Sustainable Ecotechnological Remediation and Recovery of Chromium from Wastewater Using the Floating Aquatic Plant Azolla pinnata: Crimson Publishers

Sustainable Ecotechnological Remediation and Recovery of Chromium from Wastewater Using the Floating Aquatic Plant Azolla pinnata by K Usharani in Environmental Analysis & Ecology Studies: Peer Reviewed Environmental Analysis & Ecology Studies

 

Abstract

Phytotechnology utilizes aquatic macrophytes and their microbiomes to remediate chromium-contaminated waters. This study evaluates Azolla pinnata, a free-floating fern, for its potential in sustainable remediation and recovery of Cr(VI) from synthetic wastewater. Results show successful removal of hexavalent chromium through rhizofiltration, where Azolla and algae synergistically reduce chromium concentration. Batch tests at varying initial Cr(VI) concentrations (5, 10, 25, 40, and 50ppm) using 8g of fresh Azolla biomass indicated significant chromium absorption, evidenced by a dark brown coloration. The experiment lasted 14 days, with chromium quantified via the Diphenylcarbazide (DPC) assay and measurements taken spectrophotometrically at 540nm. The findings confirm the effectiveness of this phytoremediation process and highlight the potential of aquatic plants in addressing environmental challenges while promoting a cleaner, healthier earth.

Keywords:Phytoremediation; Chromium; Azolla pinnata; Bioremoval

Introduction

Heavy metal pollution-particularly chromium-poses a global threat. Conventional treatments (ion exchange, membranes, reverse osmosis, precipitation) can be effective but cost-intensive, whereas phytoremediation offers a low-cost, nature-based solution. Azolla pinnata, which hosts Nostoc azollae, grows rapidly and is easily harvested, enabling rhizofiltration and biosorption of dissolved metals. However, the development of improved solutions is limited, particularly in developing countries, due to economic constraints and the high costs of acquiring quality equipment. Phytoremediation is a cost-effective green technology that has proven effective in various water systems, such as dams and lakes. This bioremediation process utilizes different plant species to remove, transfer, stabilize, or eliminate heavy metal deposits in water. One such plant, Azolla pinnata, is a small free-floating aquatic macrophyte that effectively absorbs heavy metals from concentrated sources.

Azolla thrives in symbiosis with cyanobacteria, which live within the cavities of its fronds. Its rapid growth rate, capable of doubling its biomass in just 2 to 4 days, along with its ability to float, makes it easy to harvest and facilitates the removal of heavy metals from contaminated water. Phytoremediation leverages these plants to degrade, stabilize, digest, or eliminate toxins and other chemical pollutants. The aquatic plants always have an extensive system of roots that helps them and makes them the primary route for the accumulation of contaminants in their roots and shoots. The aquatic free-floating plants have been revealed to be a principled prototype for ecotoxicological studies, and they have the competence to bioaccumulate or biosorb nutrients and metals [1-3].

The benefits of using plants for this purpose include economic advantages, effective harvest management, and the potential utilization of harvested products. Azolla is particularly promising for the removal of metal ions. Chromium, when present in high concentrations, poses toxic risks to humans, animals, and the environment. Specific tests for chromium have been conducted and stored for laboratory studies on phytoremediation. The aquatic plant Azolla is currently being investigated for its effectiveness in removing chromium from solutions at various concentrations, with the levels of chromium analyzed using UV spectrophotometer readings at 540nm. The nearby electroplating industry produces chromium alloys at high levels, making this study crucial for reducing chromium concentrations to safer levels.

Materials and Methods

Plant acquisition and acclimatization

Azolla pinnata was collected from the Azolla growing bed near the Horticulture Centre for the experiment. The plants were washed with running water to remove any mud particles from the roots. To support adaptation to the experimental conditions and to produce substantial biomass, the plants were grown in a plastic tray filled with tap water for approximately 15 days.

Chromium standards, controls, and analytical method

Chromium was quantified via the Diphenylcarbazide (DPC) colorimetric assay (λ=540nm), with acidification by H₂SO₄ and calibration curves from standards and blanks; measurements taken at 5min after reagent addition. A few drops of concentrated sulfuric acid were added to establish acidic conditions. The mixture was then diluted to a final volume of 50mL using distilled water. After 5 minutes, 1,5-Diphenylcarbazide (DPC) reacts selectively with Cr(VI) to afford a complex with a purplish red color. The intensity of the resulting color was measured at 540nm using a UV spectrophotometer.

Estimation of the concentration of chromium after phytoremediation treatment using Azolla pinnata

Stock solutions of potassium dichromate are prepared in water at known concentrations and quantities. Aquatic plant samples of Azolla are weighed and placed into 12-liter plastic containers filled with a specific volume and concentration of the metallic compound in water. The experimental setup is maintained under partially covered conditions. To account for evaporative losses, distilled water is added every third day to maintain a consistent water level. Water samples from the containers are collected every seventh day for analysis. The condition of the plants is visually assessed during these intervals. After 28 days, the plants are removed, weighed, and then returned to their original water source. To analyze the concentration of metal ions in the water samples collected weekly, and analyzed by a UV spectrophotometer. Based on the absorbance readings of the known solution samples, a standard graph is plotted to illustrate the relationship between absorbance and concentration. This graph is then referenced to determine the concentration of the water samples collected from the containers over time [4].

The samples were allowed to cool, and the absorbance in terms of Optical Density (OD) at 540nm was measured using a UVVisible spectrophotometer. The initial and residual concentration of chromium in terms of Optical Density (OD) value was measured using a UV-Visible spectrophotometer (UV-2600 series SHIMADZU).

From this value, the percentage chromium reduction or removal was calculated.

The percentage Chromium removal efficiency (R) was calculated using the formula,
R = (I − F) / I × 100

Where I and F are the Initial and Final concentrations of Chromium, respectively.

The Chromium Removal Efficiency (%R): This measures the percentage of the contaminant removed from the solution/soil relative to the initial amount. It is calculated using the standard formula:
(%R) = (Ci −Cf ) / Ci × 100

where (Ci) is the initial concentration and (Cf) is the final concentration.

An efficiency value of 100% was obtained when no Chromium appeared in the treated water sample (i.e., Final concentration=0). The fresh and dried biomass of A. pinnata plants was measured. Growth of the plant biomass (A. pinnata) was measured in terms of dry weight by the gravimetric method [2,3,5,6].

Estimation of pH after phytoremediation using Azolla pinnata

The pH change in treated wastewater at different concentrations and with varying time intervals, as affected by free-floating aquatic plants A. pinnata, was estimated using a digital pH meter (Scientific Tech, Advanced pH meter, model ST-2002).

Estimation of biomass after phytoremediation treatment

On the 14^th day of the experiment, the fresh and dried biomass of A. pinnata plants was measured. Each replicate was harvested, and the plants were rinsed with distilled water to eliminate any ions adhering to their surfaces. The plants were then blotted with filter paper to remove excess water. After measuring the fresh biomass, the samples were dried in an oven at 80 °C for 48 hours. The dry weights were subsequently recorded [7].

Estimation of chlorophyll after phytoremediation treatment

Approximately 1 gram of finely chopped plant material was mixed with 20 milliliters of 80% acetone and ground in a clean mortar. The mixture was then centrifuged at 500RPM for 5 minutes, and the supernatant was transferred to a 100-milliliter volumetric flask. This centrifugation process continued until the residue became colorless. The solution was then diluted to a final volume of 100 milliliters with 80% acetone, and the absorbance was measured spectrophotometrically at wavelengths of 664nm and 647nm.

Result and Discussion

Effect of chromium after phytoremediation treatment

The results indicate that phytoremediation is effective at a wavelength of 540nm, as measured using a UV spectrophotometer. When compared to standard chromium levels, Azolla pinnata effectively reduces chromium concentrations after the phytoremediation process. Based on these observations, it can be concluded that Azolla pinnata can thrive in chromium concentrations of less than 5ppm and can tolerate a maximum concentration of 50ppm (Figure 1A&1B). After seven days of treatment with Azolla pinnata, the percentage removal of chromium reached a maximum of 94.8% at a 5ppm chromium concentration and a minimum of 16.5% at a 50ppm concentration (Figure 2A). The effects of chromium through sustainable ecological phytoremediation after seven days are shown in Figure 3. After fourteen days of treatment, the percentage removal of chromium by Azolla pinnata increased, achieving a maximum bioremoval of 97.3% at 5ppm chromium concentration and a minimum of 16.7% at 50ppm (Figure 2B). The effects of chromium via sustainable ecological phytoremediation after fourteen days are illustrated in Figure 2. Compared to the results from day seven, Azolla pinnata significantly reduces chromium concentration after fourteen days of phytoremediation. These observations suggest that the reduction in chromium concentration is greater at 5ppm compared to the measurements taken on the seventh day. Removal efficiency decreased as initial Cr(VI) concentration increased, consistent with sorption site saturation and stress effects; at 40-50ppm, removal was notably reduced (≈16-43%). Additionally, Figure 3A&3B illustrates that on the 7th and 14th day, the leaves of Azolla pinnata remained fresh at a concentration of 5ppm; however, at higher concentrations, the leaves became completely dried out due to the stress caused by chromium toxicity. The highest recorded chromium removal rate was 97.34%. It is evident that as the concentration of chromium increases, the percentage of removal decreases. Figure 4 illustrates the impact of pH on chromium levels following phytoremediation, measured on the 14th day. Initially, the pH was acidic; however, after phytoremediation, it shifted to an alkaline state. According to Lindberg and Wingstrand (1985), plants can alter the pH of the surrounding water through their roots, which may help them cope with stress from heavy metals. Specifically, the roots significantly raised the rhizosphere pH under stress, especially when chromium was present in the solution. This shows the effects of chromium on biomass content after 14 days of phytoremediation.

Figure 1:Spectral observation of Chromium removal at 540nm (A) after the 7^th day and (B) 14^th day.

Figure 2:The percentage removal of chromium by Azolla pinnata (A) after the 7^th day and (B) 14^th day.

Figure 3:Effect of Chromium Phytoremediation by Azolla pinnata treatment after the (A) 7^th day and (B) 14^th day.

Figure 4:Effect of pH of the chromium after phytoremediation by Azolla pinnata treatment on the 14^th day.

The growth rate of Azolla pinnata is negatively impacted by chromium stress. According to the analysis, there is a significant increase in the percentage decrease of fresh weight, as well as a notable reduction in the dry-to-fresh weight ratio at higher chromium concentrations. The maximum percentage decrease in fresh weight was observed with a 50ppm chromium solution after 14 days, suggesting that Azolla pinnata may accumulate chromium. Additionally, a higher dry weight-to-fresh weight ratio was noted at the lower concentration of 5ppm during the same 14-day period.

Figure 5 illustrates the impact of chromium on total chlorophyll content following phytoremediation over a period of 14 days. Chlorophyll content was assessed using acetone extracts, in accordance with the Arnon method (1949). The results indicate a decreasing trend in chlorophyll A content as chromium concentrations increased. In contrast, chlorophyll B and total chlorophyll contents showed an increasing trend; however, these overall values still declined with higher chromium concentrations. Elevated levels of chromium toxicity negatively affect photosynthesis. The reduction in chlorophyll content may be attributed to decreased enzymatic activity of protochlorophyllide reductase. Additionally, the lowered levels of photosynthetic pigments in plants could be linked to lipid peroxidation occurring in the chloroplast membranes.

Figure 5:Effect of chromium after phytoremediation by Azolla pinnata on Total Chlorophyll content after treatment on the 14^th day

Figure 6A&6B illustrates the impact of chromium on carotenoid and protein content after 14 days of phytoremediation. It shows that as chromium concentration increases, carotenoid content decreases. According to Hou et al. (2007), carotenoids are nonenzymatic antioxidant pigments that protect cells from Reactive Oxygen Species (ROS) under chromium stress. The decline in carotenoid content can be attributed to metal toxicity, which may hinder the formation of photosynthetic pigments. This explains the observed reduction in photosynthetic pigments with increasing chromium concentration. Moreover, protein content also diminishes as chromium levels rise. Specifically, Azolla pinnata shows a reduction in protein content at higher chromium concentrations.

Figure 6:Effect of chromium after phytoremediation by Azolla pinnata on (A) Chlorophyll, (B) Carotenoid, and Protein content after treatment on the 14^th day.

Phytotechnology harnesses aquatic macrophytes and their microbiomes to remediate chromium-contaminated waters. This study evaluates Azolla pinnata as a free-floating fern for sustainable remediation and potential recovery of Cr(VI) from synthetic wastewater. This study decisively demonstrates the successful removal of hexavalent chromium from contaminated synthetic wastewater using Azolla pinnata, highlighting a healthy, sustainable ecotechnology approach that capitalizes on the synergistic effects of both Azolla and algae. This innovative method, known as phytotechnology, encompasses processes such as bio-removal, bioreduction, and biodegradation. It prominently features rhizofiltration, where free-floating aquatic plants, such as Azolla pinnata, play a crucial role in pollutant removal. The combined action of the algae and Azolla leads to a significant reduction in chromium concentration through rhizofiltration, as these aquatic plants effectively adsorb pollutants from their environment.

As noted by Mishra et al., this decline can be linked to lower protein levels in aquatic macrophytes, as well as increased protein degradation resulting from metabolic processes. Chromium may disrupt the balance of free nucleotides and RNA, leading to reduced RNA biosynthesis, which could explain the decrease in protein synthesis. Azolla pinnata possesses an exceptional ability to hyperaccumulate heavy metals from contaminated water bodies (Wagnar, 1997). Ex situ research conducted by Salt et al. (1995), Bennicelli et al. (2004), Jangwattana (2010), Sood et al. (2011), Deval et al. (2012), Moradi et al. (2013), Sufian et al. [8], and Thayapara et al. [9] has demonstrated the various capacities of A. pinnata to uptake and retain different heavy metal ions. These findings highlight A. pinnata’s potential for the phytoremediation of heavy metal-polluted water reservoirs. The nearby electroplating industry discharges chromium, a heavy metal, into water bodies. The dried biomass of A. pinnata can be easily transported to recycling sites for heavy metal recovery (Sood, 2011). The study of aquatic free-floating plants, including L. minor, has been revealed to be a noble model for ecotoxicological studies, and it can bioaccumulate, or biosorption of nutrients and metals, was reported earlier [1-3].

This study confirms the efficacy of this phytoremediation treatment in removing heavy metals. Additionally, it is suggested that the 33 artificial systems integrated into the lake, planted with A. pinnata, can effectively function as biological filters to remove heavy metal ions from industrial discharges. Implementing such filter systems would ensure that industrial pollutants are captured and eliminated before contaminating public water bodies [10-15]. The results unequivocally demonstrated a significant reduction in total chromium concentration in the samples compared to the controls, confirming the effectiveness of the phytoremediation process. The aquatic, free-floating plants serve as a principled model for ecotoxicological studies, possessing the ability to bioaccumulate or biosorb nutrients and metals, thereby supporting sustainable development [16-21].

Conclusion

The primary characteristics of macrophytes that exhibit strong phytoremediation abilities include rapid growth rates, high biomass, and significant adaptability to a variety of environmental conditions. A notable example is the endosymbiont Nostoc azollae found in Azolla pinnata, which plays a crucial role in assimilating atmospheric nitrogen, enhancing the plant’s overall growth. This feature contributes to the vigorous reproduction of the water fern in irrigated environments. Additionally, the free-floating nature of A. pinnata and similar macrophytes simplifies the harvesting process. Their high-water content in fresh biomass, ranging from 90% to 94%, significantly reduces volume when dried, mitigating disposal challenges for the harvested material. Furthermore, A. pinnata demonstrates an impressive ability to thrive in highly polluted waters with varying pH levels, temperatures, and salinity, making it particularly suitable for phytoremediation applications. This study concludes by advocating for A. pinnata as a viable phytoremediation agent capable of addressing heavy metal contamination in public dams and waterways. Its delicate structure is especially beneficial for use in industrial ponds and rice paddies, where pollution from waste disposal and agricultural chemicals is a significant concern. Harvesting the heavy metal-rich water ferns can be done mechanically or manually, facilitating the chemical extraction of heavy metals, such as chromium, for multiple recycling uses. Although A. pinnata may not serve as a complete solution to heavy metal pollution, it provides a promising approach to restoring water bodies contaminated with specific heavy metals by capturing them within the plant’s tissues. Azolla-based phytotechnology provides a pragmatic pathway to mitigate chromium pollution and support resource recovery; field deployment should pair with upstream source control and routine monitoring. This sustainable remediation approach not only addresses critical environmental challenges but also showcases the remarkable potential of leveraging natural solutions for a cleaner, healthier earth.

References

1. Usharani K, Arunkumar V (2023) Bioremoval and resource recovery of nutrients by phytoremediation using aquatic free-floating plants Lemna minor. Ukrainian Journal of Ecology 13(5): 13-27.
2. Usharani K, Keerthi KV (2020) Nitrate bioremoval by phytotechnology using utricularia Aurea collected from eutrophic lake of theerthamkara, Kerala, India. J Poll 6(1): 149-157.
3. Usharani K, Divya K, Sruthilaya K (2020) Combined effect of nitrate bioremoval by aquatic free-floating plant and association with filamentous Cyanobacteria. Austin Environ Sci 5(1): 1043.
4. Shekhar P, Prashik G (2016) Phytoremediation studies for removal of copper & chromium using azolla pinnata and water hyacinth. International Journal of Innovative Research in Science, Engineering and Technology 5(5): 7078-7083
5. Usharani K, Muthuchamy M, Perumalsamy L (2011) Biological removal of phosphate from synthetic wastewater using bacterial consortium. Iran J Biotechnol 9(1): 37-49.
6. Usharani K, Sruthilaya K, Divya K (2017) Determination of nitrate utilization efficiency of selective strain of bacillus sp. isolated from Eutrophic Lake, Theerthakara, Kasaragod Kerala. J Poll 3(1): 55-67.
7. Mandakini U, Bandara N, Gunawardana D (2016) A study on the phytoremediation potential of Azolla pinnata under laboratory conditions. JTFE 6 (1): 01-49.
8. Sufian J, Golchin A, Avanes A, Moradi S (2013) Potentials of azolla (Azollacaroliniana) for uptake of arsenic from contaminated waters with different levels of salinity. International Journal of Agriculture and Crop Sciences 6(12): 778-783.
9. Thayapara M, Iqbal S, Chathuranga PKD, Iqbal MCM (2013) Rhizofiltration of pb by Azolla pinnata. International Journal of Environmental Sciences 3(6): 1811-1821.
10. Annie MPA, Gilbert CS (2013) Phytoremediation: A green technology to remove environmental pollutants. American Journal of Climate Change 2(1): 71-86.
11. Abdel WR, Lubberding HJ, Alaerts GJ (1995) Copper and chromium (III) uptake by duckweed, water science and technology. Water Science and Technology 32(11): 105-110.
12. Gandhi N, Sirisha D, Chandra SKB (2013) Phytoremediation of chromium and fluoride in industrial waste water by using aquatic plant ipomoea aquatica. SPJPBS 1: 001-004.
13. Malairajan S, Alemayehu AM, Vinodhini SR (2007) Studies on the removal of hexavalent chromium from industrial wastewater by using biomaterials. EJEAFChe 6(11): 2557-2564.
14. Nelson M, Marta M, Elena M (2006) Phytoremediation and phytotechnologies: A review for the present and the future. Soil and Water Pollution Monitoring, Protection and Remediation pp. 403-416.
15. Nuzhat S, Ashok KP, Azra NK, Basharat M (2015) Heavy metal accumulation by Azolla pinnata of dal lake ecosystem, India. Journal of Environment Protection and Sustainable Development 1(1): 8-12.
16. Punita SP, Soma KM (2015) Capacity of Azolla pinnata var. imbricata to absorb heavy metals and fluorides from the wastewater of oil and petroleum refining industry at Vadodara. IJAPRR 2(1): 37-43.
17. Ranjana JT, Jeya R, Prabha JMP (2012) phytoaccumulation of chromium and copper by Pistia stratiotes and Salvinia natans (L.) all. Plant Resour 2(6): 725-730.
18. Santosh KP, Neelima M, Shivangee S (2012) Phytoremediation of chromium and cobalt using Pistia stratiotes: A sustainable approach. Proceedings of the International Academy of Ecology and Environmental Sciences 2(2): 136-138.
19. Uysal Y (2013) Removal of chromium ions from wastewater by duckweed, Lemna minor by using a pilot system with continuous flow. Journal of Hazardous Materials 263(2): 486-492.
20. Wang Q, Cui Y, Dong Y (2002) Phytoremediation of polluted waters: Potentials and prospects of wetland plants. Engineering in Life Sciences 1(2): 199-208.
21. Xiaomei L, Maleeya K, Prayad P, Kunaporn H (2004) Removal of cadmium and zinc by water hyacinth, Eichhornia crassipes. Science Asia 30: 93-103.

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Quantification of Free Cyanide (CN-) by Volumetric Analysis in Synthetic Solutions of NaCN: Crimson Publishers

Quantification of Free Cyanide (CN-) by Volumetric Analysis in Synthetic Solutions of NaCN by Ramiro Escudero G in Aspects in Mining & Mineral Science: Peer Reviewed Aspects in Mining & Mineral Science

Abstract

There are at least five standardized methods to determine cyanides in water. These procedures have certain limitations and must be applied according to the type or components in the liquid medium. What is clear is that each step of the method applied must be well understood to avoid introducing errors that could lead to the incorrect quantification of free cyanide in solution. An important application of one of the procedures is in mineral processing, specifically in the quantification of free cyanide in residual solutions, for example, during the leaching of gold ores with cyanide. In this work, unreacted cyanide was quantified with known concentrations of cyanicide elements contained in synthetic solutions (simulating a pregnant residual solution). The free cyanide was quantified by the volumetric method based on the color change of the problem solution upon adding the titration reagent AgNO3. Experimental results show that CN and the metal form the complex Me(CN)x-, and the residual CN-, determined by the colorimetric technique, is satisfactorily quantified with an error of less than 3%; however, it is established that a limitation of this technique is the correct quantification of free cyanide if the CN- is less than 0.001g/l. The correct calculation of cyanide (without reaching saturation) that must be added to ensure the dissolution of all the gold contained in a mineral, avoids cyanide passivation and excessive contamination of groundwater and soils.

Keywords:Cyanidation; Leaching; Cyanicides; Cyanide passivation; Free cyanide

Theory

There is little literature on the subject of standardized procedures applied to determine with some accuracy the cyanide in solution, which does not react after a leaching process, even in mining practice procedures are not applied to establish the amount of cyanide necessary to cyanidate the cyanicide species and the gold contained in a gold-bearing ore. The addition of cyanide is based on empirical decisions. This fact is never associated with phenomena such as cyanide passivation and low gold recoveries [1]. NMX-AA-058-SCFI-2001 [2] and ASTM D2036-09 [3], are documents that describe standard test methods for cyanides in water. These include the quantification of cyanide species after distillation, cyanides amenable to chlorination by difference, weak acid dissociable cyanides and cyanides amenable to chlorination without distillation. Common interferences in the analysis for cyanide include oxidizing agents and sulfides, aldehydes, mainly. Aspects that are not considered in these communications are the sensitivity of the techniques, especially for the relatively high and low concentrations of cyanide in solution, in addition to human errors due to the visual assessment of the person applying the technique. In the case of the dissolution of gold and cyanicide metals contained in a gold-bearing ore, the technique applied to quantify the unreacted (or residual) cyanide is the one mentioned above as weak acid dissociable cyanides, which involves the quantification of free cyanide in solution, by colorimetry.

Procedure for quantifying free cyanide (CN-) by volumetry or colorimetry

The weak acid dissociable cyanides procedure consists of the following steps:
a. Preparation of the titrating reagent (AgNO3) at 0.1 normal,
b. Preparation of the KI reagent at 5% (v/v),
c. Preparation of the solution with cyanide,
d. Titration with silver nitrate,

From (Equation 1), the constant 5.308 x 10-4 corresponds to the grams of cyanide that are associated with 1ml of AgNO3; in other words, one milliliter of silver nitrate reacts or identifies 5.308 x 10-4 grams of cyanide, according to the stoichiometric ratio of the following reaction.

Certainly, cyanide in solution can be empirically quantified by constructing calibration curves that relate the silver nitrate “consumed” or added, and the known cyanide added to the solution; that is, the grams of silver nitrate that stoichiometrically correspond to react with one gram of cyanide (98g NaCN/169.86g AgNO₃=0.576), for a certain dissolution factor of the sample with cyanide. Clearly, the limitation of this empirical quantification is that it does not consider the saturation factor of the solution with CN.

Difference between empirical and standardized methods

It is clear that the empirical method based on the development of calibration curves does not take into account the grams of silver nitrate contained in one liter of silver nitrate, nor does it consider the dilution factor.

Where the result is the cyanide saturation factor, which is equal to 0.05308g of CN- (free cyanide). This constant is substituted into (Equation 1) and together with the dilution factor, generalizes the equation to determine the cyanide that did not react in solution. In this work, cyanide in solution is quantified using empirical calibration curves and applying the equation proposed by the standardized method, which considers the saturation factor of the solution with cyanide, regardless of the degree of dissolution of the analyzed sample.

Experimental Methodology

Materials and reagents

For the quantification of free cyanide by the volumetric technique, a 25ml burette, 100ml Erlenmeyer flasks, universal support, burette clamp, 500, 100 and 50 milliliters volumetric flasks and a 100 milliliters beaker were used. The chemical reagents used were Silver Nitrate (AgNO3), Sodium Cyanide (NaCN), Sodium Hydroxide (NaOH), Potassium Iodide (KI), distilled water, Au, Ca, Mg, Cu and Fe standards-all the above reagent grades.

Quantification of free cyanide in synthetic solutions without metals

Solutions with known concentrations of CN (0.05 to 0.25g) dissolved in distilled and deionized water were prepared. The pH of each sample was set at 11.5. To quantify the CN in solution, the procedure was applied according to the standards NMX-AA-058- SCFI-2001 and ASTM D2036-09.

Quantification of free cyanide in synthetic solutions with metals

Five cyanicidal elements (Au, Ca, Mg, Cu and Fe) were used to prepare synthetic solutions with known concentrations of these elements (Me_added). The quantification was carried out in unitary and binary synthetic solutions; each determination was carried out in triplicate. In the experiments with unitary solutions, the concentration of metals was set at 50ppm in 50ml of deionized water; in the case of binary solutions, the concentration of gold was maintained at 50ppm in 25ml of deionized water, and the rest of the metals were varied from 200 to 300ppm. The percentage of cyanide associated with metals varied from 0.002 to 0.13 (Metal/CN_added ratio from 0.04 to 5), depending on the cyanide concentration in the synthetic solutions. To prevent the formation of hydrocyanic acid, the pH of the synthetic solutions was kept at 11.5; this also ensures that the cyanide dissociated from NaCN will react with the cyanicide metal.

Identification of CN-metal complexes by UV-Vis technique

Each Metal-CN solution before free cyanide quantification was analyzed using the UV-Vis technique to identify the complex species formed for unitary and binary solutions. UV-Vis Spectroscopy equipment, Perkin Elmer brand, model Lambda BIO Series 24117, was used at 200 to 800nm wavelengths.

Result and Discussion

Results of free CN- quantification, empirically through calibration curves

Figure 1 shows the results of CN- quantification by constructing calibration curves, for a dilution factor of 1.0. From the figure above, it is established that the concentration of cyanide in solution can be quantified if the milliliters of silver nitrate added during the colorimetric test are known; this applies to a dissolution factor of 1.0 and the CN range included in this test ([CN] much longer than 0.001g/l). It should be noted that some deviation from linearity is observed for the known minimum and maximum CN values. The equation shown in Figure 1 indicates that the concentration of cyanide in solution can be empirically determined by multiplying the milliliters of silver nitrate by the factor 0.0032. Applying the above relationship to the known values of CN, a certain deviation or error is observed, as the cyanide concentration decreases, as shown in Table 1. This deviation is due to the fact that the relationship between cyanide and silver nitrate is not always linear, this deviation is due to the fact that the relationship between cyanide and silver nitrate is not always linear, as shown in later paragraphs.

Table 1:[CN] known and calculated according to the equation included in Figure 1.

Figure 1:Calibration curve for the data with known concentration of CN in distilled water.

Results of free CN- quantification by volumetry. unitary and binary solutions

Identification of CN-metal complexes by UV-Vis technique: The solutions analyzed by this technique show evidence of the formation of the Me(CN)⁻ ₂ — complexes for the metals Au, Ca, Mg, and Cu, while for Iron, the species Fe(CN)⁻ ₆ — was identified. Its characteristic position at the corresponding wavelengths coincides with those reported in the literature [4-10]. An example of the spectra obtained is shown in Figure 2, which corresponds to one of the solutions with Mg. From the identification of the formation of metal complexes with cyanide, it is established that the CN determined by the volumetric technique is the residual or free cyanide, which is the one that did not react with the metals (CN⁻).

Figure 2:UV-Vis spectrum of the solution with the mixture of CN and Mg in distilled water.

Results of free CN- quantification by volumetry. All solutions

The results of free cyanide quantification by the volumetric technique in both unitary and binary solutions are presented in a previous communication [11]. By including in a single figure all the Me/CN_added ratios for all solutions, unitary and binary, the result is as shown in Figure 3. The above figure shows that all the relationships fit reasonably with the same function, which allows establishing the concentration of free cyanide for any concentration of cyanicidal metal and the cyanide added to the aqueous system. By knowing the concentration of cyanicidal metals in the system, it is possible to determine the quantity of cyanide that must be added so that the free cyanide is less than or close to 0.001g/l, which will help to avoid excess added cyanide, which can promote the passivation of the cyanidation reactions, as well as the contamination of soils and groundwater. Due to the shape of the curve in the relationship shown in Figure 3, it is established that a limitation of this technique is the adequate determination of free cyanide if the CN⁻ is less than 0.001g/l and in any case, the quantification must be carried out in aliquots with a certain dilution factor less than 1.0.

Figure 3:Free CN⁻ quantification results for all Me/CN_added ratios tested in this work.

Conclusion

After applying the standardized method of dissociable cyanides in weak acids to determine free or residual cyanides in water, the following conclusion is derived: All the Me/CN_added ratios included in this research work, reasonably adjusted to the same function that allows establishing the g/l of free cyanide in the residual solution once the cyanide reacts with the cyanicidal metals contained in the reaction system. This conclusion leads to the assertion that it is possible to avoid the excessive addition of cyanide, which inhibits the dissolution of cyanide species, passivating the reactions between cyanide and cyanicide metals contained, for example, in a mineral, as well as the high contamination of groundwater and soil. A limitation of this technique is proposed when the concentration of free cyanide is less than 0.001g/l; it is recommended that the quantification should be carried out in aliquots with a certain dilution factor less than 1.0.

Acknowledgment

Authors deeply thanks the UMSNH for the support received for carrying out this research work.

References

1. Chumpitaz PH, Rodriguez CE, Andia PG (2022) Significant reduction in cyanide consumption in gold leaching allowed considerable savings in operating costs at the La Arena Mine Processing Plant. Technical Report 2022, Panamerican Silver-La Arena Mine, Canada.
2. Standard NMX-AA-058-SCFI-2001. Official Gazette of the Federation.
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