Environmental pollutant removal tests by new materials synthesized from mineral tailings disposed in the region

Abstract

Environmental pollution is a worldwide problem. The increasing of irregular mining, and in industrial activities in the secondary sector, has also contributed to triggering these impacts in the Amazon. The dumping of waste from these industries containing a high concentration of heavy metals and dyes in effluents ends up altering the characteristics of the rivers in the region. Thus, the development of removers of these pollutants, derived from tailings, stand out in the treatment of these wastewaters. The goal of this study was to carry out new removal tests of the dyes methylene blue and methyl orange by an activated sodium vermiculite and heavy metals Cr⁺⁶, Mn⁺⁷ and Cu⁺² by a LDH Shigaite-type phase. The removal performance was quantified by UV-visible spectrophotometry. Activated sodium vermiculite removed 99% of methylene blue and was not active for methyl orange. LDH removed 97% of Cr⁺⁶ and 100% of Mn⁺⁷, with no significant results for Cu⁺².

Keywords:
adsorption; vermiculite; shigaite; dyes; heavy metals

1. Introduction

In the Amazon region, illegal mining has become one of the main causes of deforestation, soil degradation and water contamination of several rivers in the region (Ramírez, et al. 2020). The irregular disposal of waste containing high concentrations of heavy metals resulting from this disorderly extraction, establishes a cascade effect, because the bio-accumulative pollutant participates in various trophic levels of the food chain, intensify because they are not biodegradable.

Currently in the Amazon, the implementation of new ventures in the secondary sector, as is the case of textile industries, other pollutants, such as dyes, from the irregular discharge of these industries have also been intensifying and affecting the quality of surface waters. The industry’s discharge has intense coloration and is highly concentrated, preventing the penetration of sunlight, thus changing the photosynthetic capacity of the aquatic environment, as well as the amount of dissolved oxygen, creating a toxic and harmful environment for the biota (Lau, et al. 2015).

Also, in this scope of concern, Brazil being one of the main producers of leather of the world, discards residues, such as chromium (VI) from its processing into the aquatic environment, which in high concentrations can pollute rivers and groundwater in the region (Rodrigues, et al. 2019).

Nowadays, several techniques are available for the remediation of these pollutants, such as precipitation, flotation, reverse osmosis among several others. However, despite being efficient, these demand a large number of resources. Consequently, other methods are being developed in order to minimize these harmful environmental elements in a more practical and economical way, such as adsorption or removal by functionalized materials. The procedure can be selective and present the advantage of the reuse of adsorbent or the remover, and also, depending on starting material for its synthesis, they could be of lower production cost, the reason why they are receiving attention among researchers in the area (Muller, et al. 2019; Rodrigues, et al. 2019).

In Northern Brazil, due to environmental issues, research institutions have been consolidating their efforts in the development of these new materials, especially those originated from mining industry waste, nowadays stored in large dams, to act as adsorbents or removers of industrial pollutants in order to minimize the irregular disposal of waste from various companies in the industrial sector implemented in the Amazon (Silva, et al. 2018; Rodrigues, et al. 2019).

The current literature points out several materials developed by local research bodies that can act on problems related to environmental pollution, beyond the conventional use for which they were initially developed, as is the case of a sodium activated vermiculite prepared by a new synthesis process that proved to be an excellent remover of Cu⁺² from tailings dam waters, as well as a LDH type Shigaite structure that does not present yet a reported application in literature, but that presents properties related to this scope of investigation (Moraes, et al. 2019; Marinho, et al. 2021).

Thus, the goal of this study was to show the need to test the use of these materials in the removal of supposed pollutants in wastewater, simulating irregular discharges by the industrial sector, minimizing contamination by possible pollutants and suggesting new applications of these new manufactured materials on environmental impacts.

2. Materials and methods

In this investigation, two materials were chosen, synthesized from tailings and produced by research entities in Northern Brazil. The first was a sodium activated vermiculite produced from a raw vermiculite discarded in the region after its conventional use and tested after activated on Cu⁺² removal from the tailings dam waters; another was a LDH Shigaite-type phase produced from waste of manganese mining industry from the Carajás Mineral Province, Pará state, whose use has not been presented in any scientific test (Moraes, et al. 2019; Marinho, et al. 2021).

Since both materials present structures similar to natural clays, it is reasonable to assume that they also present similar properties, such as surface charges that are naturally presented by certain types of clays. As the supposed pollutants also present surface charges that contribute to the removal process, both samples were initially measured and studied for their point of zero charge (pHPZC), value of pH in which the surface charge of the sample is equal to zero for specified conditions of temperature, pressure and aqueous solution components (Regalbuto & Robles, 2004). In this analysis 20 mg of the remover were suspended in 20 mL of KCl (Dynamic) solution at a concentration of 0.1 mol.L^-1, conditioned to 12 samples with initial pH ranging from 1 to 12 and adjusted with HCl or NaOH (Merck) solution to 0.1 mol.L^-1, when necessary.

Both these materials were now directed to new tests for the removal of pollutants. Two classes of materials were used, which are currently highly investigated in adsorption processes simulating pollutants. In the first class, methylene blue (Dynamic) and methyl orange (Exodus) were used, dyes selected because of their different ionicities and wide industrial application. In the second class, the heavy metals Cr⁺⁶ (K₂Cr₂O₇, Dynamics), Mn⁺⁷ (KMnO₄, Exodus) and Cu⁺² (CuSO₄.5H₂O, Dynamics) were used. All these reagents were purchased from local trade and used as received. These materials were chosen because of their high harmful potential and the possibility to quantify their concentration by UV-visible spectrophotometry (Reasonably cheap and easy to use for research and inspection).

2.1 Dye removal

Sodium activated vermiculite was used in the dye removal tests in view of its favorable performance in the removal of cationic metals as reported by Moraes et al. (2019). The tests were performed with 7.5 mL (conc. 5, 10 and 20 mg.L^-1) of methylene blue and methyl orange solutions added in conical tubes with 10 mg of the remover. The suspension was stirred at 240 rpm/1h. in a horizontal orbital shaker (Biomixer, model TS-20000A), at room temperature and pH ≈ 7. After this period, the samples were centrifuged at 3000 rpm/min. using an analog centrifuge (Quimis, model Q222T216). The separation of the supernatant was performed with a 3 mL Pasteur pipette and the final concentration analyzed in a spectrophotometer (Varian, model Cary 50 Probe UV) at a wavelength (λ) of 665 nm for methylene blue and 464 nm for methyl orange. The pollutant removed (%) was determined using the formulae: % removed = (Ci - Cf / Ci) x 100, Ci and Cf are initial and final concentrations (g.L^-1) of the analyte. The tests were performed in triplicate, blank performed with deionized water (the same used in the preparation of the solutions) and only the average of the determined values are shown in the results (Silva, et al. 2018).

2.2 Heavy metals removal

In the heavy metal removal, an LDH Shigaite-type phase was selected, in view of the results presented in literature for this type of material (Rodrigues, et al. 2019). Solutions of K₂Cr₂O₇ (0.58 g.L^-1), KMnO₄ (0.08 g.L^-1) and CuSO₄.5H₂O (2 g.L^-1) were prepared following the best results found in the removal of these pollutants (Chen, et al. 2015; Moraes, et al. 2019; Rodrigues, et al. 2019). The procedure in these tests followed the same routine applied for the dyes, 0.5 g of remover was suspended in 10 mL of the solution of these metals and shaken (240 rpm/1h.) at room temperature and pH ≈ 5. After this stage, the mixture was centrifuged (5000 rpm/30 min.) and the supernatant separated for determination of the metal concentration. Prior to the choice of the wavelength for Cr⁺⁶, scans in the range from 320 to 540 nm were performed, being obtained the maximum absorption at 470 nm (Rodrigues, et al. 2019). A similar scanning procedure was performed for Mn⁺⁷ in the range 256 to 660 nm, with the highest absorption at 520 nm (Lee, et al. 1987). For Cu⁺², since it did not present fluctuations in the determination of the measurements; its established wavelength was 740 nm (Moraes, et al. 2019).

3. Results and discussion

3.1 Dyes removal by sodium activated vermiculite

In the dye removal tests, the results obtained were promising for methylene blue (cationic dye), with a performance of 99% of its removal, superior to that presented by raw vermiculite (97.8%) and other results found in literature for other types of removers, as was the case of a Zeolite A-LDH (62.3%) synthesized from tailings from the mineral industry (Silva, et al. 2018). For the methyl orange (anionic dye) the results were not significant for raw material, while the activated sodium vermiculite demonstrated the possible selectivity of this remover for substances that present positive surface charges.

Based on these obtained results, it is proposed that the mechanism of pollutant removal occurs due to an Na⁺ exchange (surface and interlayer) from the sodium activation process by the cationic pollutant, which is deduced as a function of the different superficial charge presented by raw and activated material, an assumption corroborated by the pHPCZ (Figure 1) determined for both compounds.

Figure 1
Zero charge point of raw vermiculite (left) and sodium activated vermiculite (right).

The pHPZC value of sodium activated vermiculite (6.93) slightly higher than raw vermiculite (6.38) corroborates that when the pH of the solution is above the pHPZC value, there is a predominance of negative charges on the remover surface, resulting in a cation exchange, which supports the fact that there is no removal of the methyl orange dye because it is anionic.

3.2 Heavy metals removal by LDH Shigaite-type phase

Heavy metal samples were tested in order to evaluate these materials in disaster management. The results were promising for Cr⁺⁶ and Mn⁺⁷, which presented removal of 97 and 100% respectively. For Cu⁺², the result was not significant, only 18% removal of this ion was observed, which at first sight suggests the possible predominance of positive charges on the surface of the remover. Thus, it is assumed that Cr and Mn should be removed in the form of dichromate (Cr₂O₇^-) and permanganate (MnO₄^-) ions, unlike Cu, which occurs as Cu⁺² ions. The efficiency of the material as remover was achieved by comparison of its performance with a hydrotalcite-hydroxyapatite doped with carbon nanotubes, which removed 76.97 % of the pollutant (Cr⁺⁶), a compound synthesized from reagents of an analytical degree of purity (Rodrigues, et al. 2019). For manganese, the results were compared with an MCM-41 functionalized with amine and nitrilotriacetic acid anhydride, also synthesized from reagents of analytical purity, showing similar results to Shigaite for this pollutant [9]. As expected by the assumption of the suggested positive surface charge for the material, Cu⁺² removal was low compared to the sodium activated vermiculite discussed in section 3.1 of this manuscript, which removed ≈ 79% of the ion (Moraes, et al. 2019).

As for the mechanism of pollutant removal, it is assumed that these occur due to surface adsorption of ions on the synthesized material. This assumption is corroborated by the pHPZC (Figure 2) determined for the remover, where solutions with pH below the PZC value denote the predominance of positive charges on its surface, in accordance with what was observed.

Figure 2
Zero charge point of LDH Shigaite-type phase.

4. Conclusion

In order to investigate the possibility of other applications beyond those initially tested on the occasion of the synthesis of new materials presented in literature, experiments were performed for the removal of supposed environmental pollutants (dyes and heavy metals).

The materials tested for this purpose were a sodium activated vermiculite used for cationic dye removal and a LDH Shigaite-type phase for heavy metal removal. Both proved effective in the newly selected tests, methylene blue was removed with a performance of approx. 99% and metals Cr⁺⁶ and Mn⁺⁷ had a removal of 97 and 100% respectively.

The selectivity of these materials for cationic and anionic substances was evidenced as a function of surface charge determined by pHPZC.

The good results obtained point to the possibility of application in real situations, thus contributing to the minimization of mining tailings in the North of Brazil.

Finally, the results also encourage their future use in various other tests for the removal of supposed environmental pollutants in order to demonstrate the effectiveness of the materials developed, especially by institutions in the Amazon with waste disposed of in the Amazonian environment itself.

• Funding Information
  There are no funders to report for this submission.

Data availability

Datasets related to this article will be available upon request to the corresponding author.

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Edited by

• Associate Editor
  Marilena Cardu

Publication Dates

• Publication in this collection
  08 Aug 2025
• Date of issue
  Jul-Sep 2025

History

• Received
  02 June 2023
• Accepted
  17 Apr 2025