Carbon Dioxide-Based Neutralization of High-Density Sludge Effluents as a Sustainable Climate and Water Quality Alternative to the Use of Strong Mineral Acids

Abstract

The neutralization of high-density sludge (HDS) effluent is a required process involved in the treatment of acid mine drainage (AMD). In their last treatment stage, effluents with high pH values are acidified to reach legal standards before being released to the environment using hydrochloric or sulfuric acid. In this investigation, CO₂ was tested as an alternative way to decrease the pH of the HDS effluent, together with an economic analysis comparing the results with the use of strong mineral acids, considering a full-scale 300 m³/h plant. HDS samples were collected from a PAN American Silver operation in Cajamarca, northern Peru. Four acidification tests were carried out on 20 L containers, with a subsequent evaluation of reaction time and CO₂ consumption to regulate the final pH of the treated solution. The results suggest that by adding CO₂ (0.5 L/min) to the solution, the pH was successfully decreased from 10–10.5 to 6.5–7.5 (which falls within the legal limits) in a matter of minutes. An average of 130 g of CO₂ was sequestrated per m³ of solution to decrease the pH within legal limits, representing around USD 0.031/m³ in terms of treatment cost for a full-scale plant. While this is more expensive than using other acids, with a CO₂ credit of USD 100/ton, sequestrated CO₂ neutralization is 12% cheaper and only 6% more expensive than using H₂SO₄ and HCl, respectively. Moreover, in terms of the costs per ton of avoided CO₂ of USD 133 and USD 262 for replacing hydrochloric and sulfuric acid, respectively, it is markedly lower than the cost of other CO₂ abatement technologies, like, for instance, solar photovoltaic panels (PV) that can cost between USD 368 and USD 684/ton of avoided CO₂ in Peru and require substantial capital investments. Moreover, the use of CO₂ implicates a series of additional safety, operational, and environmental advantages that should be considered. Therefore, the use of CO₂ to decrease HDS effluent’s pH should be further explored in Peru and elsewhere as a sustainable alternative.

1. Introduction

Water pollution from acid mine drainage (AMD) is a global concern. Surface or groundwater contamination from untreated (or not properly treated) AMD has been reported in countless geographical areas around the world, including India [1], Spain [2], Indonesia [3], Slovakia [4], Portugal [5], Brazil [6], and the United States [7]. Moreover, Peru (the home of this investigation) is not an exception; scientists such as Grande et al. [8], Sevink et al. [9], and Vriens et al. [10], among many others, have reported water contamination from AMD in this South American country.

AMD remediation costs can vary widely depending on several factors, including the scale of the affected area, the severity of contamination, and the chosen remediation techniques [11]; however, AMD treatment cost is known to be high and has been the focus of interest of researchers with the purpose of finding an economic way to improve the quality of those toxic effluents. For example, AMD can be treated using microbes [12], membranes [13], or wetlands [14]. Furthermore, the latter is often considered the most cost-effective treatment method [15]; for example, in a comprehensive study in Pennsylvania, wetlands were found to amount to USD 5700/per km of affected stream per year [16]. However, when it comes to semi-arid regions like parts of Peru, those toxic effluents are commonly treated with hydrated lime (Ca(OH)₂) in a high-density sludge (HDS) system to increase the pH until around 9.5 to 10.5, depending on the metal type and concentration, as basic fluids can maximize the metal precipitation process [17]. However, the resulting cleaner water cannot be released into the environment because it is too basic, so hydrochloric or sulfuric acid (HCl and H₂SO₄, respectively) is commonly used to decrease the pH of the effluent [18]. Moreover, this process adds chloride or sulfate ions to the fluid, which can cause adverse effects at higher concentrations in people [19], downstream water transportation structures [20,21], and the environment [22]. While many sulfate removal technologies exist such as chemical-, electrical-, biological-, and membrane-based approaches, among many others [23,24,25,26,27], these technologies are generally not implemented, as they add significant costs to the process of AMD treatment, and sulfate ion regulations are commonly not very stringent due to the moderate toxicity of the sulfate ion.

It is long known that when CO₂ is dissolved into a fluid, it reacts with water molecules, becoming carbonic acid (H₂CO₃) and making water more acidic [28]. Hence, AMD’s pH can be lowered by simply adding carbon dioxide (CO₂) into the fluid [29], which is what motivated this investigation. Moreover, the use of CO₂ can contribute to decreasing carbon emissions from mining operations and, therefore, mitigate global warming [30].

The use of CO₂ in AMD treatment is not novel. For example, the authors that used CO₂ on AMD highlight the work by Vaziri Hassas et al. [31], who used CO₂ mineralization to produce rare elements’ precipitation in AMD in Pennsylvania (USA). Similarly, Masindi et al. [32] applied CO₂ bubbling (among other techniques) to treat AMD waters and generate gypsum and limestone in South Africa. However, not much has been done in terms of CO₂ sequestration in the process of HDS effluent’s pH decrease, other than the work by Lee et al. [33], who performed a feasibility study using CO₂ injection after applying hydrated lime to increase the pH of AMD from an abandoned coal mine in Germany. The authors injected CO₂ into the AMD neutralization process to generate calcium carbonate (CaCO₃), noting that estimating the optimum CO₂ injection volume and concluding that CO₂ sequestration using the neutralization process can be considered a sustainable option. However, an economic analysis of CO₂ injection compared to hydrochloric and sulfuric acid for acidification of AMD is highly needed for further decision-making and environmental policies. Moreover, Lee et al. [33] applied CO₂ injection to HDS effluent with a pH of 12. This pH is much higher than a typical HDS effluent’s pH and may have led to an overestimated CO₂ sequestration potential as well as a prohibitively high cost of acidification. Therefore, in this investigation, CO₂ injection was applied to bring the pH of a typical HDS effluent of 10 down to legal disposable levels (following the Peruvian environmental standard DS 010-2010-MINAM), complementing an economic evaluation of the process. The main objective of our research was to determine the influence of CO₂ injection on the reduction of pH and, at the same time, assess the amount of CO₂ that can be sequestered as well as the avoided CO₂ emissions from not using other mineral acids. The second objective was to assess the operating costs of using CO₂ as opposed to hydrochloric or sulfuric acid at a future 300 m³/h AMD treatment plant at the Shahuindo Mining Unit operation in Peru.

2. Materials and Methods

The investigation took place at the Choloque subdrainage of a pilot plant, with AMD samples provided by Pan American Silver (PAS) in the Shahuindo Mining Unit operation in Peru (coordinates 9158986N-808891E). This open-pit gold and silver operation is able to produce over 36 thousand tons per day of minerals. The operation, which is considered large-scale, i.e., above 5000 tons of minerals processed per day per Peruvian law, is located in the District of Cachaci, Province of Cajabamba, Department of Cajamarca in northern Peru (blue dot in Figure 1).

The objective of the pilot plant is to determine optimal design parameters for the full-scale 300 m³/h AMD treatment plant. The pilot plant’s operating process (Figure 2) consists of the following stages: reagent preparation; contact between the acid solution and the alkalis; alkalization; flocculation; sedimentation; acidification; and filtration. For this, ten tanks are used in the pilot plant, as well as a bag filter and a filter press. The reagent preparation is subdivided into two types of reagents, alkalis (lime milk) and flocculant, for which there are two tanks of approximately 120 L for each reagent (preparation and dosing tank). The lime milk is prepared at 15% solids and the flocculant at a concentration of 0.1% (1000 ppm). The acid solution and the lime milk are mixed in a 500-L contact tank. Acid water is fed using a centrifugal pump from the acid water receiving tank (provided by PAS-Shahuindo) to the contact tank and the lime milk is fed by a peristaltic pump from the lime milk dosing tank. The mixture made in the contact tank is then transferred by a centrifugal pump to an adjacent alkalization tank, which has a capacity of 2 m³. In this tank, the mix spends a certain residence time to react and reduce the metals contained in the initial solution, bringing the pH to levels of approximately 10.5 at the tank outlet. The solution is then transferred to the flocculation tank by overflow, where it comes into contact with the flocculant, transported by a peristaltic pump from the flocculant dosing tank, forming floccules, which are essential for the solid–liquid separation stage. Flocculation takes place in a 250 L tank that also receives the recirculation of sludge from a later stage. Once the mixture is flocculated, it is transferred to the 5 m³ clarifier reactor using a centrifugal pump. The reactor has two products: (1) an underflow that is partially recirculated, while the remainder is stored for later tests; and (2) an overflow that is transferred to the holding tank of the bag filter which, using a centrifugal pump, filters the last solid residues present in the clarifier overflow solution, which is then transported to a 250 L tank where it undergoes an acidification period using CO₂ (the focus of this study). All tanks have individual frequency converters and agitators that allow the agitator rotation speed to be regulated. Additionally, the pump’s inlet flow is regulated using the rotameter and the pump outlet valves.

The lime was prepared at 15% solids by mass, named “Bendiciones de Dios” (65% CaO and 42% -m200 particle size) in 120 L tanks with the lime milk preparation for 30 min, with constant agitation to prevent segregation of suspended particles. Moreover, the flocculant used was MT-6506 at a concentration of 0.1% (weight) and a contact time of one hour under constant agitation. The operating conditions for the execution of the pilot tests developed in the Shahuindo Mining Unit—Choloque Platform and the quality of the final overflow to be treated are shown in Appendix A, while physicochemical analyses of four samples (from the feed water to be treated) are detailed in Appendix B. The consumption ratio of lime to reach pH of 10.4, 10.5, 10.6, 10.7, and 10.8 according to treatment flows were 0.43, 0.45, 0.47, 0.49, 0.51 kg/m³, considering a density of 1.11 kg/L of 15% lime milk, respectively. No preliminary screening of the coarse fractions of lime was performed in the preparation of lime milk. Needless to say, those pH values violate Peruvian environmental standards.

Four acidification tests were carried out on 20 L containers after filtering using a 15-micrometer screen due to surface total suspended solid (TSS) contents removing the slimes that are passed to the clarifier overflow to avoid the redissolution of solids when regulating the pH of the overflow solution. A subsequent evaluation of reaction time and CO₂ consumption to regulate the final pH of the treated solution to a target 6.5–8.5 range (to make sure legal limits are not violated) was performed. For each test, exact weight, CO₂ flow set with a rotameter, and reaction time were documented. Similarly, CO₂ consumption was determined by measuring the exact weight of the gas cylinder before and after the test, also documenting the residence time. Obviously, initial and final pH values were precisely measured.

3. Results

The results from the four tests are shown in

Table 1. Acidification test conditions for the four neutralized solutions tested.

Sample ID	Solution Weight (kg)	Initial pH	CO2 Flow (L/min)	Reaction Time (min.)	Initial CO2 Tank Weight (kg)	Final CO2 Tank Weight (kg)	CO2 Consumption (g/m3)	Residence Time (min.)	Final pH	pH Drop
S1	13.82	10.43	0.5	1.2	8.632	8.63	144.7	10	6.63	3.80
S2	16.11	10.39	0.5	0.8	8.612	8.61	124.18	5.3	7.76	2.63
S3	15.08	10.16	0.5	0.55	8.608	8.606	132.59	10	7.40	2.76
S4	17.05	10.42	0.5	0.9	8.612	8.61	117.32	10	7.50	2.92

, while the water quality levels of samples alkalinized using lime and samples from further acidified water using CO₂ are detailed in Appendix C and Appendix D, respectively. The evolution of pH over time is detailed in Appendix E and water quality analysis methods in Appendix F. By adding CO₂ (0.5 L/min) to the solution, the pH was successfully decreased from 10–10.5 to 6.5–7.5 (which falls within the legal limits), in 5–10 min. An average CO₂ consumption of 130 g per m³ of solution was required to decrease the pH within legal limits.

A comparison between the economic cost of acidification with CO₂ to the benchmark with HCl or H₂SO₄ is shown in

Table 2. Comparison of acidification cost with carbon dioxide, hydrochloric acid, and sulfuric acid.

Sample ID	CO2 Consumption (g/m3)	CO2 Cost (USD/m3)			HCl Equivalent (g/m3)	HCl Cost (USD/m3)			H2SO4 Equivalent (g/m3)	H2SO4 Cost (USD/m3)
		Ave.	Min.	Max.		Ave.	Min.	Max.		Ave.	Min.	Max.
S1	144.7	0.03	0.03	0.04	102.9	0.02	0.01	0.03	138.3	0.02	0.02	0.04
S2	124.2	0.03	0.03	0.03	88.3	0.02	0.01	0.02	118.7	0.02	0.01	0.04
S3	132.6	0.03	0.03	0.03	94.3	0.02	0.01	0.03	126.7	0.02	0.01	0.04
S4	117.3	0.03	0.03	0.03	83.5	0.02	0.01	0.02	112.2	0.02	0.01	0.03

. Slightly less H₂SO₄ is required for acidification [28]. Carbon dioxide and especially HCl and H₂SO₄ costs have exhibited a sharp spike since the pandemic in 2020 [29]. Therefore, the cost was calculated considering (1) an average global CO₂ price from 2020 to July 2024 of USD 0.236/kg, as well as a minimum and maximum price of USD 0.223/kg and USD 0.259/kg, respectively; (2) an average HCl price in Latin America of USD 0.18/kg, as well as a minimum and maximum price of USD 0.13/kg and USD 0.26/kg, respectively; and (3) an average global H₂SO₄ price of USD 0.161/kg, as well as a minimum and maximum price of USD 0.101/kg and USD 0.298/kg, respectively. Considering this,

Table 3. Average acidification cost and CO₂ sequestrated for a full-scale 300 m³/h AMD treatment plant.

Sample ID	CO2 Consumption (g/m3)	CO2 300 m3/h (Ton/Year)	Average CO2 Cost (USD/m3)	Average CO2 Cost with USD 100/Ton CO2 Credit (USD/m3)	Cost 300 m3/h (USD/Year)	Cost 300 m3/h with USD 100/Ton CO2 Credit (USD/Year)	HCl Equivalent (g/m3)	Average HCl Cost (USD/m3)	Cost 300 m3/h (USD/Year)	H2SO4 Equivalent (g/m3)	Average H2SO4 Cost (USD/m3)	Cost 300 m3/h (USD/Year)
S1	144.7	380.3	0.034	0.020	89,744	51,717	102.926	0.019	48,688	138.3	0.022	58,527
S2	124.2	326.3	0.029	0.017	77,017	44,383	88.330	0.016	41,783	118.7	0.019	50,227
S3	132.6	348.4	0.031	0.018	82,233	47,389	94.312	0.017	44,613	126.7	0.020	53,629
S4	117.3	308.3	0.028	0.016	72,763	41,931	83.450	0.015	39,475	112.2	0.018	47,452
Average	129.7	340.8	0.031	0.018	80,439	46,355	92.254	0.017	43,640	124.0	0.020	52,459

summarizes the average yearly cost for the full-scale 300 m³/h AMD treatment plant, assuming no downtime. Table 3 also lists the amount of CO₂ that can be sequestered per year for the full-scale plant. Unfortunately, most Latin American countries currently do not have a functioning carbon emission credit market, with a resulting average carbon tax of only USD 5/ton CO₂ compared to more than USD 100/ton CO₂ in some European countries [34]. However, this may change in the future, leading to extra costs for using hydrochloric or sulfuric acid and hence an added advantage of acidification with CO₂. Therefore, an acidification cost including USD 100/ton of CO₂ sequestrated was included in Table 3 as well.

However, by not using the other acids, the embedded CO₂ emissions incurred during the production of those acids could also potentially be avoided. The CO₂ emission factor for the production of liquid carbon dioxide is 0.82 CO₂ equivalents per kg compared to hydrochloric and sulfuric acid, which have 0.89 and 0.14 kg CO₂ equivalents per kg of acid, respectively [35]. With these emission factors, the total avoided CO₂ emissions per m³ of HDS effluent, including the CO₂ sequestrated, can be calculated (

Table 4. Average cost of avoided CO₂ emissions by using CO₂ instead of hydrochloric or sulfuric acid.

HCl Avoided Emissions (g CO2/m3)	Avoided CO2 300 m3/h (Ton/Year)	Avoided + Sequestered CO2 (g CO2/m3)	Additional Cost of Using CO2 Over HCl (USD/m3)	Cost Avoided CO2 HCl (USD/Ton CO2)	H2SO4 Avoided CO2 Emissions (gCO2/m3)	Avoided CO2 300 m3/h (Ton/Year)	Avoided + Sequestered CO2 (g CO2/m3)	Additional Cost of Using CO2 Over H2SO4 (USD/m3)	Cost Avoided CO2 H2SO4 (USD/Ton CO2)
−24	−64	105	0.014	133	−89	−234	41	0.011	262

). Lastly, taking into account the difference in cost of using CO₂ instead of the other acids, a cost per ton of avoided CO₂ can be calculated and compared with the CO₂ abatement cost of other technologies like solar photovoltaic (PV). Based on the range of the cost of utility-scale solar PV in Peru [36], the carbon-intensity of the Peruvian grid [37], and the carbon-intensity of utility-scale solar PV [38], a CO₂ abatement cost range of PV can be calculated in Peru (

Table 5. Cost range of avoided CO₂ emissions for utility-scale solar PV in Peru.

Minimum PV Cost (USD/kWh)	Maximum PV Cost (USD/kWh)	Emission Factor PV (Ton CO2/kWh)	Emission Factor Electricity Grid (Ton CO2/kWh)	Minimum Cost Avoided CO2 PV (USD/Ton CO2)	Maximum Cost Avoided CO2 PV (USD/Ton CO2)
0.07	0.13	23	213	368	684

).

4. Discussion

Carbon dioxide has been successfully used in AMD treatment [38]. The use of this greenhouse gas to lower the pH of treated AMD is promising. Lee et al. [33] were able to sequestrate 0.54 g CO₂ per kg of AMD. Our results showed that for this particular HDS effluent, about four times less CO₂ can be sequestrated, i.e., between 0.12–0.15 g CO₂ per kg of effluent (see Table 1). This still translates into 341 tons of CO₂ sequestrated/year for the full-scale 300 m³/h AMD plant. The main reason for the significantly lower CO₂ sequestration in this study, compared to the results by Lee et al. [33], is that the pH of the HDS effluent was around 10 instead of 12.5 in that study. HDS systems typically operate at a pH of 9.5–10.5, since at that pH, most metals such as iron, zinc, copper, and lead exhibit the lowest solubility [39]. Only to remove nickel and cadmium is a higher pH of 10.5–11 necessary. Operating at a pH of 12.5 is likely cost-prohibitive, and metal solubility increases again above pH 11. The CO₂ sequestration potential obtained in this study at an HDS effluent pH of 10 can therefore be considered more realistic. The fact that the total pH drop has a significant impact on CO₂ sequestration is confirmed by the largest pH drop of 3.8, with S1 having by far the largest CO₂ consumption. Further investigations should focus on how to increase the efficiency of this approach, including an optimization of the variables involved; as Lee et al. [33] recommended, reaching an optimum CO₂ injection volume is important. Unfortunately, Lee et al. [33] did not report the alkalinity of the water after alkalinization, which is another important variable to assess CO₂ sequestration potential besides pH. However, in this study, no clear trend could be discerned. S3 and S4 had very similar pH drops, but for S4, quite a bit more alkalinity was added, while it had a lower CO₂ consumption. This is a testament to the complexity of the process and the need for experimental validation.

Therefore, this investigation should be replicated in other mining operations within Peru and elsewhere to evaluate the results under other site conditions. Considering the large volumes of acid mine drainage in the world, even small specific sequestration quantities (around 130 g/m³ of AMD, according to our results) can add up to significant total CO₂ sequestration volumes. Unfortunately, there does not seem to be a good estimate available in the literature on global acid mine drainage generation, let alone average water quality characteristics to quantify the potential.

Lee et al. [33] did not include an economic analysis in their study, but considering the four times larger CO₂ consumption, it may be cost-prohibitive in their case. For the AMD in this study, USD 0.031/m³ seems reasonable, which would amount to nearly USD 80 thousand/year for the full-scale 300 m³/h plant (Table 3). Currently, using CO₂ to decrease the pH in HDS effluent is still about 55% and 82% more expensive than using sulfuric or hydrochloric acid, respectively. However, the cost of CO₂ is still decreasing from a peak during the past COVID-19 pandemic. Moreover, if a credit of USD 100/ton of CO₂ sequestrated is taken into account, using CO₂ is only 6% more expensive than using HCl and 12% cheaper than using H₂SO₄. Unfortunately, the emission factor for the production of liquid CO₂ is quite high because the main production method is as a waste product from the Haber–Bosch ammonia fertilizer process that is very energy- and emission-intensive [40]. This leads to the fact that higher CO₂ consumption compared to the other acids also leads to higher embedded CO₂ emissions, hence the negative values for avoided CO₂ emissions shown in Table 4. Nevertheless, the cost per ton of CO₂ avoided, taking into account both the sequestrated CO₂ and the higher embedded emissions, of USD 133 and USD 262/ton CO₂ for replacing HCl and H₂SO₄, respectively, is actually significantly lower than other CO₂ abatement technologies (see Table 4 and Table 5). One of the most popular CO₂ abatement technologies, namely solar photovoltaic panels, can cost between USD 368 and USD 684/ton of avoided CO₂ in Peru and require substantial capital investments. The high CO₂ abatement costs of PV in Peru (in the US, the range is USD 60 to USD 300/ton of CO₂ [41]), are mainly due to the already low carbon-intensity of the electrical grid because of the large contribution of hydropower [42]. However, additional PV will likely not replace hydropower, but gas-fired power plants with a typical emission factor of 490 tons of CO₂/GWh [43]. In this case, the range of CO₂ abatement costs drops to USD 150 to USD 278/ton CO₂, quite similar to the cost of CO₂ neutralization.

Dosing CO₂ also requires much fewer safety measures, like the secondary containment and personnel protective equipment necessary for handling strong mineral acids, and hence could lead to additional cost benefits and a less hazardous work environment for operators. Similarly, AMD treatment represents only a small fraction of the entire mining process during active operation, meaning that increasing acidification costs by less than half will not dramatically change the operation’s economic balance. However, this new practice could result in a better public image and a significant contribution to a better planet.

In addition to the above, using CO₂ also has several water quality benefits. For this AMD, the addition of 0.08–0.10 g/m³ of chlorides and 0.11–0.14 g/m³ of sulfates can be avoided, which improves water quality [44]. Adding CO₂ also buffers the water, leading to a more stable pH [45]. Similarly, dosing CO₂ is much less corrosive than dosing strong mineral acids, minimizing the release of metals through downstream pipes into receiving surface waters [46]. Most importantly, storing CO₂ in the mining process effluents, especially if it can be captured from on-site sources, could result in more sustainable mineral extraction worldwide.

The long-term advantages and potential financial incentives for CO₂ sequestration within the mining sector can be systematically classified into environmental, economic, regulatory, and reputational dimensions (e.g., [47,48]). Notably, the principal environmental benefits include the reduction of greenhouse gas emissions, along with enhancements in water quality and ecosystem health, as previously mentioned. Furthermore, economic advantages are reflected in cost reduction associated with remediation, long-term liability mitigation, and the prospective generation of revenue through carbon credits. CO₂ sequestration initiatives are capable of producing carbon credits that can be traded in carbon markets for revenue generation. Depending on the prevailing regulatory framework, mining companies have the opportunity to capitalize on their CO₂ reductions, thereby transforming a sequestration cost into a potential revenue source. Additionally, mining companies can utilize carbon credits derived from carbon capture and sequestration (CCS) to counterbalance their own direct emissions, facilitating the achievement of their internal emission reduction goals or ensuring compliance with external emissions reduction mandates.

Regarding regulatory benefits and compliance, numerous countries and regions are progressively adopting carbon pricing mechanisms, such as carbon taxes or cap-and-trade programs (e.g., [49]). CCS offers mining companies a mechanism to offset their CO₂ emissions, thereby diminishing their liability to taxes or penalties. Likewise, governments frequently extend financial incentives to entities that invest in carbon sequestration technologies. Such incentives may comprise tax credits, grants, and additional forms of financial support designed to expedite the adoption of CCS technologies. Notably, some examples of subsidy programs and incentives include the US 45Q Tax Credit [50], the Carbon Utilization Procurement Grants [51], and the EU Funding Programs (e.g., [52]). Furthermore, mining companies that implement CCS for environmental reasons may find themselves better equipped to comply with stringent environmental standards established by national or international regulatory authorities. This compliance could alleviate the risk of incurring fines or penalties for non-adherence to environmental regulations, particularly in jurisdictions with rigorous environmental protection legislation.

Equally important is that mining corporations may accrue reputational and corporate social responsibility advantages through the reduction of CO₂ emissions (e.g., [53]). Mining entities that practice environmentally responsible measures and adopt proactive approaches to diminish their carbon footprint and neutralize environmental contaminants, such as acid mine drainage, can enhance their public image. This may exert a direct influence on their marketability, attract investors with environmental inclinations, and foster robust relationships with customers and stakeholders. Furthermore, mining companies that invest in CO₂ sequestration technologies and exhibit a reduced environmental impact have the capacity to produce comprehensive sustainability reports. These reports are of increasing importance to investors, consumers, and regulators who concentrate on environmental, social, and governance criteria. This could elevate the likelihood of obtaining green investments and diminish community opposition.

5. Conclusions

The results indicate that adding CO₂ to HDS effluent resulted in disposable AMD with pH levels that fall within the Peruvian environmental standards. The carbon dioxide addition decreased the pH from 10–10.5 to 6.5–7.5 within minutes. While the 130 g CO₂/m³ is lower than a previous study that started at a rather unrealistic pH above 12, when calculating for a 300 m³/h full-scale treatment plant, this still amounts to 341 tons of CO₂ sequestered per year.

When compared to traditional HCl- and H₂SO₄-based approaches to decrease pH in AMD, the CO₂-based approach seems to be more expensive in the current economic environment (around 82% and 55% more expensive than using HCl and H₂SO₄, respectively). However, when taking into account a CO₂ credit of USD 100/ton, CO₂ neutralization is 12% less expensive than using sulfuric acid and only 6% more expensive than using hydrochloric acid. Moreover, the calculated cost per ton of avoided CO₂ of USD 133 and USD 262 for replacing HCl and H₂SO₄, respectively, is cost-competitive with the CO₂ abatement cost of solar photovoltaic panels in the Peruvian context. The advent of higher carbon taxes and cheaper carbon capture technologies that could recover CO₂ from mining operations on-site should reduce this cost dramatically in the future. Moreover, using CO₂ has numerous other benefits compared to strong mineral acids like improved water quality, a less hazardous work environment, and the decreased corrosion of downstream pipes. Therefore, this new approach should be further explored as it represents a valuable option for more sustainable mining operations overall.