Experimental study on the treatment of acid mine drainage containing heavy metals with domestic waste pyrolysis ash

Mineral resources play a pivotal role in the continued development of society. However, due to the long-term and unreasonable manner of development of mineral resources, a series of serious mining environmental problems have been induced, and acid mine wastewater is one of them (Núñez-Gómez et al. 2018). The sulfur-containing waste rocks, tailings, and other rock materials during the excavation and drilling process cause an acid-generating capacity that exceeds the acid-neutralizing capacity. These materials were oxidized by exposure to water and oxygen, and acid mine drainage (AMD) was thus produced (Kobielska et al. 2018; Lra et al. 2021). Due to its high acidity and the presence of various heavy metal ions, AMD has a great impact on the environment (Robinson & Brennan 2009). Once AMD directly flows into the natural water system, and then into the regional water system, it will lead to changes in water quality, which will not only inhibit the growth but also reproduction of organisms and hamper the self-purification of the water body (Kurniawan et al. 2006; Lim et al. 2021); at the same time, AMD containing a large number of heavy metals will lead to soil pollution and poisoning. If it is not managed properly, it causes considerable environmental degradation, water and soil contamination, severe health impacts on nearby communities, and loss of biodiversity and aquatic ecosystems (Kefeni et al. 2017).

AMD often has the characteristics of strong acidity and a high concentration of heavy metals; therefore, its treatment requires to an increase in the pH value and removal of toxic and harmful components such as heavy metals. Commonly used treatment technologies include; chemical precipitation, ion exchange, adsorption, membrane filtration, oxidation-reduction, biological methods (MacIngova & Luptakova 2012; Zhang et al. 2016), etc. For the advantages and disadvantages of different methods, see Table S1 (Supporting Information). In recent years, due to its great advantages, adsorption has become one of the most effective methods for the removal of heavy metals from wastewater, especially when large quantities of available natural materials or certain types of wastes from industrial activities may have the potential to act as low-cost adsorbents. Different studies have confirmed the effectiveness of using various environmentally available materials (e.g., dairy manure, bentonite, aluminum silicate clay, lignite and zeolite) (Motsi et al. 2009; Esmaeili et al. 2019; Es-Sahbany et al. 2021) for the removal of heavy metals from AMD. In addition, the simplicity, flexibility, and continuous operability of adsorption make it important to carry out research on the use of these materials for the treatment of AMD containing heavy metals.

Furthermore, with rapid global economic development and increasing urbanization, the massive increase in domestic waste has become a serious social problem (Han et al. 2018, 2019; Resende et al. 2019). The main method of garbage disposal in China is incineration, which produces a large amount of fly ash. However, it cannot be ignored that waste incineration generally has high r treatment costs, low efficiency, little space for resource recovery, and a large amount of land space required for incineration plants (Xue & Liu 2021). Waste fly ash contains a large number of heavy metal constituents, and the dioxins produced during incineration can cause air pollution (Huang et al. 2019a). Therefore, the pyrolysis technique has become a new method for domestic waste treatment due to its high capacity reduction ratio, low pollutant emissions, and lack of secondary pollution. Although the amount of domestic waste can be greatly reduced, some of the ash remaining in the slag will make disposal difficult. There are few studies on the application of pyrolysis ash as an adsorption material, referring to the existing applications of waste incineration fly ash in water treatment (Xue et al. 2015; Luo et al. 2019). It would be beneficial to adopt pyrolysis ash for water treatment in this paper.

In summary, this study will carry out research on the use of unmodified waste pyrolysis ash to treat synthetic solutions of AMD containing heavy metals. The effects of waste pyrolysis ash dosage, initial pH value of wastewater and reaction time on adsorption were studied through batch experiments, and the adsorption mechanism of pyrolysis ash was analyzed. It is found that waste pyrolysis ash has the advantages of environmental protection, high efficiency and low cost in the treatment of AMD, which meets the requirements of environmentally friendly adsorption materials. It provides a new method for the treatment of AMD and a new use for the solid waste produced by the domestic waste treatment plant.

The material used in the experiment is prepared from collected household waste (its physical and chemical properties are shown Table S2, Supporting Information) by cracking in a high-temperature pyrolysis device. The pyrolysis device was mainly composed of a feed preheated reaction chamber, a pyrolysis furnace, a cooling chamber, and an exhaust chamber. Before the reaction starts, the domestic waste is screened and processed to remove stones, soil, plastics, waste batteries, metal blocks, etc. The water content of domestic waste is usually relatively large before being dried; the squeezing method was used to remove 20% of the water in the waste. The initial dehydrated 10 kg waste was sent to a dryer for processing to obtain the organic waste, which prevents moisture from affecting the preheating and pyrolysis of the waste. The organic waste material was then fed into a sealed, oxygen-poor preheating reaction chamber where it was sterilized and partially decomposed at 350 °C for 1 h. Then the preheated organic waste material was fed into a sealed, oxygen-poor, high-temperature cracking section of the cracking chamber where it was completely cracked at 1,000 °C for 3 h. After sufficient smothering and cracking, the final waste treatment was achieved in a reduced and harmless manner.

With the completion of the pyrolysis reaction, the final household waste was converted to approximately 2.1 kg of ash, discharged from the bottom of the pyrolysis furnace, and was later used in experiments. After cooling and standing for 48 h, the completely cracked waste ash sample was crushed with an electromagnetic sample crusher and sieved with a 100-mesh sieve (particle diameter ≤0.154 mm). After the completion of the process, the resulting waste pyrolysis ash was sealed, and then stored in vinyl plastic bags for subsequent experiments (see Figure 1 for the preparation process).

A GC-4000 gas chromatograph was used to measure the concentration of metal ions after the reaction; an FE28-Standard pH meter was used to measure the pH of the solution before and after the reaction.

A Bruker AXS D8 Advance X-ray diffractometer was used to perform XRD analysis to characterize the types of minerals in waste pyrolysis ash; at the same time, a 250Xi X-ray fluorescence spectrometer (XRF) was used to analyze the chemical elements and forms of waste pyrolysis ash. The surface micromorphology of the waste ash samples before and after the adsorption experiment was analyzed using an S4800 scanning electron microscope (SEM), and all the gold-plated samples were sputtered before SEM imaging so to improve their conductivity and imaging quality. Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) models were used to determine the pore size distribution and specific surface area of waste ash.

To study the adsorption mechanism of heavy metals on waste pyrolysis ash, the properties of waste pyrolysis ash were determined. Its chemical composition is shown in Table 2, and its X-ray diffraction (XRD) analysis results are shown in Figure 2(a).

The XRD pattern of cracked waste ash shows that its main components are calcium zeolite (CaAl₂Si₂O₈·4H₂O) (PDF#20-4052), calcite (CaCO₃) (PDF#41-1475), gypsum (CaSO₄) (PDF#43-0606) and brushite (CaPO₃(OH)·2H₂O) (PDF#11-0293) (Lu et al. 2001). When 2θ is 25°∼28°, an obvious diffraction peak appears in the XRD pattern, that is, CaAl₂Si₂O₈·4H₂O is the main component of waste pyrolysis ash, which is a hydrated aluminosilicate mineral with a porous structure and properties such as cation exchange, adsorption, and molecular sieving (Gupta et al. 2009). The dissolution of CaO leads to an increase in alkalinity, which promotes the complete dissolution of SiO₂ and Al₂O₃, and the formation of a higher degree of aggregation of silicate anions so that the equilibrium of the components of the system develops toward the zeolite species with smaller pore sizes. On the other hand, the presence of the Ca-rich aqueous phase induces Ca²⁺ to be embedded in the lattice of some zeolites, forming a small number of aqueous calcium zeolites (Chen et al. 2017). It also shows that amorphous SiO₂ exists in the structure of waste pyrolysis ash, which exists as a silicon-oxygen tetrahedron [SiO₄]⁴⁻ structure in the pyrolysis ash of waste. The structure containing tetrahedral vacancies is considered to be the site for heavy metal adsorption (Hong et al. 2009). The diffraction peaks of the waste pyrolysis ash are less regular and have a wider peak pattern, indicating a more complex composition (Liang et al. 2020). Using XRF to determine the elemental content of the waste pyrolysis ash, the main elements were found to be Ca, Si, O and Al, and it was further demonstrated that the pyrolysis ash composition has the ability to provide alkalinity and adsorption.

SEM was used to characterize and analyze waste pyrolysis ash, as shown in Figure 2(b). The surface of the waste pyrolysis ash is loose, with many pores and an uneven distribution. Therefore, the Gibbs free energy of these parts is very high and has strong activity. These parts easily form adsorption sites, which benefit the heavy metal adsorption reaction (Nguyen et al. 2020; Dhaouadi et al. 2021).

The specific surface area of waste pyrolysis ash was analyzed by the BET method, and the specific surface area of waste cracked ash was measured to be 24.88 m²/g. The adsorption/desorption isotherm curve of waste pyrolysis ash is shown in Figure 2(c) and 2(d). According to the classification standard of the International Union of Pure and Applied Chemistry, the adsorption/desorption curve is a type IV isotherm with an H3-type hysteresis loop, which implies the presence of mesopores in the waste pyrolysis ash. H3-type hysteresis loops are common in nonrigid aggregates of lamellar particles, indicating that the material has developed slit-type mesopores (Tanhaei et al. 2019). The pore size distribution curve also shows that the pore size of waste pyrolysis ash is mainly mesoporous with a small number of macropores. The average pore size of waste cracked ash measured by the BJH method is 11.35 nm and the pore volume is 0.060 cm³/g. The rich pore size structure and large specific surface area results fully illustrate that the waste pyrolysis ash has a large specific surface area and abundant pores, which can provide more adsorption sites for heavy metals, confirming its adsorption properties and improving its adsorption capacity of heavy metals in water to a certain extent (Hong et al. 2009).

The waste pyrolysis ash selected in this experiment was a freshly prepared new material. A total of 10∼60 g/L waste pyrolysis ash was added to acid mine water to carry out the effect of pyrolysis ash addition on the release of alkalinity, and the adsorption and removal of heavy metal ions. The reaction time was 100 min, and the results are shown in Figure 3(a).

It can be seen that waste pyrolysis ash has a significant effect on the increase in the pH value of acidic water. When the dosage was 10 g/L, the pH value of the solution was increased from 2.8 to 9.23, and with the continuous increase in the dosage, the pH value of the solution reached 10.15 when the 60 g/L waste pyrolysis ash was added. This is mainly related to the participation of the main components of waste pyrolysis ash, such as CaO and CaCO₃, and the release of these alkaline materials raises the pH of raw water. In the reaction, within a certain limit, the more the dosage is, the more sufficient the neutralization reaction will be.

From Figure 3(a), it can be seen that when the dosage of waste pyrolysis ash varies from 10 to 60 g/L, the changing trend of the adsorption amount for each metal ion except for Pb²⁺ showed an increasing trend. For the adsorption of Pb²⁺ there was also an obvious trend of change. The adsorption capacity of Pb²⁺ decreased with the increase of waste pyrolysis ash, and it is possible that competitive adsorption between ions inhibits the removal of Pb²⁺. The unit adsorption amount gradually decreases, and according to the adsorption equilibrium law, the equilibrium concentration of Pb²⁺ in solution is relatively reduced and some adsorbents overlap or accumulate when the adsorbent dosage is increased, so that the active sites on the adsorbent surface that can effectively adsorb Pb²⁺ are reduced. A similar trend was also observed in adsorption studies of related heavy metal ions (Xue et al. 2015; Long et al. 2019). Cracked ash-based sorbents are mainly composed of silicates and thus have a negatively charged surface, resulting in the adsorption of metal ions. The alkalinity released by the adsorbent neutralizes the acid and inhibits the competing adsorption of H⁺ on the metal, and OH⁻ precipitates on the surface with the metal to further achieve metal removal (Zahar et al. 2015). When the dosage of waste pyrolysis ash is increased from 10 g/L to 30 g/L, the adsorption capacities of Zn²⁺, Cu²⁺, Mn²⁺, Fe²⁺ and Cd²⁺ also increased from 0.289 mg/g, 0.325 mg/g, 0.265 mg/g, 2.991 mg/g and 0.021 mg/g, to 0.331 mg/g, 0.332 mg/g, 0.329 mg/g, 3.323 mg/g and 0.027 mg/g, respectively. When the added amount exceeded 30 mg/L, the adsorption amounts of the five metal ions described above all tended to decrease. On the one hand, this may be due to the redissolution of metal precipitates when the pH of the water is too high. On the other hand, too many adsorbents exist so that the collision and contact of the adsorbent itself are intensified, and some active sites on the adsorbent are blocked, resulting in a decrease in the adsorption capacity. In addition, it might be that the charge buildup on the surface of the adsorbent is affected due to electrostatic interference between the adsorbents, thus weakening the binding sites of the ions to their surface and resulting in a decrease in adsorption capacity (Wang et al. 2013; Khalid et al. 2015).

The results of the correlation test using SPSS 22.0 showed a Pearson correlation coefficient value of 0.977 between the pH of the postreaction solution and the amount of waste pyrolysis ash dosed, with a significance probability value of P = 0.01 (Table S4, Supporting Information). This shows that the correlation between the two indicators, final pH and dosage, has a high positive correlation, which was consistent with the results obtained from our experiments (Baarda et al. 2019). The data on the correlation between waste pyrolysis ash and metal sorption capacity show that the removal of Fe²⁺ is least correlated with the amount of ash added. In general, in the presence of dissolved oxygen, most of the Fe²⁺ in water can be oxidized to Fe³⁺, which can be further hydrolyzed to Fe(OH)₃ flocs (Chaturvedi & Dave 2012) and removed by ash, while a very small fraction of unoxidized Fe²⁺ can be removed by pyrolysis ash adsorption. The adsorption capacity of Pb²⁺ had the highest negative correlation with the addition of pyrolysis ash, indicating that acidic conditions are more favorable for its removal. Pb²⁺ has the lowest heat of hydration, which makes it easier for it to shed its complex water and become naked Pb²⁺ ions, which are easily exchanged with the internal cations of the ash (Hui et al. 2005). The treatment effect of waste pyrolysis ash on heavy metals in acid mine wastewater is Cu²⁺ > Zn²⁺ > Fe²⁺ > Mn²⁺ > Cd²⁺ > Pb²⁺, and a moderate increase in the dosage can improve the adsorption efficiency. Considering the treatment effect and cost, the optimal dosage of waste pyrolysis ash is approximately 30 g/L, and the adsorption sites provided by the adsorbent reach saturation at this dosage.

The change in pH value will affect the existence of ions, which is an important factor affecting the adsorption of heavy metals. Under the conditions when the amount of adsorbent was 30 g/L and the adsorption time was 100 min, the effect of the initial pH value ranging between 1 and 7 on the experimental results was investigated (Figure 3(b)).

The final pH value of the solution after the adsorption reaction under different acid-base conditions is different (Figure 3(b)). In the process of increasing the initial pH value from 1.5 to 5.5, the pH value of the solution after the reaction increased from 9.69 to 10.03; when the initial pH was 6.7, the pH of the solution after the reaction decreased slightly. The pH value change of the whole experiment may be related to the reaction of metal ions, and alkaline substances are generated in this process, thereby increasing the pH value of the solution (Table S5, Supporting Information).

As the initial pH value increases, the adsorption capacity of waste pyrolysis ash for Zn²⁺ and Pb²⁺ shows a trend of first increasing and then gradually decreasing, but the adsorption capacity of Cu²⁺, Mn²⁺, Fe²⁺, and Cd²⁺ shows a steadily increasing trends. When the overall initial pH value changed from 1.5 and 6.7, the adsorption capacity of Fe²⁺ by waste pyrolysis ash was relatively higher than that of Cu²⁺, Mn²⁺ and Cd²⁺. The reason may be the different types and quantities of functional groups on the surface of waste pyrolysis ash and the different affinities between functional groups and metal ions (Wang et al. 2009). When multiple ions coexist, ion competitive adsorption occurs, which leads to different adsorption capacities of ions by waste pyrolysis ash. The results show that the initial pH value has a significant effect on the removal rate of heavy metals. At a solution pH below 2.8, the lower pH leads to an increase in H⁺ in solution, which competes with heavy metal ions for exchange sites in the waste pyrolysis ash, making the waste pyrolysis ash less capable of adsorption (Dai et al. 2012).

The correlation analysis of the initial pH with the pH of the postreaction solution and the adsorption capacity of the metal ions therein shows that the initial pH correlates well with each of the postreaction parameters (except Zn). The pH of the postreaction solution and the initial pH have a strong positive correlation, i.e., as the initial pH increases, the pH of the postreaction solution also increases. The adsorption capacity of Zn is less affected by the initial pH of the solution. Under low pH conditions, hydrogen ions compete for the active sites on the adsorbent surface with heavy metal ions and lead to compromised adsorption of other metal ions (Tohdee et al. 2018). As the initial pH increases, the concentration of OH⁻ in the solution increases, which will react with metal ions to form hydroxides, and the resulting precipitation promotes the removal of metal ions to a certain extent (Motsi et al. 2009). The active sites of the adsorbent gradually become negatively charged and start to gain a gradual advantage in competition and bind to a large number of functional groups on the surface of the adsorbent, but too high a pH can lead to a decrease in adsorption due to charge repulsion (Long et al. 2019). Considering the treatment effect and cost, it was found that the adsorption performance of waste pyrolysis ash for metal ions changed significantly at the initial pH value of 4.1.

The time for the adsorbent to participate in the reaction is also an important condition affecting adsorption. In this experiment, under the condition that the amount of waste pyrolysis ash is 30 g/L, the effect on the final pH value of mine wastewater and the adsorption capacity of various heavy metals was studied. The results are shown in Figure 3(c).

The adsorption process of waste pyrolysis ash to five kinds of metal ions can be divided into the fast adsorption stage and adsorption saturation stage. The intersection point of the two stages is defined as the inflection point, where the inflection points of Cu²⁺, Mn²⁺, Zn²⁺, Fe²⁺, Pb²⁺, and Cd²⁺ are at 160 min, 160 min, 180 min, 80 min, 120 min, and 140 min, respectively. Before the inflection point, the pyrolytic ash rapidly adsorbed metal ions, and after the inflection point, the adsorption and desorption processes reached dynamic equilibrium. The competitive adsorption capacity of metal ions is generally related to the hydrolysis constant, ionic radius, electronegativity, and so on (Li et al. 2021). There are two paths for Fe²⁺ in water: first, the oxidation to Fe³⁺ and then the formation of trivalent hydroxide precipitation, and second, the direct precipitation of Fe²⁺ (Badmaeva et al. 2019). Having two routes can cause it to show the fastest adsorption saturation. The ionic radii of Pb²⁺ and Cd²⁺ are larger than those of Mn²⁺, Cu²⁺, and Zn²⁺, so the hydrates formed by Pb²⁺ and Zn²⁺ have smaller radii, which are more rapidly adsorbed by pyrolysis ash. This is consistent with the research of Nagwa & Mahmoud (2016) and Charazińska et al. (2021). The greater the electronegativity of the metal is, the stronger the covalent bonding with the surface or internal oxygen atoms of the adsorbent, and the easier it will be adsorbed (Dhaouadi et al. 2021), so it is predicted that the adsorption order is selected according to the electronegativity as Cu²⁺ > Mn²⁺ > Zn²⁺, which is consistent with the results of this experimental study.

The recycling test of waste pyrolysis ash was conducted at an adsorbent dosage of 30 g/L, an initial pH value of 4.1, and an adsorption time of 120 min. As shown in Figure 3(d), the removal rates for each metal ion decreased from 98.2%, 97.7%, 96.1%, 98.3%, 74.4%, and 67.5%, to 78.7%, 85.2%, 80.2%, 82.3%, 65.1%, and 52.9%, respectively after three times of repeated use, and the overall removal effect was better when the waste pyrolysis ash was repeatedly used five times, and the removal efficiency for each metal ion, the highest removal rate of 71.3% for Cu²⁺ and the lowest removal rate of 40.8% for Cd²⁺ were maintained. In the regeneration and recycling of waste pyrolysis ash, the treatment effect for metal ions was better in the first three reactions and gradually weakened in the subsequent uses, which confirmed that waste pyrolysis ash can be reused at least three times in the treatment of acid mine wastewater and has good recycling performance.

The adsorption kinetics and linear correlation fitting results of each metal ion on the waste pyrolysis ash adsorbent are shown in Figure 4, and the kinetic parameters are listed in Table 3. The pseudo-second-order kinetics coefficients of determination (R²) for the adsorption of heavy metals on waste pyrolysis ash were all above 0.99, and the q_e values of Zn²⁺, Cu²⁺, Mn²⁺, Fe²⁺, Pb²⁺ and Cd²⁺ simulated with the model were 0.298 mg/g, 0.414 mg/g, 0.413 mg/g, 3.534 mg/g, 0.033 mg/g, and 0.034 mg/g, respectively, which were in general agreement with the experimental results. The above results suggest that the pseudo-second-order kinetic model can be used to describe the sorption of heavy metals from acid mine water by waste pyrolysis ash. The kinetic equations attributed the main cause of adsorption to the formation of chemical bonds, and the adsorption of metal ions by waste pyrolysis ash is mainly due to a large number of active adsorption sites for Si and Al, which form chemical bonds with metal ions. Additionally, in combination with the molecular structure of the adsorbent and the metal ions, it is hypothesized that the adsorption of metals by pyrolysis ash is primarily a chemisorption process, which may be either electron-sharing coordination or an electrostatic effect of electron transfer, or a synergistic effect of both (Li 2019a). As seen from Table 4, the fitted results of the intraparticle diffusion model simulated in this experiment for the six heavy metal ions showed a weak linear relationship during the adsorption process, except for Mn²⁺ and Cd²⁺ (the values of R² ranged from 0.6∼0.7), indicating that the adsorption process was controlled by more than two models (McKay 1998). The fitted straight line did not pass through the origin (Oezcan & Oezcan 2005), indicating that intraparticle diffusion is not a rate controlling step (Li 2019b; Al-Dahri et al. 2020). In summary, the adsorption of metal ions by waste pyrolysis ash adsorbents is dominated by chemisorption with the influence of intraparticle diffusion.

The adsorption isotherms and linear correlation fitting results of metal ion adsorption are shown in Figure 5. The Langmuir and Freundlich isotherm model parameters for metal adsorption by waste pyrolysis ash are summarized in Table 4. An analysis of the coefficients of determination shows that R_F² > R_L², indicating that the adsorption of heavy metals by waste pyrolysis ash is shown to be multilayered, precisely because it is composed of various substances such as calcite, calcium zeolite, and gypsum. Evidence indicates that the adsorption process is primarily a nonhomogeneous process (Fisher-Power et al. 2016), and the 1/n values for the metal ion adsorption models are all less than 2, indicating that they are readily absorbed by the waste pyrolysis ash (Wang et al. 2020). In addition, the Freundlich adsorption coefficients of the waste pyrolysis ash for Cu²⁺ were greater than those of Zn²⁺ and Mn²⁺ under the same initial concentration conditions, due to the differences in electronegativity and ionic radii of the metal ions, where the electronegativity of Cu²⁺ was 28.56 eV, which was essentially close to that of Zn²⁺ (28.84 eV) and slightly greater than that of Mn²⁺ (24.66 eV). The ionic radius of Cu²⁺ is 0.72 Å, which is smaller than that of Zn²⁺ (0.74 Å) and Mn²⁺ (0.8 Å), which means Cu²⁺ has a closer electronegativity and smaller ionic radius than Zn²⁺ and Mn²⁺, resulting in a greater electrostatic gravitational interaction and leading to an increase in its adsorption. The related adsorption of Pb²⁺ and Cd²⁺ is similar (Huang et al. 2019b). Additionally, the coefficients of the Langmuir adsorption model show that the maximum adsorption amounts of waste pyrolysis ash for Zn²⁺, Cu²⁺, Mn²⁺, Fe²⁺, Pb²⁺, and Cd²⁺ are 0.425, 0.593, 0.498, 18.519, 0.055, and 0.039 mg/g respectively, which demonstrates that there is great potential for the treatment of acid mine wastewater with waste pyrolysis ash.

As the pH of the solution rises, the metal ions are encouraged to react with OH⁻ and precipitation occurs, facilitating removal. The small particle size and large specific surface area of the waste ash allowed flocculation, and the adsorption and coprecipitation of the flocs combined to increase the removal of metal ions. Waste pyrolysis ash has a high specific surface area, a rich pore structure and a high concentration of Na and Ca, which can play a greater role in adsorption and ion exchange.

The process of treating acid mine wastewater containing heavy metals involves three main reactions: alkaline release, adsorption flocculation, and precipitation complexation. The addition of waste pyrolysis ash induces the precipitation of metal ions in water by raising the alkalinity of acidic wastewater. The generated precipitation in the form of flocs further adsorbs more metal ions through adsorption bridging and electrical neutralization, accelerating the aggregation effect and prompting rapid precipitation of flocs. In addition, the pyrolysis ash itself has a large specific surface area with a certain pore size and pore capacity, which can also adsorb the polymers of metal ions or/and the metal ions to a certain extent. Therefore, waste pyrolysis ash can improve the pH value of acidic wastewater and achieve a good treatment effect on metal ions through the combined effect of adsorption, chemical precipitation and flocculation (the process is shown in Figure 6).

In this study, the adsorption capacity of waste pyrolysis ash for metals in AMD was compared with some other adsorbents, as shown in Table 5. There may be some deviations between the experimental results due to the existence of different experimental conditions, but the maximum adsorption capacity of waste pyrolysis ash for the metals used in this experiment clearly showed good results for AMD. By applying inexpensive pyrolysed waste to AMD treatment, we cannot only solve the problem of heavy metal pollution in acidic water, but also find new ways for the use of waste. Moreover, the waste pyrolysis ash used in this study was not otherwise modified and could be widely collected. It has both economic and environmental benefits and the potential for large-scale commercial application.

In this study, ash from domestic waste pyrolysis was involved in the treatment of acid mine wastewater containing several heavy metals. Waste pyrolysis ash has a large specific surface area and is rich in alkali metal oxides, which exhibit rapid pH elevation and both pollutant adsorption and removal during the reaction. The adsorption of heavy metals by waste pyrolysis ash in acidic mineral water was obtained through batch experiments. The experimental results and data analysis led to the following conclusions.

• (1)

  The maximum adsorption amounts of Zn²⁺, Cu²⁺, Mn²⁺, Fe²⁺, Pb²⁺ and Cd²⁺ reached 0.425, 0.593, 0.498, 18.519, 0.055, and 0.039 mg/g, respectively, when the addition amount of waste pyrolysis ash was 30 g/L, the initial pH value of water was 4.1 and the reaction time was 150 min. An amount of 30 g/L is also the optimal amount of waste pyrolysis ash in the adsorption reaction. The adsorption process can be divided into a rapid adsorption phase and adsorption saturation phase, where Fe²⁺ can reach adsorption saturation the fastest.

• (2)

  The level of heavy metal ion removal is related to its competition for hydrogen ions. The correlation coefficient values of waste pyrolysis ash dosage and initial pH with postreaction solution pH were 0.977 and 0.936, respectively, with a significance probability value of P = 0.01. The results indicated that there was a high positive correlation between both initial pH and postreaction pH, and the dosage.

• (3)

  The recycling test of waste pyrolysis ash showed that the regenerated waste pyrolysis ash could be reused at least 3 times with good recycling performance in the treatment of AMD.

• (4)

  The pseudo-second-order kinetic model is the best model to describe the adsorption process of waste pyrolysis ash on heavy metals in AMD, where the adsorption is mainly chemisorption and influenced by the intraparticle diffusion effect. The experimental data obtained in the batch study are in good agreement with the Langmuir and Freundlich isotherm models and are more consistent with the Freundlich isotherm model.

• (5)

  The process of treating AMD containing heavy metals involves three main reactions: alkaline release, adsorption flocculation, and precipitation complexation. The generated precipitation in the form of flocs further adsorbs more metal ions through adsorption bridging and electrical neutralization, accelerating the aggregation effect and prompting rapid precipitation of flocs.

The results of a series of experiments showed that waste pyrolysis ash has the potential to effectively increase the pH value of acid mine wastewater and to remove heavy metals. The amount of waste storage is greatly reduced after using waste pyrolysis ash to treat AMD, which not only realizes the disposal and utilization of waste but also achieves a good treatment effect on wastewater and achieves the purpose of treating waste with waste. This study is mainly a theoretical study under laboratory conditions, with less research on the use of real AMD. Subsequent studies would further explore the metal leaching of waste pyrolysis ash, establish a complete evaluation system, give full play to the application of waste pyrolysis ash in AMD, and lay the foundation for the practical application of industrial and agricultural waste byproducts as adsorbents.

The financial support of the diffusion barrier technology for compound pollution from acidified waste rock dumps at mining sites (2019YFC1805003) is gratefully acknowledged.

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