
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

72764
10.1038/s41598-024-72764-7
Article
The effect of calcium on the removal of Cd2+ in the formation of biogenic secondary iron minerals
Geng Kanghui 14
Wang Chong 14
Wu Xianhui 14
Wei Caichun 13
Huang Haitao hthuang@glut.edu.cn

12
1 https://ror.org/03z391397 grid.440725.0 0000 0000 9050 0527 College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004 China
2 grid.440725.0 0000 0000 9050 0527 Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin, 541004 China
3 https://ror.org/03z391397 grid.440725.0 0000 0000 9050 0527 Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Areas, Guilin University of Technology, Guilin, 541004 China
4 https://ror.org/03z391397 grid.440725.0 0000 0000 9050 0527 Modern Industry College of Ecology and Environmental Protection, Guilin University of Technology, Guilin, 541004 China
14 9 2024
14 9 2024
2024
14 2149927 6 2024
10 9 2024
© The Author(s) 2024
2024
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Cadmium is a toxic heavy metal found in acid mine drainage. It hinders plant and animal growth and accumulates in human organs. In this study, through shake flask experiments, an iron-rich, sulphate-rich environment was simulated, and Acidithiobacillus ferrooxidans was used to mediate the formation of secondary high-iron minerals to explore the effect of calcium ions on the removal of Cd2+ from that environment. Four treatment systems were used: “Blank”, “Ca2+-30 mg/L”, “Fe/K = 3,Ca2+-30 mg/L”, and “Fe/K = 3”. The results showed that Cd2+ with an initial concentration of 20 mg/L was effectively removed in each treatment system. The removal efficiencies of Cd2+ in each treatment were 23.46%, 18.42%, 52.88%, and 45.76% respectively. The quantity and type of minerals determined the removal efficiency of Cd2+. The Fe/K = 3 treatment system can significantly increase the amount of mineral formation and improve the removal efficiency of Cd2+. In the Ca2+-30 mg/L, Fe/K = 3 treatment system, the biological oxidation ability was the strongest, and the removal effect of Cd2+ was the best under the combined action of K+ and Ca2+. Co-precipitation was the main way to remove Cd2+ during the formation of biogenic secondary iron minerals, and the removal amount was 5.64 to 14.83 times that of adsorption. Biogenetic secondary iron minerals showed high values in repairing heavy metal pollution. This study provides a theoretical basis for treating heavy metals in acid mine drainage.

Keywords

Acidithiobacillus ferrooxidans
Ca2+
Cd2+
Co-precipitation effect
Secondary high-iron minerals
Subject terms

Environmental chemistry
Environmental impact
Foundation of Guilin University of TechnologyGUTQDJJ 2005020 GUTQDJJ 2001013 Wei Caichun Huang Haitao http://dx.doi.org/10.13039/501100021189 Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology 1601Z005 Huang Haitao issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Long-term exploitation of mineral resources will lead to an imbalance in environmental ecosystems. Mining and grinding metal ores increase their surface area, resulting in waste weathering. The ore can easily react with oxygen and surrounding water to form metal ions and sulphuric acid1. Those products interact with bedrock, surface water, and groundwater to form a lower pH water environment, namely acid mine drainage (AMD)2. In the formation of AMD, its low pH, high heavy metal content, and sulphates greatly harm water, soil, and organisms. In China, approximately 3.5 billion tons of AMD are produced yearly, and its negative effect on the ecological environment and human health in mining areas cannot be ignored3,4.

Acidithiobacillus ferrooxidans isolated from AMD can oxidize Fe2+ and reduce inorganic sulphur, making A. ferrooxidans the driving force of ecological succession in acidic biological communities and an important model organism in astrobiology5–7. In an iron- and sulphate-rich environment, secondary high-iron minerals represented by schwertmannite and jarosite are easily formed under the mediation of A. ferrooxidans. Those secondary iron minerals have poor crystallinity, high surface activity, and a rich tunnel structure, which can effectively remove heavy metals in water through redox reaction, adsorption, and co-precipitation8. The main mechanisms for the removal or passivation of heavy metals in water by biogenic, secondary high-iron minerals are as follows: A. ferrooxidans consumes H+ and oxidizes Fe2+ to Fe3+ during the growth process, and Fe3+ is hydrolysed and mineralized. In that process, Fe3+ and heavy metal ions with approximate charge and ionic radius are co-fixed in secondary high-iron minerals by isomorphism to effectively remove metal ions9. The removal effect of trace heavy metal ions from an acidic water system is closely related to that of secondary iron minerals. In the process of biomineralization, many factors, such as reaction temperature, pH, seed species, anion and cation species, and concentration, can affect the formation of secondary iron minerals10–12. Studies have shown that when the Fe/K molar ratio is 3, the conversion of schwertmannite to jarosite in the biosynthetic secondary high-iron mineral system is effectively promoted, and the ability to form crystalline jarosite minerals is the strongest13,14.

Cadmium is a common, high-concentration, toxic heavy metal in AMD. It has high mobility and a long half-life (10 to 30 years). It can enter the ecological cycle through the atmosphere, water, soil, and food, endangering the growth of animals and plants and accumulating in human kidneys, liver, bones, and other organs, causing irreversible damage to those organs. and cadmium is even carcinogenic15,16. Secondary iron minerals can be used as an effective adsorbent to remove Cd2+ from contaminated water. Fan et al.17 found that the adsorption of Cd2+ by schwertmannite was 110 mg/g and the desorption of Cd2+ accounted for 50–80% of the total adsorption when the Cd2+ dosage was 1 g/L, the initial pH was 8.0 and the temperature was 25 °C.

Secondary high-iron minerals such as schwertmannite and jarosite are effective in the removal of Cd2+. Given the high concentration of Ca2+ in the soil and water environment in the iron-rich and sulphate-rich environments of karst areas18, the effect of Ca2+ on the removal of Cd2+ in the process of A. ferrooxidans catalysing the synthesis of secondary high-iron minerals requires further investigation. Therefore, in this study, A. ferrooxidans was used to synthesize secondary minerals. The effects of the Ca2+ and Fe/K molar ratio on the changes of pH and the Fe2+ oxidation rate were investigated, as were the total Fe precipitation efficiencies of culture systems and the removal effect of Cd2+ in the process of synthesizing secondary high-iron minerals. That provides a reference for the repair and treatment of heavy metal wastewater.

Materials and methods

Instruments and reagent

The instruments used were as follows:

A TS-2102 multifunctional, constant temperature shaker (Shanghai Tiancheng Experimental Instrument Manufacturing Co., Ltd.)

An Allegra 64R refrigerated high-speed centrifuge (Beckman, USA).

A UV-5800PC UV spectrophotometer (Shanghai Yuanxi Instrument Co., Ltd.)

A PHS-3 C digital precision acidity meter (Shanghai Leici Factory).

An HH-8 water-bath heating pot (Jincheng Hailan Instrument Factory).

A PinAAcle 900 atomic absorption spectrometer (Shanghai PerkinElmer).

The preparations of media were as follows:

9 K liquid medium: 3.0 g (NH4)2SO4, 0.1 g KCl, 0.5 g K2HPO4·3H2O, 0.5 g MgSO4·7H2O, and 0.01 g Ca(NO3)2·4H2O were dissolved in 1 L deionized water, and the pH was adjusted to 2.5 with 1:1 H2SO4.

Modified calcium-free 9K liquid medium: Ca(NO3)2·4H2O was not added, and the remaining constituents were consistent with the preparation method of the 9 K liquid medium.

Ca2+solution: 8.85 g of Ca(NO3)2·4H2O was dissolved in 1 L of deionized water to prepare a calcium stock solution with a Ca2+ concentration of 1500 mg/L.

Cd2+solution: 1.05 g of Cd(NO3)2 was dissolved in 100 mL of deionized water to prepare a Cd2+ stock solution with a Cd2+ concentration of 5 g/L.

K2SO4solution: 10.66 g of K2SO4 was dissolved in 200 ml deionized water and the K2SO4 solution was 53.31 mg/mL.

Preparation of resting cells of A. ferrooxidans

The strain used in the experiment was A. ferrooxidans (ATCC 23270), which was collected from acid mines. The A. ferrooxidans was inoculated in 9 K liquid medium at pH 2.5 in a 180 r/min, 28 °C constant temperature shaker, and the culture was stopped at the later stage of exponential growth (approximately 2 to 3 d). The medium was cultured and activated twice by the above method and filtered by qualitative filter paper, then the precipitate was removed. The filtrate was the inoculation liquid required for that experiment. The bacteria were collected by centrifugation (relative centrifugal force 10,000×g, 4 °C, 10 min), washed 3 times with acidic deionized water (pH = 1.5) to remove impurity ions, and suspended with acidic deionized water (pH = 2.5). The concentration ratio was 50 times, and the cell density was approximately 108 cells/mL.

Adsorption of Cd2+ by biosynthesis of secondary iron minerals

Twelve 500 mL conical flasks were filled with 12.5 mL of modified calcium-free 9 K liquid medium, 1 mL of A. ferrooxidans concentrate, 1 mL of Cd2+ solution, and 11.12 g of FeSO4·7H2O. The conical flasks were divided into 4 groups of treatments, and 3 parallel samples were set up in each group, with one group as the “Blank” group. The 4 treatments were recorded as “Blank”, “Fe/K = 3”, “Ca2+-30 mg/L”, and “Fe/K = 3,Ca2+-30 mg/L”. In the Ca2+-30 mg/L treatment, 5 mL of Ca2+ solution was added to make the Ca2+ concentration 30 mg/L. In Fe/K = 3, 20 mL of K2SO4 solution was added to make the Fe/K molar ratio of the system 3. Five mL of Ca2+ solution and 20 mL of K2SO4 solution were added to Fe/K = 3, Ca2+-30 mg/L. Deionized water was added to all conical flasks, and the volume of each solution system was 250 mL. The pH of the above solution systems was adjusted to 2.50 ± 0.02 with 1:1 H2SO4, and the solutions were placed in a 180 r/min multifunction constant temperature shaker at 28 °C to shake the culture. At 6, 12, 24, 36, 48, 72, and 96 h of culture, 2 mL supernatant was filtered through a 0.22 μm filter membrane and stored. The concentration of Cd2+ in the supernatant was determined, and the pH of the solution systems was monitored. Fe2+ and total Fe were determined.

At the end of the culture, it was filtered by neutral qualitative filter paper and washed 2 to 3 times with deionized water. The collected minerals were dried in an oven at 60 °C, weighed, and stored for later use.

Desorption of Cd2+ from secondary iron minerals

Of the aforementioned dried and preserved mineral, 0.5 g was placed in a 50 mL volume polyethylene plastic sealing bottle. Dilute hydrochloric acid of pH = 2 was added at a solid–liquid ratio of 1:10, and the bottle was placed in a constant temperature shaker (28 °C, 180 r/min) for 24 h to remove the adsorbed Cd2+. After filtration with a 0.45 μm filter membrane, the filtrate was saved, and the concentration of Cd2+ in the filtrate was determined by atomic absorption spectrometer. The filtered residue was dried in an oven at 60 °C, then 0.2 g of the dried filter residue was dissolved in 2 mL of concentrated hydrochloric acid. After the filter residue was completely dissolved under water-bath heating conditions, the solution was diluted by a certain multiple to determine the concentration of Cd2+ in the solution. The amount of heavy metal removed by mineral co-precipitation and adsorption was calculated.

Determination method and data analysis

Total Fe and Fe2+ were determined by phenanthroline colorimetry. When the total Fe was determined, Fe3+ was reduced to Fe2+ by hydroxylamine hydrochloride, then determined by phenanthroline colorimetry. Equations of Fe2+ oxidation efficiency and total Fe (TFe) precipitation efficiency are1 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{\text{Fe}}^{2+}\:\text{oxidation efficiency}\left({\%}\right)=\left({\text{Fe}}_{initial}^{2+}-{\text{Fe}}^{2+}\right)/{\text{Fe}}_{initial}^{2+}\times100$$\end{document}

2 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\text{T}\text{Fe}\:\text{precipitation}\:\text{efficiency}\left({\%}\right)=\left({\text{TFe}}_{initial}-\text{TFe}\right)/{\text{TFe}}_{initial}\times\:100.$$\end{document}

The concentration of Cd2+ in the solution was determined by the atomic absorption spectrometer. The removal efficiency of Cd2+ was calculated in accordance with the change of Cd2+ concentration in the solution before and after adsorption. The equation is3 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{\text{Cd}}^{2+}\text{removal}\:\text{efficienc}\text{y}\left({\%}\right)=\left({\text{Cd}}_{initial}^{2+}-{\text{Cd}}^{2+}\right)/{\text{Cd}}_{initial}^{2+}\times\:100.$$\end{document}

The final experimental data were analysed using Microsoft Excel 2021 and Origin 2022.

Results and analysis

Changes in pH and oxidation efficiency of Fe2+ in the solution system

In general, A. ferrooxidans relies on the oxidation of Fe2+ to obtain energy, and its oxidative metabolism law contains 2 stages of acid effect: (1) the acid consumption process of the biological oxidation of Fe2+ and (2) the acid production process of Fe3+ hydrolysis mineralization19. The relations between pH and culture time of the 4 groups of treatment systems are shown in Fig. 1. The pH changes of the 4 groups of treatment systems first increased and then decreased, which was consistent with the law of acid consumption and acid production in the process of bacterial biological oxidation. However, the groups’ rates of increase and decrease in pH differed. The pH of the 2 groups of Blank and Ca2+-30 mg/L increased to the highest value at 6 h, whereas the 2 groups of Fe/K = 3 and Fe/K = 3,Ca2+-30 mg/L increased to the highest at 12 h. The pH of the treatment system added with K+ decreased significantly faster than that of the other 2 groups. Among them, Fe/K = 3,Ca2+-30 mg/L had the most significant effect on promoting acid production relative to the Blank group, and the pH decreased to 1.77 at the end of the culture. However, the pH change of the system with only a Ca2+ addition showed a slight inhibition. The pH decline rates of the 4 treatment systems from highest to lowest were Fe/K = 3,Ca2+-30 mg/L > Fe/K = 3 > Blank > Ca2+-30 mg/L.

Fig. 1 Changes in pH in each treatment system.

The change of Fe2+ oxidation efficiency in each treatment system during the culture process is shown in Fig. 2. The typical microbial growth curve is generally divided into lag, logarithmic growth, stable, and decline phases. The 4 cycles of bacterial growth states are the growth and reproduction induced state, rapid growth, slow growth, and gradual apoptosis20. Because the Fe2+ oxidation process gives A. ferrooxidans energy, the higher the oxidation efficiency, the stronger the bacterial activity, so the Fe2+ oxidation of a reaction system can indirectly reflect the overall oxidation capacity of the organism. Figure 2 shows that the changes in Fe2+ oxidation efficiency in the 4 groups were basically the same: it increased significantly from 0 to 48 h, and the time of entering the logarithmic growth phase was similar. However, the activity and oxidation abilities of the bacteria were significantly different. Among them, the Fe/K = 3, Ca2+-30 mg/L treatment system had the highest Fe2+ oxidation efficiency, and the Fe2+ oxidation efficiency of the Fe/K = 3 treatment system was second only to the former. When cultured for 48 h, the Fe2+ was completely oxidized in the treatment of Fe/K = 3, Ca2+-30 mg/L, whereas the oxidation efficiency of Fe2+ in the treatment of Blank and Ca2+-30 mg/L was less than 80%. The complete oxidation time of Fe2+ was greatly delayed, and the treatment of Fe/K = 3 could also be completely oxidized at 72 h. The above results showed that relative to the Blank group, the oxidation of Fe2+ in the system showed slight inhibition when only Ca2+ was added. K+ can significantly promote the biological oxidation ability of bacteria14. When K+ and Ca2+ worked together in the system, the biological oxidation ability of Fe2+ was the best.

Fig. 2 Change of Fe2+ oxidation efficiency in each treatment system.

Changes in total Fe precipitation efficiency and Cd2+ removal efficiency in the solution systems

In the process of bacterial bio-oxidation, Fe2+ is converted into Fe3+, which reacts with monovalent cations such as K+, Na+, NH4+, H3O+, and SO42– in a solution system to form the iron mineral, and the total Fe in the system is precipitated21. The dynamic change of total Fe directly reflects the formation of secondary high-iron minerals: the change of total Fe precipitation efficiency in each treatment system is shown in Fig. 3. During the lag phase of bacterial growth for 0 to 24 h, some bacteria were not adapted to the environment and died, reducing the number of bacteria. The oxidation of Fe2+ was generally weak, and the total Fe precipitation efficiencies of the 4 treatment systems were only slightly different. When cultured for 96 h, 55.09% and 58.62% of iron was converted to a precipitate in the treatment systems of Fe/K = 3 and Fe/K = 3, Ca2+-30 mg/L respectively, whereas the total Fe precipitation efficiencies in the treatment systems of Blank and Ca2+-30 mg/L were only 36.30% and 33.95% respectively. Studies have shown that metal ions are combined mainly with trivalent iron oxides rather than with divalent iron compounds22. The oxidation efficiency of Fe2+ in the 2 systems with K+ was high, and the rapid conversion of Fe2+ to Fe3+ led to a sufficient supply of Fe3+, promoting total Fe precipitation. The quantity of the sediment obtained from each treatment system is shown in Fig. 4. In the biosynthetic secondary high-iron mineral system, the total Fe precipitation efficiency was positively correlated with the quantity of secondary high-iron minerals. The final total Fe precipitation efficiency of Fe/K = 3, Ca2+-30 mg/L was the highest, which corresponded to the highest quantity of secondary high-iron minerals, which was 3.4 g. The presence of K+ and Ca2+ significantly promoted ferrous oxidation and hydroxyl ferrous sulphate formation. Promoting the formation of secondary high-iron minerals can correspondingly improve the fixation efficiency of metals.

Fig. 3 Changes in total Fe precipitation efficiency in each treatment system.

Fig. 4 Quantity of precipitates obtained from each treatment system.

Heavy metals can be co-precipitated with iron under acidic conditions or combined with secondary iron minerals to achieve effective removal23. Figure 5 shows the changes in Cd2+ removal efficiency. The characteristic curve of Cd2+ removal in the process of the biosynthesis of secondary iron minerals was roughly S-shaped, and the removal process could be divided into rapid removal and removal equilibrium stages. The first stage occurred at 0 to 48 h, and the removal efficiency of Cd2+ increased rapidly with the prolongation of culture duration. The second stage showed that the removal efficiency of 48 to 96 h gradually tended to balance and finally reached the maximum removal efficiency. The above shows that when the bacteria were in the logarithmic growth stage, Fe2+ was rapidly converted into Fe3+ and then hydrolysed and mineralized, and secondary high-iron minerals contributed greatly to the fixation of Cd2+. At the end of the culture, the removal efficiencies of Cd2+ in the 4 groups of Fe/K = 3, Ca2+-30 mg/L, Fe/K = 3, Blank, and Ca2+-30 mg/L were 52.88%, 45.76%, 23.46%, and 18.42% respectively. It can be seen that the removal effect of Cd2+ in the 2 groups with added K+ was significantly better than that in the other 2 groups, and the removal effect of the Fe/K = 3, Ca2+-30 mg/L treatment system was the best. Studies have shown that when the molar ratio of Fe2+ to K+ is controlled to be 3, the mineral phase of biogenic secondary high-iron minerals gradually transitions from schwertmannite with poor crystallinity to jarosite with good crystallinity24. A Pearson correlation analysis of the total Fe precipitation efficiency and Cd2+ removal efficiency during the formation of secondary high-iron minerals is shown in Table 1. The results show that there was a significant positive correlation between the 2 (average correlation coefficient r was 0.916). In summary, the types of secondary high-iron minerals affected the removal efficiency of Cd2+, and the amount of mineral production was also directly related to that efficiency.

Fig. 5 Removal efficiency of Cd2+ in each treatment system.

Table 1 Pearson correlation analysis of total Fe precipitation efficiency and Cd2+ removal efficiency in each treatment.

	Blank	Ca2+-30 mg/L	Fe/K = 3	Fe/K = 3, Ca2+-30 mg/L	
Correlation coefficient r	0.829	0.921	0.937	0.975	
Average correlation coefficients	0.916	

Removal of Cd2+ by biogenic secondary iron minerals

Figure 6 shows the removal of Cd2+ by adsorption and co-precipitation in each treatment system. The figure shows that the amounts of Cd2+ removed by surface adsorption per unit mass of minerals in the 4 treatment systems of Blank, Ca2+-30 mg/L, Fe/K = 3, and Fe/K = 3, Ca2+-30 mg/L were 0.048 mg/g, 0.041 mg/g, 0.035 mg/g, and 0.036 mg/g respectively. The amounts of Cd2+ removed by co-precipitation were 0.285 mg/g, 0.231 mg/g, 0.457 mg/g, and 0.534 mg/g respectively. The removal amount of Cd2+ by co-precipitation was 5.63 to 14.83 times that of adsorption, indicating that the removal of Cd2+ by biogenic secondary iron minerals was dominated by co-precipitation. By comparing the Cd2+ removal in each treatment in Table 2, it can be seen that the maximum difference between the mineral fixation efficiency and the actual average removal efficiency of each treatment appeared in the Blank group at 5.63%, and the minimum appeared in the Fe/K = 3,Ca2+-30 mg/L group at 2.15%, which also shows that the removal effect of heavy metals was related to the type of secondary high-iron minerals.

Fig. 6 Amounts of Cd2+ removed by mineral adsorption and co-precipitation.

Table 2 Comparison of Cd2+ removal in each treatment.

Group	Unit mineral holding capacity (mg/g)	Total holding capacity (mg)	Heavy metal dosage (mg)	Mineral retention efficiency (%)	Actual average removal efficiency (%)	The ratio of co-precipitation to adsorption removal	
Blank	0.333	0.869	5.00	17.38	23.46	5.94	
Ca2+-30 mg/L	0.272	0.688	5.00	13.75	18.42	5.63	
Fe/K = 3	0.492	2.075	5.00	41.49	45.76	13.06	
Fe/K = 3,Ca2+-30 mg/L	0.570	2.537	5.00	50.73	52.88	14.83	

Discussion

Differences in biological oxidation and mineralization ability of each treatment system

In this experiment, the characteristic curves of pH, Fe2+ oxidation efficiency, and total Fe precipitation efficiency of the 4 treatment systems were basically the same: the difference was the strength of bacterial biological oxidation ability and mineralization ability. The Ca2+-30 mg/L treatment system showed slight inhibition relative to the Blank group. The reason might have been that in the presence of the 9 K medium, due to the addition of Ca2+ and various ions in the medium, the osmotic pressure of cells increased, bacterial tissues and organs were damaged, and normal physiological functions were affected25. The 2 groups of treatment systems with Fe/K = 3 were superior to the other 2 groups in terms of biological oxidation capacity and metallogenic quantity. Studies have shown that monovalent cations affect the formation of biogenic secondary high-iron minerals. The abilities of different cations to form mineral are very different, and K+ has the greatest ability to form mineral14. The biogenetic secondary high-iron mineral formation system Fe/K = 3 effectively promoted the formation of minerals and improved the quantity of mineral formation. At the same time, the generated schwertmannite was gradually transformed into jarosite24. Also, the biological oxidation ability of A. ferrooxidans was different under different K/Ca molar ratios. When K/Ca = 3, the total Fe precipitation efficiency was the highest, and the mineral state was the best, which helped improve the crystallinity of secondary high-iron minerals26. In that experiment, the treatment system of Fe/K = 3,Ca2+-30 mg/L showed obvious advantages in pH, Fe2+ oxidation efficiency, and total Fe precipitation efficiency. The Fe/K = 3 was second only to it, indicating that when K+ and Ca2+ worked together, the biological oxidation ability was the strongest in the formation of biogenic secondary high-iron minerals, and the amount of mineral formation was significantly increased.

Differences in Cd2+ removal effects in treatment systems

The higher Fe2+ oxidation efficiency and Fe3+ supply efficiency in the process of bio-mediated formation of hydroxyl ferric sulphate minerals are beneficial to the formation of minerals and the increase in particle size27. Complex heavy metals can be combined and effectively removed from acidic wastewater by adsorption or co-precipitation28. The final mineralization qualities of the 4 treatment systems in this experiment, from most to least effective were Fe/K = 3, Ca2+-30 mg/L > Fe/K = 3 > Blank > Ca2+-30 mg/L, which corresponds to the removal efficiency of Cd2+. The correlation coefficient analysis showed that there was a significant positive correlation between mineral production and heavy metal removal. In the Ca2+-30 mg/L treatment system, the biological oxidation ability decreased due to the increased osmotic pressure, the Fe3+ supply was insufficient, and the mineralization amount was small, which in turn affected the subsequent Cd2+ removal effect. Fan C29 studied the Fe2+-mediated transformation of Ca2+-adsorbed schwertmannite and the subsequent calcium redistribution behaviour. The results showed that Fe2+-mediated adsorption of Ca2+ by schwertmannite blocked the mineral surface sites and slightly inhibited the transformation of mineral species. Their research can provide evidence for the further weakening of the adsorption capacity of Cd2+ in the Ca2+-30 mg/L treatment system. The Fe/K = 3 treatment system had strong Fe2+ oxidation ability, sufficient Fe3+ supply, high total Fe precipitation efficiency, and increased Cd2+ removal. The prominent mineral-forming ability of K+ weakened the effect of trace Ca2+ on biomineralization, and the CaSO4·2H2O produced by the system could also act as a seed to promote the formation of minerals30. In addition, the concentration of monovalent cations determines the type of hydroxyl ferrous sulphate precipitation31. The apparent structure and specific surface areas of different mineral species differ, resulting in large differences in the removal ability of heavy metals32.

Conclusion

Acid mine drainage is rich in Fe2+, Fe3+, and SO42+, as well as many dissolved toxic metals and nonmetals, posing a serious environmental threat. In this study, the iron- and sulphate-rich environment of acid mine drainage was simulated. The effects of Ca2+ on pH, Fe2+ oxidation efficiency, TFe precipitation efficiency, and secondary iron mineral content under various conditions were investigated by shaking cultures, and the removal effect of secondary iron minerals on Cd2+ was compared and analysed. Based on those experimental results, the main conclusion can be summarized as follows.

The changes in pH, Fe2+ oxidation efficiency, and total Fe in each treatment system can correspond to each other. The biological oxidation ability and mineralization amount from greatest to least were determined to be Fe/K = 3,Ca2+-30 mg/L > Fe/K = 3 > Blank > Ca2+-30 mg/L. In the modified 9 K medium, adding Ca2+ alone had a slight inhibitory effect on the biological oxidation ability, whereas the combined action of K+ and Ca2+ promoted the oxidation of Fe2+ and increased the total Fe precipitation.

In the modified 9 K medium, only the addition of Ca2+ would weaken the cell oxidation ability and inhibit the formation of minerals. The low amount of mineral production and the occupation of adsorption sites by Ca2+ led to poor removal of Cd2+. Controlling the formation system of biogenic secondary high-iron minerals Fe/K = 3 can effectively increase mineral production. The removal efficiency of Cd2+ in the Fe/K = 3 treatment system was 45.76%, and the removal effect of Cd2+ in the Fe/K = 3, Ca2+-30 mg/L treatment system was the greatest at 52.88%.

The quantity and type of minerals directly affected the efficiency of Cd2+ removal. The removal of Cd2+ during the formation of biogenic secondary high-iron minerals was dominated by co-precipitation, and the removal amount was 5.63 to 14.83 times that of adsorption.

Acknowledgements

The assistance provided by Guangxi Engineering Research Center of Com-prehensive Treatment for Agricultural Non-Point Source Pollution has been instrumental in facilitating this research experiment.

Author contributions

K.G.: Writing-Original Draft, Writing-Review & Editing, Data Curation, Formal analysis, Investigation. C.W. (Chong Wang): Validation, Investigation. X.W.: Validation. C.W. (Caichun Wei): Resources, Funding acquisition. H.H: Conceptualization, Methodology, Resources, Writing-Review & Editing, Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology (1601Z005) and Supported by Foundation of Guilin University of Technology (GUTQDJJ 2001013, GUTQDJJ 2005020).

Data availability

All data generated or analysed during this study are included in this published article.

Competing interests

The authors declare no competing interests.

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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