF values relative to the GD values, while the suspended sand values were proportional to the EF values in all areas except for the Wenge Bridge in the Lake Shore network. Therefore, the AE values are better able to assess the sorption of Ni by suspended sand at each point in the area than the GD values (Fig. 6-d). The highest Ni sorption by suspended sand particles was found in the slow-flowing plain river network (EF = 4.28; AE = 4.42).