Properties of soil sample
The FTIR spectrum of OS, CS and NS is given in Fig. 1. The transmittance peak at 3443 cm-1, 1636 cm-1 and 1034 cm-1 represent the stretching of -OH, C=C / C=O and Si-O-Si, respectively. The XRD pattern of OS, CS and NS (Fig. 2) showed that all soil samples were composed of saponite, kaolinite, rodolicoite and vanadium oxide. The FTIR spectrum and XRD pattern of OS, CS and NS indicated that the coating of Cd/naphthalene did not change the surface functional group and the mineralogical composition of calcareous soils. Table 1 is the zeta potential of OS, CS and NS. The OS, CS and NS are all negatively charged in the condition of this work, and the presence of n-hexadecane increased the negative surface potentials of OS, CS and NS since surface coating by hydrophobic compounds can change the zeta potential of porous media (Song et al., 2011; Yang et al., 2017b).
Table 1 The zeta potential of all soil samples
concentration of n-hexadecane
|
Zeta potentials (mV)
|
CS
|
OS
|
NS
|
0 mg/L
|
-15.84
|
-20.76
|
-23.03
|
100 mg/L
|
-29.11
|
-23.47
|
-24.63
|
The DOM concentration of CS (313.10 mg/kg) and NS (331.30 mg/kg) was higher than that of OS (88.35 mg/kg) (Table S1), suggesting that microbial communities in calcareous soil may respond to Cd/naphthalene (Schwarz et al., 2011; Wang et al., 2020a). DOM may control the fate of n-hexadecane in calcareous soil due to its amphipathic molecules. Fluorescence spectra of OS, CS and NS (Fig. 3) showed the intensity of tyrosine-like fluorescent peak in CS/NS were more significant than in OS, suggesting that CS/NS have more aromatic ring amino acids (Yan et al., 2019). The intensity of tryptophan-like fluorescent peak in CS/OS was stronger than in NS. Fulvic acid relates to the carbonyl and carboxyl groups, and it can increase the solubility of hydrocarbon (Lu et al., 2013). Clearly, the intensity of fulvic acid peak in CS was greater than in OS/NS. Fluorescence components of DOM in OS, CS and NS were shown in Fig. 4. It can be seen that the humic acid content of NS was smaller than that of OS/CS. It may result in the greater adsorption capacity of NS since the humic acid can generally inhibit the adsorption of hydrocarbon on porous media (Wang et al., 2017).
Adsorption and desorption kinetic of hexadecane on calcareous soil samples
N-hexadecane may be firstly trapped in calcareous soil through molecular adsorption, and later desorption may constitute the rate determining step. All samples were fitted well by the pseudo-first-order dynamics model with R2> 0.9 (Fig. S3 and Table S2). Adsorption equilibrium time for n-hexadecane on OS, CS and NS were all 4 h, suggesting that the presence of Cd/naphthalene cannot change the adsorption equilibrium time. It may be causing by the homologous properties of calcareous soil samples (Yang et al., 2013). The adsorption rate constant (k1) of OS was higher than that of CS/NS, which means Cd/naphthalene can increase the adsorption efficiency of n-hexadecane on calcareous soils. Desorption equilibrium time for n-hexadecane on OS, CS and NS were all 1 h, indicating that Cd/naphthalene cannot change the desorption equilibrium time. The k1 of n-hexadecane adsorption on OS was lower than CS/NS (Table S2), which means Cd/naphthalene can reduce the desorption efficiency of n-hexadecane on calcareous soils. In all cases, adsorption efficiency was higher than desorption efficiency, suggesting the desorption behavior of n-hexadecane cannot greatly influence the adsorption behavior in this work. On the whole, Cd/naphthalene can improve the adsorption capacity and decrease the desorption capacity of n-hexadecane on calcareous soils, but they cannot change the equilibrium time.
Effect of pH on the adsorption isotherm of n-hexadecane on the calcareous soil samples
In most cases, Freundlich (Fig. 5 and Table 2) is better than Langmuir (Fig. S4 and Table S3), which means the adsorption behavior of n-hexadecane on soil samples may not dependent on monolayer adsorption. In Table 2, Kf value (at pH=7) of n-hexadecane adsorption on soil samples was lower than that of on loess soil (Jiang et al., 2016) and modified diatomite (Xu et al., 2020). It can be summarized that Cd/naphthalene cannot obviously improve the adsorption efficiency of n-hexadecane on calcareous soils in comparison with other soils. At pH=7, Kf value of OS was lower than that of CS/NS, suggesting that Cd/naphthalene can improve the higher n-hexadecane adsorption capacity on calcareous soil. Previous studies have also found that adding metals can increase the adsorption of hydrocarbon on porous medias (Saeedi et al., 2018). Cd-coating porous medias can enhance the adsorption of anionic and neutral hydrocarbon compounds since the polar functional group in porous media was complexation with Cd (Wang et al., 2017). However, in this study, the surface functional group in calcareous soils did not change after Cd/naphthalene coating, therefore the improved adsorption efficiency may not absolutely depend on function group’s change. Furthermore, the aggregation and hydrophobicity of porous media causing by Cd-coating can increase the adsorption efficiency of hydrocarbons (Nguyen et al., 2013). The negative charge of porous media surface can be shielded after coating naphthalene (Yang et al., 2017a), therefore the adsorption of n-hexadecane was inhibited. Moreover, the n value of all cases was lower than 1, indicating that adsorption sites in the surface of all soil samples (OS, CS and NS) were limitation although Cd/naphthalene can open more adsorption sites for n-hexadecane.
Table 2 The parameters of Freundlich
Soil sample
|
pH
|
Kf a
|
1/n b
|
R2
|
OS
|
5
|
0.10±0.15
|
2.30±0.26
|
0.95
|
7
|
0.00±0.01
|
2.10±0.36
|
0.98
|
9
|
0.02±0.03
|
2.76±0.32
|
0.98
|
CS
|
5
|
0.26±0.38
|
1.89±0.33
|
0.95
|
7
|
0.05±0.08
|
2.27±0.42
|
0.95
|
9
|
0.01±0.02
|
2.48±0.25
|
0.98
|
NS
|
5
|
1.27±1.40
|
1.56±0.25
|
0.95
|
7
|
0.82±0.82
|
1.65±0.22
|
0.96
|
9
|
0.06±0.09
|
2.17±0.30
|
0.95
|
a Kf is the coefficient of Freundlich which positively associated with adsorption capacity.
b n is the sorption intensity.
In recent years, rainwater with higher pH value (pH=8.0) was found in a typical karst area (Zeng et al., 2020). Furthermore, previous studies have found that the adsorption of diesel oil on loess soil was weakened by increasing pH (Jiang et al., 2016) since pH improve the dissolution and dispersion of diesel oil (Delle Site, 2001; Pradubmook et al., 2003). However, the adsorption of naphthalene on biochar colloid had almost no effected by pH (Yang et al., 2017b) due to the little dissociation degree of nonpolar hydrocarbons in polar solution (Grządka, 2011). Therefore, the potential influence of pH on the n-hexadecane adsorption was investigated in this study. As shown in Fig. 5 and Table 2, it can be observed that the elevated pH (from 5 to 9) caused decreasing adsorption efficiency of n-hexadecane on OS, CS and NS. In this study, pH value was set from 5 to 9, thus the adsorption of n-hexadecane on each soil sample at pH=5 was the maximum. The adsorption of n-hexadecane on OS was weaker than CS/NS at pH=5 and pH=7. The Kf value of OS was higher than CS in alkaline environment (at pH=9). The n-hexadecane adsorption on NS remained stable from 5 to 7, but it was changed from pH=7 to pH=9. Some studies reported that the release of Cd/naphthalene in soils depended on pH since the bonds between contaminations and soils were broken by pH (Kicińska et al., 2022; Yang et al., 2001). Furthermore, the release of Cd/naphthalene in soils may open more adsorption sites for n-hexadecane. Moreover, the presence of impurities in n-hexadecane, such as long-chain carboxylic acids (Fang et al., 2015), and the unbalanced hydrophobic/hydrophilic properties at oil/water boundary in solution (Li and Bhushan, 2015) may influence the adsorption of hydroxyl ions (OH-), thus n-hexadecane carries negative surface charge. The electronic mobility of the n-hexadecane and the adsorption sites on soil samples may change along with the increasing pH at aqueous phase (Kim et al., 2012; Li and Bhushan, 2015). Therefore, pH can influence the adsorption behavior of n-hexadecane on soil samples.
Effect of flow velocity on transport of n-hexadecane in the calcareous soil samples
The BTCs of n-hexadecane in the calcareous soil samples with the addition of various flow velocity are presented in Fig. 6. As shown in Fig. 6, all BTCs were symmetric in shape, implying that there was the physical equilibrium transport in column. The maximum value of C/C0 were 28 %, 35 % and 48 % in OS, CS and NS with the set flow velocity of 1 mL/min, respectively, suggesting n-hexadecane may more effectively breakthrough the column of CS/NS than that of OS. Similarly, the maximum value of C/C0 in CS/NS was higher than in OS at the set flow velocity of 2 and 4 mL/min. The two kinetic sites model of Hydrus-1D well described the BTCs of n-hexadecane in each soil sample with different Darcy velocity according to the higher correlation coefficient (R2) between observed and simulated data (Table 3). The characteristic of BTCs for n-hexadecane in all soil sample column can be reflected by Smax and Katt of Hydrus-1D (Table 3). In all case, the Smax value of OS was higher than that of CS/NS, which means greater irreversible retention of n-hexadecane in OS. The greater value of Katt for CS/NS at all Darcy velocity reflects a rapid release process of n-hexadecane. The mass recovery rate of n-hexadecane in effluent are showed in Table 3. There is a higher mass recovery rate for n-hexadecane in the effluent of CS/NS packed column than that of OS packed column. On the whole, the transport of n-hexadecane in soil samples followed the order of CS/NS > OS at the same Darcy velocity, which means Cd/naphthalene can improve the transport efficiency of n-hexadecane. In Table 2, negative charge density of CS/NS was more than that of OS. The more negative charge of soil samples, the stronger electrostatic repulsions between the soil samples and the negatively charged n-hexadecane were, which is likely responsible for promoting the transport of n-hexadecane in CS/NS packed column (Wu et al., 2020). Furthermore, although naphthalene-coating cannot dramatically change the surface negative charge of calcareous soils, nonpolar naphthalene can lead to charge-shielding (Yang et al., 2017b).
The previous studies have proved that the transport of petroleum hydrocarbons with negative charge were inhibited by the decreasing pH due to the reduction in electrostatic repulsion (Cai et al., 2017; Wang et al., 2020b). Our previous batch experiments also showed that the electronic mobility of the n-hexadecane increased along with the increasing pH at aqueous system (Fig. 5 and Table 2). Moreover, the unique hydrological and geological structures in karst areas lead to variable flow velocity, and pollutants are more easily to rapidly transport in the pipeline systems of karst (Jiang et al., 2018). Therefore, this section only investigated the potential influence of flow velocity on the n-hexadecane transport in Cd-/naphthalene-contaminated calcareous soils. In Fig. 6, high value of C/C0 for n-hexadecane in OS, CS and NS were observed along with high flow velocity, which means the amount of n-hexadecane transport increased with the increasing flow velocity. Similar results had observed by some studies (Alazaiza et al., 2021; Alazaiza et al., 2020). In Table 3, it should be pointed out that the value of Smax decreased with high Darcy flow (0.69 cm/min) while the value of Katt was to be the minimum at Darcy flow of 0.12 cm/min. It suggested that smaller Darcy velocity may improve the irreversible retention but inhibit the reversible retention of n-hexadecane in calcareous soil samples. The mass recovery rate of n-hexadecane transport increased when the set flow velocity reached 4 mL/min (Table 3), indicating the transport of n-hexadecane can be improved. Overall, n-hexadecane was more likely to break through CS/NS column with a high flow velocity.
Table 3 the parameters of fitted model
Column No.
|
Soil sample
|
Set flow (mL/min)
|
Darcy velocity (cm/min)
|
The mass recovery rate (%)
|
Smax1 (mg/g) a
|
Katt (min-1) b
|
R2
|
1
|
OS
|
1
|
0.12
|
30.96
|
0.3384
|
0.0224
|
0.9711
|
2
|
OS
|
1
|
0.12
|
27.81
|
0.3896
|
0.0279
|
0.9401
|
3
|
OS
|
2
|
0.31
|
32.87
|
0.2855
|
0.0825
|
0.9827
|
4
|
OS
|
2
|
0.31
|
35.14
|
0.2639
|
0.0861
|
0.9429
|
5
|
OS
|
4
|
0.69
|
42.08
|
0.2368
|
0.0886
|
0.9818
|
6
|
OS
|
4
|
0.69
|
40.03
|
0.2100
|
0.0827
|
0.9598
|
7
|
CS
|
1
|
0.12
|
30.80
|
0.2512
|
0.0172
|
0.9719
|
8
|
CS
|
1
|
0.12
|
28.99
|
0.2368
|
0.0216
|
0.9768
|
9
|
CS
|
2
|
0.31
|
35.36
|
0.1639
|
0.0377
|
0.9771
|
10
|
CS
|
2
|
0.31
|
37.51
|
0.1468
|
0.0361
|
0.9524
|
11
|
CS
|
4
|
0.69
|
57.68
|
0.1270
|
0.0652
|
0.9429
|
12
|
CS
|
4
|
0.69
|
54.21
|
0.1011
|
0.0681
|
0.9577
|
13
|
NS
|
1
|
0.12
|
43.98
|
0.2377
|
0.0125
|
0.9681
|
14
|
NS
|
1
|
0.12
|
40.11
|
0.2033
|
0.0153
|
0.9767
|
15
|
NS
|
2
|
0.31
|
56.98
|
0.1309
|
0.0394
|
0.9888
|
16
|
NS
|
2
|
0.31
|
57.19
|
0.1214
|
0.0365
|
0.9459
|
17
|
NS
|
4
|
0.69
|
58.66
|
0.1160
|
0.0490
|
0.9695
|
18
|
NS
|
4
|
0.69
|
56.98
|
0.1270
|
0.0424
|
0.9872
|
a the maximum solid-phase retention capacity of n-hexadecane in attachment site.
b the attachment rate.
Effect of flow velocity on retention of n-hexadecane in the calcareous soil samples
As shown in Fig. 7, the maximum concentrations of n-hexadecane were retained in -12 cm, suggesting that n-hexadecane may be migrated downward by gravity force after leaching. The mass recovery rate of n-hexadecane effluent in CS/NS packed column was higher than in OS packed column (Table 3). Clearly, Cd/naphthalene reduced the retention content of n-hexadecane at various set flow velocity. Previous studies found that soil components such as SOM play an important role in retaining petroleum hydrocarbons (Adam et al., 2002; Cai et al., 2019). The DOM concentration of CS (313.10 mg/kg) was higher than that of OS (88.35 mg/kg) (Table S1), and the intensity of fulvic acid peak in CS was stronger than in OS (Fig. 3). It may be explained that the transport of hydrophobic substance was enhanced by increasing the concentration of fulvic acid (Dong et al., 2021; Sojitra et al., 1996; Yu et al., 2011). Moreover, batch experiments showed a higher n-hexadecane adsorption efficiency of NS than that of OS, since soil samples can expose additional attachment sites to n-hexadecane in shaking condition. However, adsorption sites are not available in column experiment, and the n-hexadecane transport may more affect by hydrodynamics (Wang et al., 2020b). In Fig. 7, when the flow velocity was 1 mL/min, the retention content of n-hexadecane in each soil column reached maximum. The effect of flow velocity on n-hexadecane retention is more pronounced at 4 mL/min. In Table 3, the mass recovery rate of n-hexadecane in effluent improved when flow velocity raised from 1 mL/min to 4 mL/min. Some studies also reported an increasing velocity result in an easier transport of contaminations in soil (Jiang et al., 2019; Yang et al., 2020). Macroscopically, the increasing flow velocity caused the decreasing residence of n-hexadecane in OS, CS and NS column, respectively.