Experimental study on phosphate rock modified soil-bentonite as a cut-off wall material Open Access

The main sources of heavy metal pollution such as mining and smelting in mining areas, metal processing, electroplating and other industrial activities, cause pollution to soil and water as well as to groundwater, surface runoff and other geological processes (Bempah 2013; Ngole-Jeme & Fantke 2017; Huang et al. 2019; Zhang et al. 2021). Heavy metal pollution in the soil-water system is difficult to reverse, and contaminated soil is difficult to be repaired by itself (Sun 2005). Once lead enters an organism, it will cause chronic poisoning symptoms, and damage many organs of the human body. Once organs and tissues are damaged they cannot be repaired (Zhang & Gao 2015; Liu et al. 2018; Qaisar et al. 2019).

The mainstream method of making soil cut-off wall materials at home and abroad is to mix Bentonite with its high water absorption and swelling properties into in-situ soil, so as to improve its permeability (Koerner & Daniel 1993). Natural soil mixed with bentonite is an excellent material for building soil cut-off walls (Lundgren 1981; Chang et al. 2020). The adsorption characteristics of a soil cut-off wall for heavy metal ions are obviously affected by chemical composition. Under normal circumstances, the lower the pH, the weaker the adsorption capacity of soil materials to heavy metal ions. When encountering acidic environments such as acid rain, the adsorption capacity will decrease, and there will be obvious desorption, which can easily cause secondary pollution (Zeng 2008; Huang et al. 2009; Wang 2014). Therefore, it is necessary to study the material modification of the soil barrier to improve its compatibility in acid contaminated sites and improve the performance of the barrier.

Phosphate rock is an efficient passivator which is widely used in soil heavy metal remediation and will not cause damage to the environment (Xu et al. 2011). Phosphate, as a common soil heavy metal remediation and stabilization material, has a good adsorption effect on various typical heavy metal pollution ions such as Pb²⁺, Cd²⁺, Cr⁶⁺, Cu²⁺, Zn²⁺, As⁵⁺, etc. in a large pH range (Alam et al. 2001; Sone & D'Souza 2006; Elouear et al. 2008; Miretzky & Fernandez-Cirelli 2008). High-grade rock phosphates (>30% P₂O₅) have already been utilized for the removal of heavy metal ions and have been very effective (Ma et al. 1993, 1995; Rechert & Binner 1996). However, high-grade rock phosphates are not commercially viable due to their high cost (Sone & D'Souza 2006). Among the available phosphorus-containing substances at present, phosphate rock has many applications and and is widely researched due to its large output and low price (Wang 2008). Pb²⁺, Cu²⁺ and Zn²⁺ in a solution can reach equilibrium in only half an hour adsorbed by phosphate rock, and the removal rate is up to 100% (Sone & D'Souza 2006). The removal rates of soluble Cu²⁺ and Zn²⁺ in soil can reach up to 80 and 69% by using phosphoric acid and phosphate rock, respectively (Cao et al. 2009). The dispersion effect of soluble phosphate on lead-contaminated clay-bentonite cut-off wall material was studied through a sedimentation test, and it was found that phosphate can change the disordered arrangement of clay particles and make their more compact (Yang et al. 2015). Therefore, phosphate rock has the potential to improve the traditional cut-off wall material in environments polluted by heavy metals.

The main objective of this study was to investigate the effect of phosphate rock on the adsorption performance of traditional cut-off wall material. Pb²⁺ was used as a typical type of heavy metal contaminant. Both equilibrium and non-equilibrium adsorption batch tests were carried out, and the influence of various experimental conditions including contact time (t), initial solution concentration (C₀), temperature (T) and pH was discussed. Microscopic analyses were conducted to explore the adsorption mechanism.

PSB was prepared with three types of soils, i.e. phosphate rock (P), clayey sand (S) and Na-bentonite (B). The clayey sand was sampled from Xiangtan, Hunan Province, in the middle of China. The phosphate rock was sampled from Jinan, Shandong Province, in eastern China, with a light gray colour and fines smaller than 0.075 mm in diameter. The Na-bentonite was sampled from the Gaomiaozi area of Inner Mongolia, in the north of China. The basic parameters are shown in Tables 1 and 2.

Liquid materials in this study include Pb(NO₃)₂, NaOH and HNO₃ solutions. Pb²⁺ was used as the heavy metal of interest. Pb²⁺ solution (1,000 mg/L) was prepared by dissolving lead nitrate into deionized water. Pb²⁺ solutions of other concentrations used in this study were diluted from the Pb²⁺ solution (1,000 mg/L). NaOH and HNO₃ were used for pH adjustment to study the adsorption characteristic of Pb²⁺ onto PSB at different pH values. All chemicals used were of Analytical Reagent (AR) grade.

The liquid-plastic limit of soil samples was measured by a digital display soil liquid-plastic limit combined tester (LG100D: Shanghai Kanglu Instrument Equipment Co., Ltd) and a disc liquid limit tester (CSDS-1). The supernatant was centrifuged by a high-speed freezing multi-tube centrifuge (TGL16M). The concentration of Pb²⁺ in the solution was measured by flame atomic absorption spectrophotometer (AA-7002A). The mineral composition of the soil was analyzed by X-ray diffractometer (Philips APD-10). The chemical composition of the soil was analyzed by X-ray fluorescence (panalytical Axios FAST). Scanning electron microscopy (FEI INSPECT F50) was used to analyze the microstructure of Xiangtan clayey sand and Gaomiaozi bentonite. Fourier transform infrared spectroscopy (Nicolet Avatar 360) was used to analyze the functional groups of soil samples before and after adsorption.

In all batch tests, 1 g soil samples were mixed with 100 mL of Pb²⁺ solution at a shaking rate of 180 rpm. The adsorption materials are clayey sand, SB and PSB. Normally SB contains 4–6% bentonite (Yeo et al. 2005; Malusis et al. 2009; Hong et al. 2011; Malusis & McKeehan 2013). In this study, 5% of bentonite was used in both SB and PSB. In PSB, the amount of phosphate rock is 10%.

In kinetic adsorption batch tests, the initial concentrations of Pb²⁺ solution were 200, 400 and 800 mg/L, and shaking time was 10 min, 30 min, 1 h, 2 h, 3 h, 4 h, 6 h, 8 h and 24 h at room temperature. In isothermal equilibrium adsorption batch tests, the initial concentrations of Pb²⁺ solution were 50, 100, 200, 400, 600 and 800 mg/L, the shaking time was 24 h and temperatures were 288 K, 298 K and 308 K. The 24 h period was determined from preliminary tests to be sufficient for achieving equilibrium conditions.

Figure 1 shows the variation curve of the adsorbed amount of Pb²⁺ onto S, SB and PSB from 10 min to 24 h. The initial concentrations C_i of Pb²⁺ solution were 200 mg/L, 400 mg/L and 800 mg/L, respectively. The adsorption rate of PSB in the first 60 min is slower than that of S and SB when that initial concentration of Pb²⁺ solution is 200 mg/L, but the equilibrium adsorption time of PSB is longer. When the initial concentration of Pb²⁺ solution is 800 mg/L, the adsorption rate of PSB is faster than that of S and SB in the first 60 min, and the 3 soil samples basically reach adsorption equilibrium at 120 min, which shows that the influence of phosphate rock on the adsorption rate of S is related to the concentration of Pb²⁺ solution. The initial concentration of Pb²⁺ increases, and the adsorption rate of Pb²⁺ onto PSB also increases. It can also be seen from Figure 1 that the incorporation of phosphate rock can significantly increase the adsorption amount of clayey sand. When the initial concentrations are 200 mg/L, 400 mg/L and 800 mg/L, the adsorption amounts are, respectively: for S, 11.47, 20.58 and 30.47 mg/g; for SB, 13.43, 21.92 and 33.31 mg/g; and for PSB, 19.3, 35.59 and 47.49 mg/g. The higher the initial concentration of Pb²⁺ solution is, the more the adsorption amount of PSB than that of S and SB at adsorption equilibrium. The analysis shows that the incorporation of phosphate rock leads to an increase in H₂PO₄⁻ and a consequent increase in the anionic electrical properties on the surface of the soil particles, resulting in increased adsorption of cations, and the new anions will also form complexes with apatite mineral surface groups.

The adsorption kinetic data of PSB were analyzed and the adsorption mechanism was explained by using a pseudo-first-order kinetic model, pseudo-second-order kinetic model, intraparticle diffusion model and a liquid film diffusion model.

The fitted curves of equilibrium adsorption data by the pseudo-first-order kinetic model and pseudo-second-order kinetic model are shown in Figures 2 and 3, respectively. Table 3 shows the adsorption kinetic parameters of PSB. The correlation coefficients of the pseudo-second-order kinetic model are slightly greater than for the pseudo-first-order kinetic model. For the initial concentration of 800 mg/L Pb²⁺ solution, the maximum adsorption amount calculated by the pseudo-second-order kinetic model is 48.08 mg/g, which is the closest to the measured adsorption amount of 47.49 mg/g. The pseudo-second-order kinetic rate parameter k₂ increases from 0.0015 to 0.0026 when the concentration of Pb²⁺ solution increases from 200 mg/L to 800 mg/L, indicating that the adsorption rate increases with the increase of the initial concentration, which is consistent with the experimental results. The pseudo-second-order kinetic model can better simulate the adsorption process of Pb²⁺ onto PSB.

The fitted curves of equilibrium adsorption data by liquid film diffusion and intraparticle diffusion models are shown in Figures 4 and 5, respectively. Table 3 shows the intercept C ≠ 0, indicating that curves fitted by the intraparticle diffusion model do not pass through the origin and there is a boundary layer control effect in the adsorption process. Intraparticle diffusion is not the only control step of the adsorption rate in the adsorption reaction; liquid film diffusion is also involved in controlling the adsorption process simultaneously (Reczek et al. 2020). The correlation coefficient of the liquid film diffusion model was greater than that of the intraparticle diffusion model, indicating that the control of adsorption rate by liquid film diffusion was greater than that by intraparticle diffusion. As the initial concentration increased from 200 mg/L to 800 mg/L, the intercept increased from 0.132 to 9.84, indicating that the control of the adsorption rate by liquid film diffusion increased with the increase of the initial concentration.

The isothermal adsorption curves of S, SB and PSB at temperatures of 288 K, 298 K and 308 K, are shown in Figure 6. It can be found that with the increase of the equilibrium concentration, the equilibrium adsorption amount of S, SB and PSB increases rapidly and finally reaches the adsorption equilibrium. The incorporation of phosphate rock significantly increases the equilibrium adsorption amount of the mixed materials. When the temperature is 288 K, the equilibrium adsorption amount of PSB is 49.15 mg/g, reaching 145 and 159% of the equilibrium adsorption amounts of SB and S, respectively. With the increase of temperature from 288 to 308 K, the equilibrium adsorption amount decreased slightly for the three soil samples. This shows that the adsorption process is an exothermic reaction, and increasing the temperature will inhibit the adsorption of Pb²⁺ onto PSB, and decreasing the temperature is beneficial to the adsorption of Pb²⁺ onto PSB.

The parameters of the adsorption model fit are shown in Table 4. The parameter 1/n calculated by the Freundlich model fit ranged from 0.1 to 1, which indicates that the adsorption process of Pb²⁺ is prone to occur at the current test temperature. The fitted curves for the Langmuir and Freundlich models are shown in Figures 7 and 8, respectively. When the temperature is 288, 298, and 308 K, the correlation coefficients fitted by the Langmuir model are 0.993, 0.948, and 0.947, respectively, and the correlation coefficients fitted by the Freundlich model are 0.842, 0.805, and 0.864, respectively. The Langmuir model fits the experimental data better compared to the results fitted by the Freundlich model. When the temperature is 308 K, the adsorption amount fitted by the Langmuir model is 42.17 mg/g, which is close to the measured adsorption amount of 41.47 mg/g. This also further shows that the Langmuir model can better describe the adsorption reaction process of PSB on Pb²⁺ and, according to the theory of the Langmuir model, the adsorption potential on the surface of soil particles is uniformly distributed and it is a monolayer adsorption.

The adsorption isotherm test data of PSB is plotted with ln K_D against T⁻¹ in Figure 9. The entropy change ΔS and enthalpy change ΔH of the thermodynamic parameters of adsorption reaction can be obtained from the intercept and slope, and the calculation results are shown in Table 5.

It can be seen from Table 5 that the reaction enthalpy change ΔH= − 12.602 KJ/mol in the adsorption process of Pb²⁺ onto PSB; since this value is <0 it indicates that the adsorption process is an exothermic reaction process, in which low temperature is more conducive to the adsorption of PSB, which is consistent with the test results. The absolute value of enthalpy change |ΔH| < 40 KJ/mol indicates that physical adsorption plays an important role in this adsorption process. The entropy change ΔS = −60.93 J/mol K indicates that there is a decrease in the entropy of the solid-liquid system and a decrease in the system freedom as the adsorption reaction proceeds.

pH has a great effect on the adsorption ability of soil towards heavy metals (Khan et al. 1995; Elzahabi & Yong 2001; Mohammed-Azizi et al. 2013). The pH studies were conducted in a temperature-controlled shaker using 100 ml of Pb²⁺ solution and a fixed PSB dosage of 1 g. The initial Pb²⁺ concentration was 400 mg/L, the shaking time was 24 h and the temperature was 25 °C. It can be seen from Figure 10 that the adsorption of Pb²⁺ onto the three soil samples increased with the increase of the initial pH, and the addition of phosphate rock significantly increases the adsorption capacity of these soils. When the initial pH value is 2, the adsorption capacity of soil samples increases to 245% with 10% phosphate rock. When the pH is low, the competitive adsorption of H⁺ and Pb²⁺ leads to a lower absorption of Pb²⁺ onto the soil samples.

There are a small amount of stacked sheet and layered structures on the surface of the soil particles before adsorption in Figure 12(a), which is beneficial to the adsorption of heavy metal ions. After the adsorption of Pb²⁺, as shown in Figure 12(b), the degree of irregularity on the surface of soil particles increased, the depressions and gaps increased, and a large number of flaky structures appeared on the surface and edges of the soil particles. The analysis shows that the surface complexation of Pb²⁺ on the soil particles led to the generation of a large number of polymeric sediments.

It can be seen from Figure 14 that the antisymmetric stretching vibration of -OH is approximately 3,450 cm⁻¹, the H-O-H bending vibration of adsorbed water is approximately 1,642 cm⁻¹, the stretching vibration peak of the CO32- is approximately 1,450 cm⁻¹ and the anti-symmetric stretching vibration of Si-O-Si, which is strong and steep, is approximately 1,075 cm⁻¹. The peaks at 788 cm⁻¹ and 545 cm⁻¹ are the symmetric stretching vibration peaks of the Si-O bond. And there is a weak peak at 1,095 cm⁻¹, which is the stretching vibration peak of PO₄³⁻.

Comparing the changes of peaks in infrared spectra before and after adsorption, it can be seen that the positions of the peaks has not changed. However, the intensity of the peaks is obviously enhanced, such as the absorption peak of Si-O-Si near 1,075 cm⁻¹, the absorption peak of -OH near 3,450 cm⁻¹, and a weaker stretching vibration peak at 1,095 cm⁻¹, which is the PO₄³⁻ group in the phosphate rock. This indicates that -OH and PO₄³⁻ are on the surface or between layers of the adsorbent participate in the adsorption process.

The adsorption characteristics of Pb²⁺ in phosphate rock modified soil-bentonite as a cut-off wall material were investigated by two batch tests, together with SEM, XRD and FT-IR analysis. The main conclusions are as follows: (1) The adsorption capacity of soil samples could be significantly enhanced by the incorporation of phosphate rock. (2) The pseudo-second-order kinetic model has the best fitting correlation with the experimental data, which indicates that intraparticle diffusion and liquid film diffusion are involved in controlling the adsorption process simultaneously. (3) The thermodynamics indicate the adsorption process of Pb²⁺ on PSB was an exothermic process. (4) The incorporation of phosphate rock improves the adsorption performance of the mixed material for Pb²⁺ in the acidic environment, and the maximum adsorption amount of PSB can be 154% greater than that of SB. (5) Microscopic mechanism analysis shows that quartz, calcite and hydroxyapatite minerals in the modified materials play a role in the adsorption of Pb²⁺, and -OH and PO₄³⁻ are on the surface or interlayer of the adsorbent participate in the adsorption process of Pb²⁺.

The experimental results provide a reference for the application of PSB in vertical cut-off walls. The adsorption performance of PSB for other pollution components, and the interception ability and long-term service performance of modified materials with different blending ratios need further systematic experimental research.

The authors are grateful for the financial support of the National Natural Science Foundation of China (Grant No. 41702329), the Scientific Research Foundation of Hunan Provincial Education Department (Grant No. 17 B097), Department of Natural Resources of Hunan Province (Grant No. 2020-15) and the Key Laboratory of Soft Soils and Geoenvironmental Engineering of the Education Ministry of China (Grant No. 2016P05).

The authors declare that they have no conflicts of interest regarding the publication of this paper.

All relevant data are included in the paper or its Supplementary Information.