Study on the mechanism and kinetics of manganese release from waste manganese ore waste rock under rainfall leaching

Manganese released from the piled manganese ore wastes is a great threat to the local ecosystem and human health. The mechanism and dynamic characteristics of manganese release from the manganese ore wastes were studied based on the static and dynamic experiments. The concentration of manganese in the leaching solution under the intensive state is twice that resulted from the static state; the manganese release from the waste rock increased with the increase of the solid-liquid ratio and reached 922.3 mg/L when the solid-liquid ratio was 1:5. When the particle size of waste rock was less than 180 μm, the release amount of manganese was the largest and reached 491.3 mg/L. When the pH was 7 and the rainfall intensity was 80 mL/h, the increase of leaching time contributed to the rapidly decreased amount of manganese released, and the leaching process reached equilibrium gradually. The cumulative release of manganese increased with the increase of rainfall duration. In the dynamic leaching process, the change of pH and EC of the leachate had nothing to do with the initial pH of leaching agent but has a close relationship with the hydrolysis of minerals in waste. According to the experimental results, it was found that the double constant equation model fitted the kinetic process of release process better. The purpose of this study was to provide a scientific basis for the assessment and control of manganese pollution in soil and groundwater in manganese mining area.


Introduction
Manganese (Mn) is a kind of global pollution substance Ren et al. 2015;Ren et al. 2014), which has biodegradability, strong accumulation potential and high toxicity Peng et al. 2018;Zhou et al. 2017b).
According to the water quality standard of the World Health Organization (WHO), the content of manganese in drinking water should not exceed 0.1 mg/L. Water with high manganese content has a strong corrosion capacity to the production equipment and does great harm to human health.
Manganese (Mn) is an important strategic resource, and its application value in the national economy and steel industry is irreplaceable. Microwave heating is a new green method . The ability of fast heating dielectric materials can make effective use of manganese resources. The mixture can be used as a very good microwave absorbing material (Kl et al. 2020). The anode slime with high manganese content produced in the process of electrolytic manganese has excellent microwave absorption performance (Omran et al. 2019a). Therefore, the utilization of manganese resources is increasing with the development of economy.
Many waste rocks were produced in the process of continuous mining in manganese ore area, resulting in the formation of rock storage yard (Ren et al. 2014;Wang et al. 2014). Under the effect of rainfall leaching, the manganesecontained leachate from waste rock storage yard migrates with the rainfall infiltration and surface runoff and enters into the soil, surface, and underground water of the mining area, resulting in manganese pollution of the ecological system. Therefore, the migration of manganese in the regional water environment of the mining area can be a great threat to the environment and human health, which is a key problem to be solved urgently in national economic and social development (Lin et al. 2014;Ren et al. 2017;. In order to prevent manganese pollution in mining area, a good waste dump must be chosen. First, the slag yard should be built in places without spring water to prevent the slag reservoir from seeping for a long time. Second, the filter press workshop should be built in the position close to the slag storage, so as to reduce the probability of pollution of surface runoff during transportation. Third, improve the filter press equipment, reduce the moisture content of waste slag, and eliminate water flushing slag. The release of heavy metals from soil, sludge, dust, and other solid media by simulated rainfall completed by the researchers mainly focused on the influence of a series of factors (such as the application of microwave heating) on waste leaching mode Emilia et al. 2016;Guo et al. 2014;Han et al. 2017;Herndon et al. 2018;Kukurugya et al. 2017;Li et al. 2019a;Li et al. 2019b;Liang et al. 2016;Zehang et al. 2018;Zhang et al. 2018;Zhang et al. 2019a;Zhang et al. 2019b;Zhou et al. 2017a;. For example, Yuan et al explored the leaching law of heavy metals in Taolin lead-zinc tailing area in Hunan Province under the simulated acid rain leaching. It was shown that the larger the solid-liquid ratio and the smaller the pH value of simulated acid rain in the static leaching experiment, the larger the leaching concentration of heavy metals. In the dynamic leaching experiment, the leached concentrations of Zn and Mn were decreasing with time, while the leached concentrations of Pb and Cu were increasing and then decreasing with time (Yuan and Liu 2012). Hu et al studied the leaching characteristics and changes of antimony bearing ore layer in China. The results presented that the release of heavy metals from the medium antimony mine was mainly determined by the type of ore, as well as the type of Sb and the pH of the solution ). Zhang et al have made research on leaching of heavy metals from antimony tailings by the microbial sulfur-oxidizing bacteria which have achieved certain research results. It was proved that the presence of sulfur-oxidizing bacteria would promote the leaching of heavy metals from antimony mine tailings and aggravate the environmental pollution in the mine area (Zhang et al. 2014). Omran et al explored the application of microwave heating. It was studied that calcium-zirconia was prepared by microwave sintering and melting zirconia ceramic material, which can replace the traditional sintered brick (Omran et al. 2019b). However, there were few studies on the release characteristics and cumulative leaching mechanism of manganese under the continuous rainfall. In view of this situation, the aim of this study is to obtain the leaching characteristics of manganese from manganese ore waste rock, to explore the influence of the disturbance ratio, solid-liquid ratio, particle size, rainfall intensity, and the pH of rainfall on manganese release, and to establish a dynamic model of the manganese release. The application of this model is beneficial for understanding the mine area pollution prevention and treatment of manganese and other related heavy metal, and more conducive to the sustainable development of non-ferrous metal mining areas. Therefore, it is an important scientific basis for establishing the evaluation and control system of manganese pollution in soil and groundwater of manganese mine area to master the mechanism and dynamic characteristics of Mn release under leaching of manganese ore waste rock.

Experimental materials
The waste rock samples were collected from the Hongqi mining area, Xiangtan City, Hunan Province. The samples collected at the site were mixed evenly and then dried in the laboratory to remove the large waste rock, the biological debris, and other unrelated substances. Then, the waste rock samples were ground in the grinder. After air drying, the samples were graded by the stainless steel screen (830, 380, 250, 180 μm, respectively), and the pretreated waste rocks with different particle sizes were stored for use.

Static leaching simulation experiment
The static leaching experiment device uses 1000 mL wide mouth bottle and rotary oscillator. According to the nature of rainwater in Hunan Province, the H 2 SO 4 and HNO 3 mixture with a mass concentration of 10% (V/V: 1/1) was added into the ultrapure water to prepare the simulated acid rain. Additionally, NaOH (10%, mass concentration) solution was used to adjust to the required pH (pH = 5.0). During the experiment, 5 mL samples were obtained from the flask after standing for 10 min every 24 h. The samples were filtered using 0.45 μm filter membrane, and the concentrations of Mn in the samples were determined by flame atomic absorption spectrophotometer. The experimental period was 24 days. Parallel double samples were used in the same condition experiments.
The effects of different disturbance ratio on Mn release from waste rock were completed according to the following procedure: 10g waste rock with grading mesh less than 180 μm was added into a 1000 mL wide mouth bottle and mixed with the 200 mL leaching solution (pH = 5.0). The 24-day static leaching experiments were carried out by placing the wide mouth bottle under the static conditions on the experimental platform. Five grams, 10 g, 20 g, and 40 g of waste rock with grading mesh number less than 180 μm were respectively added into the 1000 mL wide mouth bottles to study the effects of different solid-liquid ratio on Mn release efficiency. Two hundred milliliters mixed leaching solution (pH=5.0) was added into the bottles to ascertain that the solid-liquid ratios were 1:5, 1:10, 1:20, and 1:40, respectively. Besides, 10 g of waste rock with 380-830 μm, 250-380 μm, 180-250 μm, and~180 μm waste rock were adopted to study the effects of different particle sizes on Mn release efficiency. All the experiments were completed under the oscillation condition of 150 R/min.

Simulated dynamic leaching experiment
A self-made column was used to simulate the dynamic leaching process. The experimental device includes water storage beaker, peristaltic pump, self-made leaching column, etc., as shown in Fig. 1. The water storage beaker is mainly used to store the leaching solution required by different leaching conditions every day. The inlet of the peristaltic pump provided power of the dynamic leaching procedure and controlled the intensity of the simulated rainfall. The self-made leaching column is the main place for leaching reaction, and the inner diameter of the hollow cylinder is 5 cm, and the lower end was sealed and equipped with a water collection device.
The annual pH of acid rain in Hunan Province is maintained at 4.06~6.36. The average value is 4.98 in the acid rain control area of the sulfuric acid type. Therefore, the pH of the simulated rainwater was adjusted by the mixture of H 2 SO 4 and HNO 3 (V/V: 3/1) and NaOH solution. According to the rainfall statistics of Xiangtan Meteorological Bureau in recent 10 years (2010-2019), the runoff loss of the rainfall (about 30%) was considered in the experiments, and the simulated rainfall under different conditions was formulated as follows: The average annual rainfall in recent 10 years is 1383.2 mm; after deducting the influence of surface runoff (30%), the monthly average rainfall from January to December is 43.4 mm, 42.9 mm, 98.0 mm, 86.0 mm, 141.4 mm, 168.9 mm, 98.7 mm, 77.6 mm, 63.6 mm, 27.5 mm, 78.6 mm, and 41.8 mm. The inner diameter of the self-made leaching column is 5 cm. The results showed that the average monthly leaching amount was 115 mL and that in rainy season (from May to July) was 268 mL.
Before the formal experiment, the leaching columns needed to be treated to some extent. Some auxiliary facilities needed to be added using the waste rock up of down the leaching column. From the bottom to the top, there were 2~3 layers of filter paper, 1 layer of non-woven fabric, 2~3 cm high fine quartz sand, waste rock, 1 layer of non-woven fabric, and 2~3 cm fine quartz sand. Among them, the bottom quartz sand acts as a supporting layer, and the bottom non-woven fabric and filter paper were used to prevent the loss of waste rock in the leaching process. The top quartz sand played the role of the uniform distribution of the simulated rainfall. Two hundred fifty grams of manganese-contained waste rock was respectively weighed and filled into the waste rock layers in the leaching columns and vibrated gently to make it dense. Then 500 mL ultrapure water was slowly added and naturally drained for 24 h. In the experiments, the volume of filtrate was determined as 1~2 mL resulted from evaporation per day. The leaching solution was collected at the bottom of the leaching column, and the manganese concentrations were de- Fig. 1 The dynamic leaching experimental device termined. After each leaching, it was placed naturally until the next day. The specific experimental steps were as follows: The effects of different rainfall intensities on Mn release efficiency from manganese-containing waste rock were completed according to the following procedure: the simulated rain water with pH = 5.0 was prepared by adding 268 mL leaching solution to simulate the rainfall in rainy season every day. The leaching cycle was 20 days. The influent flow was controlled to simulate three levels of rainfall intensity: 80 mL/ h, 200 mL/h, and 400 mL/h. When studying the effect of different rainfall pH, the simulated rainwater with pH of 3.0, 5.0, 7.0, and 9.0 were prepared by the above preparation method, and 115 mL leaching solution of simulated monthly average rainfall was added every day. The leaching cycle was 20 days. The influent flow was controlled to 400 mL/h. The effect of different rainfall duration on Mn release from manganese ore waste rock is as follows: the simulated rainwater with the pH of 5.0 was prepared according to the method mentioned above, and the influent flow was controlled to 400 mL/h by peristaltic pump. The added simulated rainfall was 1280 mL for 1 day. The leaching cycle was 12 days, and the leaching time was from 13:30 to 16:12 P.M. every day. The concentrations of heavy metals in the leachate obtained at the bottom of the column were determined.

Analysis methods
Manganese content was determined using the system of the nitric acid, perchloric acid, and hydrofluoric acid. High concentrations of Mn were determined using flame atomic absorption spectrometry (AA-7000), and low concentrations were determined using atomic fluorescence spectrometry (AFS-9700). The crystal structure of waste rock was determined by X-ray diffraction (Bruker D8 Discover) with CuKa radiation (40 kv, 40 mA). The pH value was determined using soil pH method NY/T 1377-2007. The total conductivity of leachate was measured using the ddbj-350 conductivity meter.

Phase composition analysis
The crystal structure of the waste rock was qualitatively analyzed by XRD. The XRD pattern of the waste rock is shown in the Fig. 2. The results show that the waste rock mainly includes quartz, gypsum, and Al 2 O 3 . Other minerals may be undetectable due to their low content.

Static leaching simulation experiment
Effect of disturbance ratio on manganese release from manganese ore waste The effect of the disturbance ratio on manganese release efficiency from manganese ore waste resulted from the static leaching experiment is presented in Fig. 3. The manganese concentration in the leaching solution increased with the increase of time and then tended to balance gradually and kept fluctuating in a small range. Under the oscillation condition, Mn had a certain leaching concentration on the first day of leaching and then increased slowly in the next 6 days. After that, the manganese concentration in the leaching solution rapidly increased to the maximum value and reached the equilibrium concentration on the 15th day. The concentration in the leaching solution fluctuated in a small range with time after it had the basic leaching concentration on the first day. It can be seen that the concentration of manganese in the leaching solution in the oscillation state is obviously higher than that in the static state, and it was twice that under the static conditions.
On the first day of the experiment, the acid (pH = 5.0) in the solution neutralized with the alkaline substance in the waste rock, which promoted the manganese release quickly into the leaching solution. Afterwards, the solution became in an alkaline state (pH is between 6 and 8) under the action of alkaline minerals. The disturbance increased the concentration gradient of manganese in leaching solution and promoted the precipitation of manganese. Simultaneously, the hydraulic shearing and the friction collision between particles were enhanced when the solution is agitated. The micro-balance formed on the surface of manganese ore waste rock particles could be destroyed under the synergistic effect of hydraulic shear effect and friction collision, causing the release and migration of manganese from waste rock into solution. In the static experiments, manganese was mainly released by acid-based neutralization of waste rock in the early stage, and then further diffusion of manganese was hindered at the surface equilibrium. Therefore, manganese was easier to be precipitated under the oscillating condition than the standing condition.
Effect of different particle size and solid-liquid ratio on manganese release from manganese ore waste According to Fig. 4, the effect of particle size was slight when the particle size was between 180 and 830 μm. The leaching concentration and dissolution rate of manganese in the leaching solution resulted from the three different particle sizes remain basically unchanged and were less affected by the particle size. When the particle size was less than 180 μm, the release rate and leaching concentration of Mn were significantly higher than those in the other three reaction conditions. In addition, smaller particle size possessed the larger specific surface area and contributed to the better contact area of solid and liquid in the experiment. Consequently, In the natural mine environment, microbial action could accelerate the change of instability, and the surface organic matter could form a layer of weathering resistant surface zone, which hindered the further change of particle size.
As shown in the Fig. 5, the leaching concentration and dissolution rate of manganese changed little when the solidliquid ratio was 1:20, 1:10, and 1:5. Nevertheless, the leaching concentration of manganese was obviously lower than the other three conditions when the solid-liquid ratio was 1:40, and the leaching process reached equilibrium soon in the early stage of the experiment. The reason could be that when the solid-liquid ratio was small to a certain extent, the effect of manganese concentration gradient played a leading role in the dissolution and release process; there are enough leachant available for the leaching of Mn particles. Because the basic concentration of manganese in the waste rock with small solid-liquid ratio was lower than that of large solid-liquid ratio, the concentrations at the solid-liquid ratio of 1:40 were obviously lower than that of other conditions. In the higher solid-liquid ratio, other factors played a leading role and were affected less by the solid-liquid ratio.

Dynamic leaching simulation experiment
Effect of different rainfall intensity on manganese release from waste rock It could be seen from Fig. 6 that the concentration of manganese in the leaching solution was extremely high on the first day of leaching and then rapidly decreased. The release concentration reached the minimum value on the seventh day of leaching, and the manganese content in the residue after leaching was extremely low.
The impact of different rainfall intensities on manganese release was slight, which was mainly reflected in the dissolution concentration in the first 2 days. In the early stage of small rainfall intensity (80 mL/h), manganese was released from waste rock. There was little difference between the rainfall intensity of 200 mL/h and 400 mL/h, but the release concentration of 200 mL/h was slightly greater than that of 400 mL/h. Due to the limited infiltration capacity of the dense leaching column, different surface rainfall ponding will be formed at different rainfall intensities. Therefore, there was no ponding on the surface of 80 mL/h leaching column and a very shallow ponding on the surface of 200 mL/h leaching column. Accordingly, a certain height of water column will appear on the surface of the filler and the trickling speed was 400 mL/h (the maximum is about 5~8 cm). Under the experimental conditions, when the surface ponding was formed, the experimental conditions would transform from the leaching to a soaking stage of slow seepage. Meanwhile, the erosion and shear effects of the rainwater on the surface ore decreased. In the case of no ponding, the accumulated voids of ores were significantly larger than those of the water bearing ores, and the oxygen in the air would be deeper in the leaching column than in other conditions, which would promote the oxidation of sulfide and the release of manganese.
Under different rainfall intensities, the pH of leaching solution was between 6.5 and 8.0, which was basically neutral and slightly alkaline (Fig. 7a). Alkaline substances in minerals could react with the acid in rainwater to form alkaline leaching solution.
The conductivity of leaching solution reflected the difficulty of charge flow in the solution. Those were the comprehensive embodiment of the number of ions and charges in the solution. In the experimental cycle, the curve of conductivity with time under different intensities is shown in Fig. 7b. It could be seen from the figure that the EC resulted from the different rainfall intensities (80 mL/h, 200 mL/h, and 400 mL/ h) in the experiment decreased from 3.10, 2.84, and 2.82 to 1.17, 1.16, and 0.74 MS/cm, respectively. In the first 5 days, the three rainfall intensities maintained a high conductivity and then decreased rapidly. The conductivity remained basically unchanged on the 17th day. In the early stage of the experiments, when acid rain reacted with the alkaline minerals on the surface of the waste rock violently during the leaching process, Na + , K + , Ca 2+ , and Mg 2+ in the waste rock were dissolved and released into the pore water. Therefore, the conductivity of the leachate in the initial stage was large, and the dissolution rate was high in the previous days. With the increase of leaching time, acid-based neutralization promoted the dissolution of alkaline substances in the acid-based reaction process of acid rain water, and the alkaline minerals increased the pH of pore water in the leaching column (Fig. 7a). The upper mineral reacted with the acid, and the leachable ion material decreased with the increase of leaching time. Meanwhile, in the deep layer of the column, the alkaline environment was formed gradually due to the influence of alkaline minerals, which promoted the adsorption and precipitation of various ions which reduced the total amount of ions in the leachate. The daily conductivity of leachate kept a small range of change and continued to extend with time. The release of Mn under different rainfall pH is shown in Fig.  8. The concentration of manganese was extremely high on the first day of leaching and then rapidly decreased. The release concentration reached the minimum value which was 0.128 mg/L on the 12th day of leaching, and the manganese content after leaching was extremely low. Under different rainfall acidity, the leaching concentration of manganese in the leachate was the highest on the first day, decreased rapidly in the following 4 days, and then gradually decreased to the lowest value day by day. In the experiment, the concentration of manganese in the leaching solution in four kinds of rain water with different pH is pH = 7.0 > pH = 9.0 > pH = 3.0 > pH = 5.0, which showed that the concentration of manganese increased with the decrease of pH in the simulated acid rain and increased with the decrease of pH in alkaline rain water. Simultaneously, the dissolution concentration of manganese under the alkaline condition is higher than that under the acid condition. Under the acidic conditions, manganese-bearing minerals are easy to dissolve, resulting in a high concentration in the leachate. Under the alkaline condition, the experiments showed that when the pH of manganese ion solution was higher than 8.0, obvious precipitation appeared in the solution. In this experiment, manganese was easier to be released from the minerals under alkaline conditions. However, when the alkalinity was strong, it was easy to form precipitation, which made the content of manganese ions in the pore water decrease. Because the alkaline substance dissolution rate of manganese after precipitation was relatively slow, the concentration of the dissolved manganese ion in the solution was higher when the pH value of simulated alkaline rainwater was lower.
No matter the initial rainwater was alkaline or acidic, the pH in the final leachate was between 6.5 and 8.0 under different rainfall pH conditions (Fig. 9a). The existence of minerals in manganese ore waste rock had a good buffer effect. The neutralization of alkaline minerals in minerals and the oxidation of sulfide minerals played a very good role as a buffer, so that the pH of rainwater was between 3 and 9, and the pH of effluent was between 6.5 and 8.0.
In the experimental process, the curve of conductivity with time in the leachate under different rainwater pH conditions is shown in Fig. 9b. The pH of the four kinds of rainfall in the experiment (pH: 3, 5, 7, 9) decreased from the initial 3.03, 2.77, 5.25, and 3.24 to 1.73, 1.74, 1.29, and 1.23 MS/cm, respectively. In the acidic condition, the conductivity of leachate maintained high in the first 7 days and then decreased slowly, while in the alkaline condition, the conductivity decreased rapidly from the first day. The conductivity of the leachate under acidic condition was significantly higher than that under the alkaline condition, which might react with the acid and alkaline minerals in rainwater violently and release various ions of alkaline minerals. However, ions were not easy to release under alkaline conditions. Moreover, iron and aluminum could adsorb ions and form precipitation in pore water under alkaline conditions, which could also reduce the total number of ions in the leachate.
In order to understand the crystal structure and phase composition changes of manganese-containing waste rock under the action of rainwater, X-ray diffraction (XRD) was used to analyze the residue. The peak intensity of basic minerals such  Fig. 9 Effect of different rainfall pH values: a pH of the leachate and b EC of the leachate as quartz and gypsum decreased obviously under the condition of acid leaching (pH = 3.0 and 5.0), which was due to the reaction of the acid and alkaline minerals in the simulated acid rain water, which led to the mineral dissolution and loss. The dissolution of the alkaline minerals made the pH of leachate increase gradually, and the drainage was alkaline. When the pH was 9.0, the peak intensity of each mineral increased obviously which could be related to the reactive deposition of materials under alkaline conditions.
The XRD analysis presented in Fig. 10 showed that the main component was quartz in the manganese ore waste. When the simulated rainwater passed through the surface of waste rock, the acid substance reacted with the alkaline mineral of waste rock and consumed H + in the aqueous solution. Accordingly, the pH of the waste increased and made the leaching liquid neutral or alkaline. The main basic minerals and acid reaction formulas were shown as the following equations listed: KAl Manganese release characteristics of manganese ore waste rock under different rainfall duration The cumulative amount of manganese precipitated during leaching is shown in Table 1 and Fig. 11. Under the same leaching speed and total amount of leaching solution, the daily release amount and cumulative release of manganese in different leaching time (rainfall duration) per day were also different. The cumulative release of manganese increased with the increase of leaching time. The cumulative release value was 143.96 mg/L and 156.23 mg/L when the leaching time (rainfall duration) was 1.6 h and 3.2 h, respectively. In the first 5 days, the daily leaching amount of manganese for 3.2 h was significantly greater than 1.6 h. It showed that the increase of rainfall duration was more conducive to the dissolution of manganese. The increase of early rainfall duration could significantly increase the total amount of manganese release, especially in the natural environment of alternation of dry and wet. Meanwhile, long rainfall duration was more conducive to the dissolution and release of manganese in the early rainfall of waste rock. Xiangtan is a rainy and long-term city in Hunan Province, which increases the probability of manganese pollution, so it is necessary to pay more attention to the protection of mining environment.

Study on release kinetics of manganese under rainfall leaching
According to the above experimental methods and ignoring the role of microorganisms, the rainfall leaching reaction of manganese ore waste can be divided into the following two  categories. The first was physical reaction. At the beginning of the rainfall process, the waste slag exposed on the ground was covered with a large amount of adsorbed soluble minerals and salts. At the initial moment of leaching, it was hit and quickly washed away into the leachate by rain water under the action of shear force. The manganese concentration in the leachate increased rapidly. The other was chemical reaction. With the continuation of the rainfall, the soluble minerals adsorbed on the surface were exhausted. Then, the ore surface inside the waste began to contact with the rain water. Under the action of rain water and air, the surface was oxidized and the manganese was released. At the same time, when the surface oxidation was carried out, the alkali ions in the waste could be replaced and made the leachate alkaline. Meanwhile, a variety of microgalvanic reactions also could lead to the precipitation of large amount of manganese. Therefore, the manganese concentration in the leachate might increase to the maximum value.
The release kinetic of manganese from the manganese ore waste (particle size = 160 μm, solid-liquid ratio = 1:20) in the process of rainfall leaching was simulated using the release kinetic model of the double constant and the Elovich equation. The results are shown in Fig. 12 and Table 2. Both the double constant and Elovich equation could be used to describe the release mechanism of manganese ore waste rock under the rainfall leaching, and the fitting results of double constant equation model were better than that of the Elovich equation.

Conclusion
In this study, the leaching release characteristics and kinetics of manganese under simulated rainfall leaching were studied. The results of static leaching experiments showed that the disturbance state could increase the concentration gradient of manganese in leaching solution and promote the release of manganese. When the solid-liquid ratio of waste rock was 1:5, the maximum release of manganese was 922.3 mg/L. The maximum amount of manganese precipitated out of manganese ore waste rock was 491.3 mg/L when the particle size was less than 180 μm. The dynamic leaching experiments showed that with the rainfall intensity of 80 mL/h and pH of 7, the largest early release of manganese was 152.3 mg/L and 926.5 mg/L, respectively. As the duration of the rainfall increased, the manganese release increased from 143.9 to 156.2 mg/L. The double constant equation model and the Elovich equation model were introduced to describe the leaching release kinetics of manganese waste rock, and the fitting results showed that the double constant equation model could better describe the release process of manganese waste rock. This study proved that manganese in waste rock can be released effectively by rainfall leaching. The pollution degree of heavy metals in manganese waste rock to soil groundwater system can be quantitatively predicted by kinetic model.
Author contribution B R and X W contributed to the study design. Measurement preparation, experiments, data collection, and analysis were performed by X W. The first draft of the manuscript was written by X W. Y Z checked the quality of the English and critically revised the work. Y Z and Y S commented on previous versions of the manuscript and provided valuable reviews. All authors read and approved the final manuscript.  Data availability All data generated or analyzed during this study are included in this published article.

Declarations
Ethics approval and consent to participate Not applicable.
Consent for publication Not applicable.

Competing interests
The authors declare no competing interests.