Correlation analysis between the content of each component and the adsorption capacity of the sediment in the direction of water flow was carried out. The correlation between the adsorption amount and the OM content, CEC, and iron and aluminium oxide contents did not correspond to the change trend of the adsorption amount. It can be seen from Table 5 that at 0–10 cm, the adsorption of Cu in the sediment had a high correlation with aluminium oxide (0.907), but the change trend of the adsorption amount was not significant. At 0–10 cm, the correlation of adsorption capacity with CEC was 0.943, but the adsorption capacity did not increase with increasing CEC. At 10–20 cm, the adsorption of Zn was correlated with OM (0.604), but it did not increase with increasing OM.
These results showed that a high content of a certain sediment component does not mean that this component is the only factor driving the adsorption process. The overall effect may be due to the change in the surface potential of sediment particles. Studies have shown that (14 after removing iron and aluminium oxides in soil, the amount of soil adsorption of Pb2+ and Cd2+ increases. This increase occurs because Pb2+ and Cd2+ can interact with the remaining citrate in solution to form acid salt complexes. The complexes are adsorbed on the surface of the soil particles, increasing the negative potential. Another factor may be that even if the content of a certain component is high, other intertwined components may block or overlap adsorption point sites. A study found 15 that iron oxide can mask some charge sites of soil particles.
Table 5
Correlation between adsorption capacity and sediment components at three depths along the direction of water flow
Depth
|
Cu
|
Zn
|
|
OM
|
CEC
|
Iron oxide
|
Aluminium oxide
|
OM
|
CEC
|
Iron oxide
|
Aluminium oxide
|
0–10 cm
|
0.626
|
0.619
|
0.721
|
0.907
|
0.799
|
0.943
|
0.499
|
0.518
|
10–20 cm
|
0.044
|
-0.297
|
-0.2
|
0.037
|
0.604
|
-0.444
|
0.411
|
0.217
|
20–30 cm
|
-0.699
|
0.361
|
0.177
|
0.147
|
0.115
|
-0.108
|
0.479
|
0.489
|
To further explore the relationship between OM content, CEC, iron and aluminium oxide contents and adsorption capacity in the direction of water flow, redundancy analysis (RDA) was applied using Cu and Zn as species and the OM content, CEC, and iron-aluminium oxide content in the sediment as environmental factors. The analysis results showed that the correlation between the adsorption of Cu and Zn and each component of the sediment was different.
Figure 5 shows Cu, iron oxide and aluminium oxide at angles less than 90°, indicating that iron and aluminium were correlated more strongly with Cu adsorption than were other components of the sediment, while OM may be more strongly correlated with Zn adsorption.
Tables 6 and 7 show that the contribution rates of OM and aluminium oxide in the adsorption process were higher than those of CEC and iron oxide. The contribution rate of OM in the entire adsorption process reached 72%, with a maximum significant difference of 0.002. Based on the distribution characteristics of the adsorption amount with increasing distance from the river and the redundancy analysis results for the sediment components and adsorption amounts, the adsorption of Cu in the Dianchi Lake Estuary is strongly affected by iron and aluminium oxides, while the adsorption of Zn is more affected by OM and CEC.
Table 6
Redundancy analysis parameters for sediment components involved in adsorption
|
Interpretation %
|
Contribution rate %
|
pseudo-F
|
P
|
OM
|
20.9
|
72.0
|
18.5
|
0.002
|
CEC
|
< 0.1
|
0.3
|
< 0.1
|
0.928
|
Aluminium oxide
|
7.7
|
26.6
|
7.5
|
0.008
|
Iron oxide
|
0.3
|
1.1
|
0.3
|
0.738
|
Table 7
Correlation between sediment components and adsorption capacity
|
|
OM
|
CEC
|
Aluminium oxide
|
Iron oxide
|
Cu
|
Correlation
|
0.126
|
0.173
|
0.318
|
0.243
|
|
Significance
|
0.222
|
0.092
|
0.007
|
0.04
|
Zn
|
Correlation
|
0.441
|
0.309
|
0.357
|
0.372
|
|
Significance
|
0.000
|
0.002
|
0.002
|
0.001
|
Different sediment components adsorb different amounts of Cu and Zn. It has been reported 16 that OM and iron and manganese oxides have a relatively high level of adsorption of Cu. Feng Jun et al. 17 also found that iron oxide strongly adsorbs Zn.
In summary, the components of the sediment are entangled with each other, shielding or overlapping adsorption sites, so the adsorption amount at each depth is not consistent with the OM content, CEC, iron and aluminium oxide contents or the change trend of the adsorption amount and the adsorption capacity does not increase with increases in OM content, CEC, or Fe-Al oxide content. The sediment components exhibit differential adsorption of heavy metals: iron-aluminium oxide contributes more to the adsorption of Cu(Ⅱ), and OM and iron oxide contribute more to the adsorption of Zn(Ⅱ).
Different components of sediment have different adsorption properties for heavy metals; iron and aluminium oxides contribute more than other components to Cu adsorption, while OM and iron oxide contribute more to Zn adsorption.
Previous research results showed that there is a certain linear positive correlation between the OM content, CEC, and metal oxide contents in sediments and the adsorption amount 18–20. The results are shown in Table 8. The R2 between each component and the amount of adsorption is not high, indicating that the amount of adsorption does not depend on a single component, but is the result of the combined effect of multiple components.
Table 8
Regression equations between sediment components and adsorption capacity
|
Cu
|
Zn
|
OM
|
Y=-5.064E-006x3-0.001x2-0.043x-0.696
R2 = 0.031
|
Y = 6.259E-007x3 + 0.064x-1.422
R2 = 0.181
|
CEC
|
Y=-0.014x2 + 0.278x-1.165
R2 = 0.042
|
Y = 0.002x3-0.079x2 + 0.979x-3.243
R2 = 0.277
|
Fe
|
Y=-1.371E-007x3 + 8.939E-005x2-0.013x + 0.031
R2 = 0.110
|
Y=-7.179E-008x3 + 5.867E-005x2-0.009x-0.08
R2 = 0.188
|
Al
|
Y=-0.001x2-0. 099-1.517
R2 = 0.098
|
Y=-6.344E-005x3-0.06x2-0.159x + 0.886
R2 = 0.128
|
In the adsorption of Cu and Zn, as the equilibrium adsorption concentration increases, the competition for adsorbate molecules to occupy the adsorbent sites becomes more intense. When a high-binding-energy adsorption site is close to its full energy, non-specific adsorption increases, and the adsorption rate gradually slows 21. The iron-aluminium oxides and OM in group B, group C, and group D masked adsorption sites; thus, the K values were much lower in these groups than in group A (400.3). As the adsorption equilibrium concentration increased, the number of adsorption sites of groups B, C, and D decreased faster than those of group A, and the growth rate of the adsorption capacity slowed faster than that of group A. Relevant studies have shown 22 that as the equilibrium concentration increases, the adsorption capacity increases, and the adsorption curve shows a sharp increasing trend at low concentrations. However, the adsorption potential is limited, and when the concentration of heavy metal ions is high, the electrokinetic potential and electrical properties of colloidal particles are reduced, which reduces the stability of heavy metal-colloid-soil aggregates, and the curve gradually flattens 23. The decrease in adsorption capacity with the increase in adsorption equilibrium concentration may be caused by the decrease in adsorption sites and the limited adsorption capacity.
In practice, the surface of sediment particles is uneven, which makes the number and distribution of adsorption sites uneven. The adsorption isotherms of the sediments for Cu and Zn were more consistent with the Freundlich adsorption isotherm model than the Langmuir model, indicating that the adsorption process follows multi-layer adsorption. The surface of sediment particles is not uniform, so the fit of the Freundlich isotherm adsorption model aligns with reality; Mustapha 's et al. 24 research results are similar. The order of the adsorption amounts of the samples corresponds to the order of their K values (because some samples could not be fitted by the Langmuir isotherm adsorption model, the Freundlich adsorption isotherm model K value was used for comparison), which is similar to most research results 25. The K value was used as an index to measure the strength of the adsorption capacity: the larger the value was, the greater the adsorption force of the sediment for Cu and Zn. Among groups B, C, and D, the K values of group C (0.17 and 0.83) were the largest, and the K values (0.09 and 0.27) of group D were the smallest, indicating that iron-aluminium oxides and OM are important influencing factors in the adsorption process and that iron-aluminium oxide has a stronger adsorption capacity than OM for heavy metals.
In the past, studies on changes in adsorption characteristics before and after the removal of soil and sediment components have focused on the changes in the isotherm adsorption equation or kinetic adsorption equation of each single component 26, but they have not considered combined effects. Individual compound effects and isolated studies of inorganic colloids (minerals) or organic colloids (OM) cannot reflect true sediment systems.
The contribution rate of each component to the adsorption of Cu and Zn in the sediments was calculated according to Eq. (3):
G = (Q not removed -Q removal)/Q removal *100% (3)
where G is the rate for a particular component, (Q not removed) is the adsorption without removal of that component and (Q removal) is the adsorption with the removal of that component. To obtain the quantitative relationship between the fitting curve and GOM−IAO, GOM and GIAO, in Fig. 6, the contribution rate of OM (GOM) plus the contribution rate of iron and aluminium oxide (GIAO) was used as the X-axis, and the contribution rate of OM-iron and aluminium oxide complexes (GOM−IAO) was used as the Y-axis. The clear positive linear correlation between GOM−IAO, GOM and GIAO indicates that OM-iron-aluminium oxide composites play a role in the adsorption of Cu and Zn. The simple addition of iron and aluminium oxides results in a certain quantitative relationship for the adsorption of Cu (4):
GOM−IAO= (GOM+GIAO) *0.4-2 (4)
as well as for the adsorption of Zn (5):
GOM−IAO= (GOM+GIAO) *1.18–3.35 (5)
The organic/inorganic composite content was not equal to the sum of OM and metal oxides, presumably because hydrogen bonding, ion exchange and hydrophobic forces, such as anion adsorption mechanisms, embedded the composites in the mineral surface and between layers of swollen clay mineral crystals 27,28, affecting the mineral cementation degree and thus colloid stability 29. In addition to directly participating in the formation of complexes, the strong surface activity of iron and aluminium oxide can form bridges with OM and stabilize colloids through coordination exchange or the formation of ionic bonds 30. In this way, these components combine to form organic and inorganic complexes that form the core and structure of the soil 31–33.
To further compare the adsorption capacity differences between OM-iron-aluminium oxide composites, OM and iron-aluminium oxide, we compared the four groups of sediment samples under different pH values and adsorption quantity changes (Fig. 7). We found that at different pH values, the sediment Cu and Zn adsorption performance for group A was greater than that of the other three groups. Similar results were reported by Perez-Novo et al. 34.
In group A, OM and iron and aluminium oxides formed organic-inorganic complexes, increasing the sediment surface area and surface activity and enhancing the adsorption capacity 35. In addition, Mg(II) and Fe(II) compounds in sediments may be dissolved due to the presence of a large amount of H+ in low-pH environments, thus competing for adsorption sites, or Cu could form hydroxylates with increasing pH 21,36,37. In group D, OM and iron and aluminium oxide were removed simultaneously, which greatly reduced the number of active sites on the sediment surface and exposed the silicic skeleton of the sediments, making adsorption more susceptible to the influence of pH value.
The surface properties of iron oxides change when OM and iron and aluminium oxides form organic-inorganic complexes. First, the decrease in zeta potential indicates that the negative charge on the surface increases, which is conducive to improving the adsorption of cations. Zhou et al.38 found that a large amount of dissociated humic acid may cover the surface of iron oxide, reducing its surface electric potential and making the zeta potential drop. Second, OM and iron aluminium oxide have a large number of adsorption sites 39. OM adsorbs heavy metals by means of ion exchange, surface complexation and precipitation with carboxyl, hydroxyl and other functional groups in OM with a large number of negative charges, while humic acid and fulvic acid in OM adsorb heavy metals through complexation 11,40. Metal oxides, represented by iron and aluminium oxides, have variable charges, which can be replaced by H+ ions through the surface -OH groups and adsorbed on negatively charged sites 41 or react with surface groups to form complexes 42.
In summary, the organic-inorganic composites in the sediments did not correspond to the simple addition of OM and iron-aluminium oxides. Their contribution rates to the adsorption of Cu and Zn were GOM−IAO= (GOM+GIAO) *0.4-2 and GOM−IAO= (GOM+GIAO) *1.18–3.35. When organic-inorganic complexes are formed, the zeta potential decreases, the surface negative charge increases, and the number of adsorption sites such as functional groups and variable charges increase, making the adsorption capacity of organic-inorganic complexes for Cu and Zn significantly higher than those of OM and iron and aluminium oxides.