Novel scheme for synergistic purification of copper mine tailings and orthophosphate Open Access

Over the past 20 years, China has discharged or stacked nearly 60 billion tons of tailings, and more than 1.5 billion tons of tailings are discharged directly into the environment each year (Zeng et al. 2021). China is rich in mineral resources, with more than 8,000 state-owned mining enterprises and 230,000 collective mines producing a large amount of tailings and mining waste annually. The abandoned tailings may pollute farmland soil, surface water, groundwater, sediments, and surrounding plants and cause serious harm to the surrounding environment and the health of local residents (Wang et al. 2019). Currently, there are more than 12,000 copper tailing (CT) reservoirs in China; since 2009, the cumulative total of CTs in China has exceeded 10,000 metric tons (Mt) (Deng et al. 2017). China's copper industry has undergone rapid development in the twenty-first century, mainly reflected in the continuous expansion of industrial scale, the continuous strengthening of enterprises, and significant progress in some areas of technology. During ore dressing, the raw ore is usually processed through crushing, grinding, and chemical flotation to separate the minerals in the raw ore. However, the majority of CTs are produced after ore dressing, and most of these CTs are directly stacked in tailing reservoirs without advanced treatment or utilization. However, the tailings in reservoirs are easily eroded by wind and rain during the long stacking process. In addition, during the process of field stacking, the heavy metal elements in the tailings may leak into the surrounding environment, causing pollution and severe ecological damage. The stacking of CTs may lead to the heavy metal pollution of groundwater, which will increase water shortage pressure in China. Moreover, the direct stacking of tailings requires a large amount of land, which will indirectly reduce agricultural production and further aggravate global food shortages. An analysis of long-term monitoring data from tailing dams has shown that the stability of these tailing dams is weak, and dam breaks can lead to serious disasters that harm human beings, the economy, and the environment (Dong et al. 2020; Lam et al. 2020; Yin et al. 2020). CTs are composed of fine-grained soil and the by-products of metallurgical processes. They release Cu, Cd, Cr, Pb, and other harmful heavy metals into the surrounding areas, causing harm to local land resources. These heavy metals spread through the food web, with potential toxic effects on animals, plants, and humans. Such toxicity is of particular concern (Munir et al. 2020). Therefore, while seeking better beneficiation technologies to reduce the heavy metal content in tailings, it is urgent to find a more appropriate treatment and disposal method for the large amount of tailings that has already been produced.

Currently, the resource utilization of CTs is mainly reflected in the use of construction materials for buildings and adsorbent material for sewage treatment (Esmaeili et al. 2020; Munir et al. 2020; Zeng et al. 2021). The utilization of CTs as building materials, such as bricks and supplementary cementitious materials, has achieved the resource utilization of CTs to a certain extent, but the technical aspects require further improvements. In addition, there may be a risk of heavy metal leaching when using CTs as building materials. Therefore, the management, disposal, and utilization of CTs pose critical challenges for the environmental governance of countries that have large copper production facilities.

Chemical washing is a feasible remediation technology for heavy metal–contaminated soil. Although chemical washing technology is an efficient, time-saving, and widely used method, its efficiency largely depends on the washing agents used. For different classes of washing agents, their remediation effects on heavy metal pollution vary. These effects mainly include passivation and sequestration. Some materials contain carboxyl and phosphonic acid groups that can form stable compounds with metal ions and thus stabilize them. However, after washing by some agents, the heavy metals in soils may transform from stable fractions to labile fractions, resulting in the increased fluidity and biological toxicity of heavy metals (Feng et al. 2020). The choice of washing agent is therefore very important. Phosphorus-containing substances have a good passivation effect on heavy metal ions in the soil, which has a great influence on changes in speciation (Gao et al. 2020; Feizi & Jalali 2021; Raklami et al. 2021; Teng et al. 2021; Wang et al. 2021). They can form relatively insoluble metal-phosphate deposits with heavy metals, thus immobilizing the heavy metals and reducing the bioavailability of heavy metal ions in the soil (Gao et al. 2020). In this study, a solution containing orthophosphate (such as KH₂PO₄) was used as the chemical washing agent. During the washing process, not only were the heavy metals in the tailings passivated but the nutrients P and K were also added to the barren tailings during the washing process.

The heavy metal ions in CTs, such as Cu²⁺, have an effective affinity with phosphate (Song et al. 2016). The author has used unmodified CTs and thermally modified CTs to study the adsorption of phosphate, and the results have shown that thermally modified CTs have a better adsorption ability for phosphate (Zhou et al. 2018; Zhou et al. 2019). Jin et al. (2021a) modified CTs with lanthanum hydroxide (La(OH)₃) and used this to adsorb phosphate. They found that La(OH)₃-modified CTs displayed great improvement in adsorption performance for phosphate. Hence, the synergistic effect of CTs and phosphate washing solution can be leveraged to mitigate some of the environmental impacts associated with the disposal of CTs. However, after washing, some heavy metals are deposited in the tail water because of the adsorption, hydrolysis, and co-precipitation of metal ions (Tchatchouang et al. 2021), which may cause heavy metal pollution in the water. Therefore, some negative ecological effects may result from this treatment method. For the remediation of heavy metal pollution in the soil of tailing reservoirs, it is difficult to accomplish remediation goals using a single remediation technology. A combination of methods should be used to improve the effects of remediation. In this study, a soil column washing system was established to simulate the tailing environment. By utilizing the synergistic purification effect between phosphorus-containing substances and CTs, washing experiments were conducted using an orthophosphate-containing solution. The tail water was collected, and its water quality indexes were evaluated. Finally, three water hyacinth materials were used for the advanced treatment of the tail water so as to reduce the harm caused by heavy metal ions to the environment. This study not only provides an effective method for the recovery and utilization of solid waste from CTs, thereby reducing the harm of CTs to the environment, but also provides a new method for phosphorus removal that makes full use of the ‘purification synergy’ of the two pollutants. This method realizes the concept of ‘treating waste with waste’ and achieves the goal of cleaner production.

The CTs and water hyacinth used in this study were collected from a concentrating mill in Tongling and the pond inside Anhui Polytechnic University in Wuhu, Anhui Province, China, respectively.

All chemicals used were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). The potassium dihydrogen phosphate (KH₂PO₄), 98% sulfuric acid (H₂SO₄), ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O), potassium antimony oxide tartrate (K(SbO)C₄H₄O₆·1/2H₂O), ascorbic acid, sodium hydroxide (NaOH), hydrochloric acid (HCl), and nitric acid (HNO₃) were all of analytical pure grade. All of the solutions required for the experiment were prepared with self-made ultrapure water.

For the thermal modification of the CTs, they were placed in a muffle furnace and heated for 2 hours at 340 °C (Zhou et al. 2018). The dried water hyacinth stem and root tissues were crushed by a plant crusher. They were then screened by 100 mesh, placed into an air-blowing drying oven, and dried to a constant weight to obtain water hyacinth stem and root powder. The preparation method used for water hyacinth biochar (WHBC) is described in a previous report (Zhou et al. 2020). The water hyacinth between 0.5 and 1.0 cm was placed evenly into a high-temperature quartz boat and placed in a constant-temperature area of the temperature control tube furnace. According to the optimal conditions determined by the results of the previous report, the pyrolysis parameters were set as follows: the initial temperature was 25 °C, the pyrolysis temperature was 425 °C, the residence time was 3.09 h, and the heating rate was 19.65 °C/min.

The eluvial column used in the experiment was made of a plexiglass material. The soil column was 8.0 cm in diameter and 30.0 cm in height. At the bottom of the soil column, two small holes with a diameter of 0.5 cm were provided to discharge the leachate. From the top to the bottom, the soil column consisted of one layer of 1 cm of geotextile to ensure that the leachate was evenly sprayed to the bottom of the tailings; 10 cm of thermally modified CTs, the main part of the soil column, which mainly reacted with orthophosphate; 1 cm of geotextile to effectively separate the upper CT sand from the lower fine sand; 6 cm of fine sand to better simulate the natural tailing reservoir environment; 1 cm of geotextile to separate the upper fine sand and the lower quartz sand; 3.0 cm of quartz sand; 1 cm of geotextile; 3.0 cm of gravel; and 1 cm of geotextile. The quartz sand and gravel served primarily as filters. The final filling height was 27 cm. The CT sand was placed on the top layer in order to be closer to the natural tailing reservoir environment and to ensure that the tailings and orthophosphates were in full contact, as if the fine sand layer was placed on the top, the orthophosphates could be intercepted. The soil column structure is shown in Figure 1(a), and the soil column washing experimental device is described in Figure 1(b).

The phosphorus-containing chemical washing agent consisted of 50 mg/L orthophosphate (KH₂PO₄) (Zhou et al. 2019). The orthophosphate solution was placed into a container and set on an iron platform. The flow rate was controlled to 0.80 cm/h, and the flow rate of leaching was controlled by the infusion flow rate regulator. The washing experiment was conducted in the soil column. The tail water was collected using a sampling flask and used as the experimental solution for subsequent advanced treatment experiments. The concentration of heavy metal ions in the tail water of the column outlet was determined by a Shimadzu ICPE-9000 inductively coupled plasma emission spectrometer, the concentration of phosphate in the tail water was determined by the molybdate blue spectrophotometric method, and the pH of the tail water was determined by a PHS-25 pH meter.

The characterization methods used for the water hyacinth materials are detailed in previous research (Zhou et al. 2018; Zhou et al. 2020). Scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) analysis was conducted using an S-4800 scanning microscope and an energy dispersive X-ray spectroscope (Hitachi, Japan) to examine the surface topography and the element types and contents of three water hyacinth materials. The materials were scanned and photographed 2,000 times and 5,000 times, respectively, under the condition of 5 kV high-acceleration voltage and vacuum extraction. The specific surface areas of water hyacinth materials were determined using a NOVA 2000e analyzer. Nitrogen adsorption-desorption was carried out in 77 K liquid nitrogen. The specific surface area and pore size distribution of the adsorbed material were calculated according to the peak areas of adsorption and desorption. The functional groups of materials were analyzed by a Shimadzu IRPrestige-21 transform infrared spectrometer (Shimadzu, Japan). Mix 1 mg sample powder with KBr at a mass of 1:200. After pressing, the samples were placed into the sample chamber and scanned at the wavelength range of 400–4,000 cm⁻¹. The crystallinity changes of the three materials were characterized by X-ray diffraction (XRD) with a Bruck-D8 series X-ray (powder) diffractometer (Bruck, Germany). A 0.2 g sample of powder was taken and placed into the sample chamber after tablet pressing and scanned in the range of 10°–80°.

After chemical washing, the pH, orthophosphate concentration, and heavy metal ion concentrations in the tail water were determined. The specific water quality indexes of the tail water are shown in Table 1, and the water quality index results are the average of triplicate experiments.

It can be concluded from Table 1 that the concentration of PO₄³⁻ in the tail water was 0.15 mg/L, which was lower than the first-level discharge standard of 0.5 mg/L in the ‘Comprehensive Wastewater Discharge Standard’ (GB8978-1996) and met the discharge standard. This standard is the national standard of China and is applicable to the management of pollutant discharge in existing water bodies nationwide. The results indicated that the CTs had a good interception effect on the phosphorus-containing wastewater and could reduce the risk of eutrophication. The pH of the tail water was 3.70, which belonged to the highly acidic wastewater category, and this pH was consistent with acid mine water exudate from actual tailing reservoirs. The concentrations of Cu²⁺, Cd²⁺, and Zn²⁺ in the tail water were 130.25, 8.10, and 21.50 mg/L, respectively, and other heavy metal ions were not detected. According to the ‘Comprehensive Sewage Discharge Standard’ (GB8978-1996), the prescribed concentrations for Cu²⁺, Cd²⁺, and Zn²⁺ are 1.0, 0.1, and 2.0 mg/L, respectively, and the concentrations of Cu²⁺, Cd²⁺, and Zn²⁺ greatly exceeded the prescribed standards. In general, the CT tail water was highly acidic. Hence, there is an urgent need for advanced treatment measures.

Following the use of chemical washing with orthophosphate, most of the phosphate was trapped in the tailings, and the phosphate concentration that leaked out was lower than the discharge standard. However, a portion of the heavy metal ions in the tailings leached into the tail water. This further confirms that during the process of tailing stacking and treatment, some heavy metal ions will leach out and cause heavy metal pollution to the surrounding environment. Therefore, it is necessary to conduct advanced treatment of these heavy metal ions to reduce harm to the environment.

The removal effects of the three materials on Cu²⁺ in the tail water are shown in Figure 2(a) and 2(b). It can be seen from Figure 2(a) that with an increase in the dosage of materials, the concentration of Cu²⁺ in the effluent showed a downward trend, and the removal rate of Cu²⁺ increased. The maximum removal rates of the stem powder, root powder, and WHBC were 63.89, 67.34, and 99.48%, respectively, and the effluent concentrations of Cu²⁺ were 47.03, 42.53, and 0.68 mg/L, respectively, when the adsorbent dosage was 20.0 g/L. It can be seen that the Cu²⁺ removal ability of WHBC was better than those of the stem and root powder. After advanced treatment using WHBC, the concentration of Cu²⁺ in the effluent was 0.68 mg/L, lower than the 1.0 mg/L in the secondary standard of the ‘Comprehensive Wastewater Discharge Standard’ (GB8978-1996).

Statistical Product and Service Solutions (SPSS) software was used to analyze the significance of the experimental results. A significance analysis of the Cu²⁺ removal efficiency of the three materials was conducted. The analysis results are described in Table 2. According to the analysis results, the Cu²⁺ removal efficiency of WHBC was significantly higher than those of the stem powder and root powder under different dosage conditions. The Cu²⁺ removal ability of stem powder was slightly better than that of root powder at medium and low dosages, while the Cu²⁺ removal ability of stem powder was lower than that of root powder at a high dosage. Overall, the significance analysis also showed that WHBC had a better removal effect than the stem powder and the root powder.

The Cd²⁺ removal effects of the three materials are displayed in Figure 3(a) and 3(b). As shown in Figure 3, compared with WHBC, the stem powder and root powder had poor removal effects on Cd²⁺ in the tail water. When the amount of adsorbent was 20.0 g/L, the removal rates of Cd²⁺ were only 48.02 and 54.39% for stem and root powder, respectively, and the effluent concentrations were 4.21 and 3.69 mg/L, respectively. The Cd²⁺ removal efficiency of WHBC increased with an increase in the dosage. When it increased to 20.0 g/L, the R% of Cd²⁺ in the tail water by WHBC was 94.94%, and the concentration of Cd²⁺ in the effluent was 0.41 mg/L, which was higher than the maximum allowable discharge standard of 0.1 mg/L (GB8978-1996). However, this would reduce the pressure for subsequent processing and the environmental risk.

The significance analysis results of the adsorption effect of Cd²⁺ are shown in Table 3. In general, the removal rate of WHBC on Cd²⁺ was significantly better than that of the stem and root powder at all adsorbent dosages. When the dosage was 0.5–2.0 g/L, the Cd²⁺ removal effect of root powder was lower than that of the stem powder. When the dosage was higher than 5.0 g/L, the use of the root and stem powder for the Cd²⁺ removal effect did not show obvious differences, and the Cd²⁺ removal effect of the root powder was slightly higher than that of the stem powder. In addition, under the condition of a dosage of 20.0 g/L, the root powder was superior to the stem powder for the removal efficiency of Cd²⁺. The significance analysis showed that WHBC had a better removal effect than the stem powder and the root powder.

After the tail water was treated with the three materials, the effluent concentrations and removal efficiency of Zn²⁺ were measured (Figure 4(a) and 4(b)). With increasing dosage, the Zn²⁺ removal rates of the three materials were also gradually enhanced. When the dosage reached 20.0 g/L, the Zn²⁺ removal rates of the stem powder, root powder, and WHBC were 43.41, 48.84, and 94.84%, respectively, and the effluent concentrations were 12.17, 11.0, and 1.11 mg/L, respectively. After treatment with the stem and root powder, the effluent concentrations were all higher than the 5.0 mg/L of the third-level discharge standard of the ‘Comprehensive Wastewater Discharge Standard’ (GB8978-1996), while the effluent concentration after WHBC treatment was lower than the 2.0 mg/L of the first-level discharge standard of the ‘Comprehensive Wastewater Discharge Standard’ (GB8978-1996).

The significance analysis results of the removal of Zn²⁺ from the tail water by the three materials are shown in Table 4. Under all the dosage conditions, the Zn²⁺ removal effect of WHBC was significantly stronger than those of the stem powder and root powder, and the removal effect of the root powder on Zn²⁺ was obviously better than that of the stem powder. According to the significance analysis, the Zn²⁺ removal effects of the three materials on the tail water were WHBC>root powder>stem powder.

To further demonstrate that biochar had a better treatment effect on tail water than stem powder or root powder, the properties of the three materials before the treatment of tail water were characterized.

The pH of the three water hyacinth materials was determined by pH meter (PHS-25, LEICI, Shanghai, China). The materials were mixed with ultrapure water at a mass ratio of 1:10 after oscillating at a speed of 150 r/min at 25 °C for 24 h. They were then filtered and the pH of the filtrate was measured.

The pH values of the stem powder, root powder, and WHBC were 6.53, 7.65, and 9.73, respectively. The pH values of the root powder and WHBC were in the alkaline range, which to some extent provided a good alkaline environment for the precipitation of Cu²⁺, Cd²⁺, and other heavy metal ions, improving the heavy metal removal performance. Biomass materials, such as biochar prepared with aquatic plants as raw materials, are generally alkaline and are used to treat acidic wastewater without washing to neutralize it, thus reducing the use of neutralizing agents and saving the cost of wastewater treatment. Therefore, whether to treat the biomass material can be decided according to the pH of the wastewater. In the treatment of acidic mine wastewater (tail water) or acidic industrial wastewater containing heavy metals, if the biomass material prepared is alkaline, no additional technical means are required to alter the pH so as to neutralize the acidic wastewater and reduce the treatment cost.

At a room temperature of 25 °C, the Ksp values of Cu(OH)₂, Cd(OH)₂, and Zn(OH)₂ were 5.0×10⁻²⁰, 2.2×10⁻¹⁴, and 7.1×10⁻¹⁸, respectively, and the K_w was 1.0×10⁻¹⁴. The concentrations of Cu²⁺, Cd²⁺, and Zn²⁺ in the tail water were 130.25, 8.10, and 21.50 mg/L, respectively. According to Equation (2), the pH values of the Cu(OH)₂, Cd(OH)_2, and Zn(OH)₂ precipitate were calculated to be approximately 5.70, 9.24, and 7.17, respectively. For Cu²⁺, the pH values of precipitation were all lower than those of the three water hyacinth materials; in particular, they were much lower than that of WHBC. Cu precipitates were found in all three materials, but more precipitates were found in WHBC. For Cd²⁺, the pH value of the precipitate was slightly lower than that of WHBC, so only a small amount of Cd was precipitated in WHBC. The pH value of Zn²⁺ precipitation was lower than those of the root powder and WHBC, but higher than that of the stem powder, so no Zn²⁺ was precipitated from stem powder. In general, the pH values of the three heavy metal ions precipitated in the tail water were all lower than that of WHBC.

The pH_pzc of water hyacinth materials was determined by a Zetasizer Nano ZEN3690 (Malvern Panalytical, UK) and measured using 0.01 M NaCl aqueous solutions in the pH range from 2.0–11.0. These pH values were fixed with 0.1 mol/L HCl and NaOH aqueous solution. The suspension was injected into the sample tank with a syringe, the zeta potential was measured in the sample tank, and the curve of the zeta potential changing with pH was drawn. When the zeta potential was 0, the corresponding pH was the point of zero charge of the material (pH_pzc) (Melliti et al. 2021).

The zeta potentials of the water hyacinth materials are displayed in Figure 5. According to the zeta potential analysis, the pH_pzc values of the stem powder and the root powder were not found in the measured pH range, and the pH_pzc of WHBC was 2.67. The variations of zeta potential of the three water hyacinth materials may have been caused by the cracking of cellulose, hemicellulose, and lignin in water hyacinth after pyrolysis, which leads to the change of pH and surface functional groups, and further changes in the zeta potential. When the pH was higher than 2.67, the surface of WHBC was negatively charged, which favored the adsorption of positively charged heavy metals onto the WHBC surface (Melliti et al. 2021).

The SEM images of the stem powder, root powder, and WHBC at the same magnification are shown in Figure 6. The images showed that there were many fine particles on the surface of the stem powder that were smooth, and no obvious pore structure was found. However, the surfaces of the root powder and WHBC were rougher than the stem powder. In general, materials with rough surfaces are more likely to adhere to pollutants, so the rough surface is more conducive to the attachment of heavy metal ions. Therefore, structures of the root powder and WHBC are more conducive to the adhesion of heavy metal ions. There were more lamellar stacks and thinner lamellae in the WHBC, which may have been due to the rupture of the aliphatic hydrocarbon group and the lipid C=O functional group under the protection of the aromatic nucleus. These lamellar structures were different in size and formed more holes (shown in the red circle in Figure 6). In the case of an adsorption reaction, these structures could provide more sites for heavy metal ions and improve the removal effect. After high-temperature pyrolysis, cellulose, hemicellulose, and lignin in WHBC were cracked, which made the pore structure of WHBC more developed. Some spherical crystals appeared and the surface became rougher, which was more conducive to the attachment of heavy metal ions. In addition, after high-temperature pyrolysis, the dendritic cellulose in water hyacinth will shrink, making the material surface become rough. Microspherical hemicellulose melts and foams, and its pore structure expands continuously. With the increase of temperature, organic components in lignin gradually exfoliate, pore structure becomes more developed, and spherical crystals appear. These changes improve the biochar's ability to capture and adsorb heavy metals.

On the basis of the SEM analysis, the stem powder, root powder, and WHBC were analyzed using X-ray energy spectra (EDS), and the element types and contents of the materials were determined. The EDS of the three materials are shown in Table 5. The results showed that the three materials all contained the elements C, O, Cl, and K. In addition to the above elements, the root powder contained small amounts of Si (3.61%) and Al (2.20%). The WHBC contained Mg (3.49%), Ca (6.23%), and P (8.03%), while the stem powder did not contain Si, Al, Mg, Ca, or P elements.

According to the data in Table 6, the surface areas of the stem powder, root powder, and WHBC were 0.216, 325.262, and 4.299 m²/g, respectively. The surface area of the WHBC was smaller than that of the root powder, probably because the stem accounted for a larger proportion in the raw material.

The nitrogen adsorption-desorption curves of the three materials are shown in Figure 7(a) and 7(b). As shown in Figure 7(a), the curves of the stem powder and the WHBC conformed to a type-IV with H3-type hysteresis loop. Type-IV isotherms are derived from mesoporous adsorbent materials with multilayer adsorption and capillary condensation phenomena (Kaur et al. 2020). According to the analysis in Figure 7(b), the nitrogen adsorption-desorption curve of the root powder basically conformed to the type-IV with H3-type hysteresis regression curve, which also belonged to the mesoporous adsorbent materials, but the degrees of adsorption and desorption were similar.

The qualitative differences in the surface functional groups of the three materials were analyzed using FTIR. The spectra are shown in Figure 8(a). The peak shapes of the three materials had many similarities, and all had stretching vibrations near 3,419 cm⁻¹ (-OH), 2,369 cm⁻¹ (C≡C), 1,645 cm⁻¹ (-COO-), 1,089 cm⁻¹ (-CO), and 832 cm⁻¹ (-CH) (Rahman et al. 2019). However, the stretching vibration peaks of the stem powder and root powder were near 2,800 cm⁻¹ and 2,900 cm⁻¹, respectively, and were absent for the WHBC. These two peaks were aliphatic C-H antisymmetric stretching vibrations that weakened or even disappeared in WHBC, indicating that the pyrolysis and dehydration of aliphatic groups occurred after high-temperature pyrolysis. Additionally, the transmittances of the stem powder and the root powder near 2,369 cm⁻¹, 1,645 cm⁻¹, 1,089 cm⁻¹, and 832 cm⁻¹ were higher than those of the WHBC, and their intensity in the WHBC decreased. For WHBC, the bands corresponding to the C-O stretching frequencies (1,645 cm⁻¹) of carboxyl, aldehyde, ketone, or ester groups, C-H bending (1,380 cm⁻¹) in alkanes and alkyl groups, C-H bending in alkenes (832 cm⁻¹), and out-of-plane N-H bending vibrations (672 cm⁻¹) gradually weakened or disappeared, suggesting that aliphatic compounds were highly aromatized during the carbonization process (Jin et al. 2021b). A new peak at 872 cm⁻¹ appeared in the WHBC. This peak may be associated with the stretching and bending motions of CO₃²⁻, which generates precipitates with heavy metal cations. This may also have been a reason why the adsorption performance of WHBC was stronger than those of the stem powder and root powder.

X-ray diffraction (XRD) was used to identify possible crystalline precipitates formed on the surface of the material. The XRD patterns of the stem powder, root powder, and WHBC are shown in Figure 8(b). As can be seen from Figure 8(b), the stem powder was dominantly amorphous with two broad peaks at 2θ values around 14.94° and 22.02°. This was likely due to the crystallinity possessed by cellulose. These two broad peaks disappeared in the WHBC, indicating that cellulose in water hyacinth decomposed under pyrolysis. Compared with the stem and root powder, the intensity of the WHBC at 2θ=28.50° and 40.62° was enhanced. This indicated that the crystallization degree of WHBC was significantly enhanced, which was consistent with Deng et al. (2019). In addition, the results showed that the pyrolytic reaction improved the graphite crystal structure and graphitization degree of the biomass. As shown in Figure 8(b), WHBC had a peak at 2θ=29.44°, while there was no such peak in stem powder or root powder. Combined with EDS analysis, these findings suggest that the peak here may represent the presence of Ca compounds, such as CaC₂O₄ or CaCO₃, which can precipitate with heavy metal ions. According to the XRD and EDS analyses, the WHBC was further confirmed to contain K, Ca, and Mg. These elements can react with heavy metal ions.

In this study, a novel soil column washing system was established to simulate the environment of CTs, and an orthophosphate solution was used to wash the thermally modified CTs. The passivation effect of orthophosphate on heavy metals in CTs reduces the bioavailability of heavy metals in CTs and alleviates pollution due to CTs that occurs during the landfill process. In addition, the adsorption and interception of orthophosphate by thermally modified CTs reduces the concentration of orthophosphates in the tail water, which helps alleviate the water eutrophication caused by phosphate. The tail water was collected, and three water hyacinth materials were introduced to remove the heavy metal ions in the tail water. The 20 g/L WHBC solution showed effective removal for heavy metals. After advanced treatment by WHBC, the concentration removal rates (R%) of Cu²⁺, Cd²⁺, and Zn²⁺ were 99.48, 94.94, and 94.84%, respectively, and the concentrations of the three heavy metal ions in the effluent were 0.68, 0.41, and 1.11 mg/L, respectively, which greatly reduced their potential threat to the environment. This scheme took advantage of the passivation of heavy metals by phosphorous substances combined with the affinity effect of heavy metals and phosphorous substances. It also took advantage of the synergistic purification effect of both to remove heavy metals and phosphates at the same time. Then, WHBC was used to remove heavy metals in the tail water to reduce subsequent heavy metal pollution to the greatest extent. This scheme not only reduces the bioavailability of heavy metals in CTs but also reduces the risk of water eutrophication and improves soil fertility in CT reservoirs.

Of course, this study has some limitations. For example, when washing the CTs, a speciation analysis of the heavy metal ions was not conducted. In future research, the forms of heavy metal ions will be analyzed and determined to obtain more theoretical support for tailing remediation projects. Future research could also focus on the extraction of heavy metals from saturated stem powder, root powder, and biochar for use as resources. This would avoid secondary pollution and provide a source of heavy metals as resources for secondary use. In addition, our research is still in the laboratory stage, and practical engineering applications are still a long way off.

This research was supported by the National Natural Science Foundation of China (51808001); the Natural Science Foundation of Anhui Province, China (1708085QB45, 1808085QE146, 2008085ME159); and the Anhui Polytechnic University Scientific Research Project (Xjky2020169); and Natural Science research project of Anhui Universities (No. KJ2021A0505). We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

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