Transition of CO₂ from Emissions to Sequestration During Chemical Weathering of Ultramafic and Mafic Mine Tailings

Abstract

Weather-enhanced sulphide oxidation accelerates CO₂ release into the atmosphere. However, over extended geological timescales, ultramafic and mafic magmatic minerals may transition from being sources of CO₂ emissions to reservoirs for carbon sequestration. Ultramafic and mafic mine tailings present a unique opportunity to monitor carbon balance processes, as mine waste undergoes instantaneous and rapid chemical weathering, which shortens the duration between CO₂ release and absorption. In this study, we analysed 30 vanadium-titanium magnetite mine tailings ponds with varying closure times in the Panxi region of China, where ~60 years of mineral excavation and dressing have produced significant outcrops of mega-mine waste. Our analysis of anions, cations, saturation simulations, and ⁸⁷Sr/⁸⁶Sr; δ¹³C and δ³⁴S isotopic fingerprints from mine tailings filtrates reveals that the dissolution load of mine tailings may depend significantly on early-stage sulphide oxidation. Despite the abundance of ultramafic and mafic minerals in tailings, dolomite dominates chemical weathering, accounting for ~79.2% of the cationic load. Additionally, due to sulphuric-carbonate weathering, the filtrates undergo deacidification along with sulphide depletion. The data in this study suggest that pristine V-Ti-Fe tailings ponds undergo CO₂ emissions in the first two years but subsequently begin to absorb atmospheric CO₂ along with the filtrates. Our results provide valuable insights into monitoring weathering transitions and carbon balance in ultramafic and mafic rocks.

1. Introduction

The weathering of ultramafic and mafic magmatic minerals (e.g., olivine, pyroxene, and serpentine) plays a critical role in regulating atmospheric CO₂ balance [1,2,3,4,5]. Weathering-derived sulphuric acid (H₂SO₄) can preferentially dissolve carbonates, leading to net CO₂ release, which provides a temporary source of CO₂ to the Earth’s atmosphere [6]. However, the timescale of sulphide oxidation to carbonate dissolution remains uncertain, hampering our understanding of carbon budgets.

Sulphuric acid is a key driver of chemical weathering reactions in river basins [7,8], deep groundwater [9], mountainous erosion [10], and glacial weathering [11]. Studies reveal that modern global sulphide oxidation fluxes range from 0.64 to 1.26 Tmol/year [8] and that sulphide-derived acid could release more CO₂ into the atmosphere–ocean system than is consumed by silicate weathering [7]. Conversely, chemical weathering is generally recognized as a process that converts atmospheric CO₂ into carbonates [12,13,14,15,16]. Further clarity on these processes requires quantification of the role of sulphide oxidation in surficial chemical processes.

The chemical weathering of ultramafic and mafic mine tailings waste offers an analogue for deciphering the timescale of sulphide oxidation and chemical weathering during their transition. Minerals for CO₂ storage of tailings mainly include olivine, serpentine, pyroxene, and amphibole [17,18,19,20,21,22,23]. The secondary minerals include magnesite (MgCO₃·3H₂O), dypingite (Mg₅(CO₃)₄(OH)₂·5H₂O) [24], hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O) [25,26], aragonite, and iron dolomite [27,28]. Factors such as milling, high permeability, and increased surface area significantly enhance the reactivity of mine waste [29,30,31], intensifying the water–tailings reactions and leading to enhanced CO₂ fixation efficiency in the tailings waste. For instance, the Baptiste iron-nickel mine in Canada can offset 42%–53% of the mine’s annual CO₂ emissions [30], while iron-nickel mine tailings in New Caledonia can offset 90% of the annual CO₂ emissions of mining operations [29]. BHP Group Limited reports that its tailings CO₂ storage capacity is 10 times greater than the company’s CO₂ emissions [32]. Other notable examples include the annual storage capacity of peridotite in Oman, which is about 10⁴–10⁵ tons [17]; the annual CO₂ storage capacity of Diavik diamond tailings in Canada, which is about 6300 tons [19,33]; and the annual CO₂ storage capacity of PGE tailings in South Africa, at about 32 million tons, accounting for 43.6% of the country’s total CO₂ emissions [34]. In China, the Jinchuan copper-nickel mine tailings CO₂ storage capacity is 40,000 tons per year [26].

Accelerated weathering of tailings waste can lead to the exhaustion of sulphides within years, further expediting the transition from CO₂ emissions to fixation. This shortened timescale enables the investigation of the entire weathering process of ultramafic and mafic magmatic mine tailings. By doing so, (1) the chemical weathering processes involved in the sulphide oxidation of fresh tailings waste can be identified; (2) the timescale of CO₂ released to sequestration of mine tailings can be quantified; and (3) potential weathering indices that may facilitate predictions regarding the transition from CO₂ emissions to sequestration can be explored. This study aims to investigate these aspects for a series of tailings ponds located in the Panxi region of China. In the following sections, we describe this study area, sampling and analysis methods, and outline a series of findings that improve our understanding of weathering transitions and carbon balances in ultramafic and mafic minerals.

2. Study Area

The Panxi region, situated within the Anning River valley of Sichuan Province, China, hosts large quantities of outcropping ultramafic and mafic mine waste produced over ~60 years of V-Ti-Fe ore excavation and mineral dressing. The ore is associated with Emeishan ultramafic and mafic magmatic intrusion in Southwest China [35]. It is estimated to contain approximately 6.7% of global vanadium (V) and 35.2% of global titanium (Ti) resources [36]. Ore minerals typically include titanomagnetite, ilmenite, apatite, pyrrhotite, and pentlandite. Gangue minerals include clinopyroxene, plagioclase, hornblende, and olivine. During beneficiation, large quantities of tailings waste are discharged into storage facilities (tailings ponds) in the form of slurry. As of 2017, mine tailings waste had accumulated to ~3.1 billion tons [37]. Tailings waste primarily consists of ultramafic and mafic minerals, such as pyroxene, plagioclase, amphibole, and olivine. Tailings have a range of applications, including roadbed landfill, construction bricks, and fireproof materials. Furthermore, they possess the potential to sequester an estimated ~8.45 million tons of atmospheric CO₂ annually [37]. Analogous cases have been studied in Platinum Group Element (PGE) mines in South Africa [34,38].

The Panxi region is characterised by several north–south oriented reverse faults. The lithological distribution in the valley comprises a Proterozoic crystalline basement, Cenozoic terrestrial red sedimentary rocks, Yanshanian granites, Palaeozoic and Mesozoic sedimentary cover, Neo-Palaeozoic rift basic intrusive rocks, Neo-Palaeozoic rift intrusive rocks, Late Palaeozoic rift basalts, Proterozoic diorites, and Proterozoic granites [39]. The topography is dominated by alpine valleys, with mine waste sites predominantly concentrated along the Anning River Valleys. The region’s climate is influenced by the Indian Ocean monsoon, southeast and plateau monsoon, and it experienced glacial activity during the Quaternary period [40]. The local annual rainfall is ~849.4 mm/year [41]. Rainfall is most intense during the summer months, while precipitation is less frequent in other seasons.

3. Methodology

3.1. Tailings Ponds Status

Figure 1 shows 94 mine area sites (including open-cut pits, milling infrastructure, waste rock dumps, and tailings storage facilities) identified in the Panxi region using remotely sensed datasets. These sites were validated through field investigations and were found to occupy 7665.5 ha [42,43,44]. Filtered water samples were obtained from 30 mine tailings ponds situated in the vicinity of Xichang (Taihe Town) and Panzhihua Cities (Baima, Xinjiu, Hongge, Yinjiang towns). The standardized construction of these tailings ponds minimizes interference factors, facilitating research on tailings chemical weathering processes. Moreover, the diverse operational histories of these ponds provide an ideal framework for investigating tailings chemical weathering across various temporal scales. Field investigation revealed that nine tailings ponds were actively receiving tailings, while the remaining ponds were either temporarily or permanently shut down (

Table 1. Hydrogeochemical characteristics of tailings ponds filtrates and background waters across V-Ti-Fe mine sites, Panxi region, Sichuan, China.

Tailings Ponds	Latitude	Longitude	Closure Time	Occupied Land	Volume	Temperature	TDS	pH	Ca2+	Mg2+	Na+	K+	Sr2+	Fe	SO42−	HCO3−	Cl−	NO3−	87Sr/86Sr	δ34SV-CDT	δ13CDIC-VPDB	Std. Dev	Saturate Index (SI)	SI	CO2 Degas (−) or Absorb (+)	CO2 Degas or Absorb
			Days	105 m2	105 m3	°C	mg/L		μmol/L											‰			FFeOOH	Fe2O3	Tons/Year
Background-1	27°7′22″ N	102°7′54″ E					194.4	6.50	1193	808	173	91			132	3921	81	0
Background-2	27°7′28″ N	102°6′8″ E					138.1	7.21	921	538	86	17			98	2734	50	41
Taihe	27°55′41″ N	102°8′18″ E	0	534	200	20.2	962	8.11	6944	5483	1506	327	18	0.04	11,118	5304.3	1318	96	0.7071	5.3	−13.14	0.03	3.87	8.71	−27,879	Degas
Desheng	27°2′15″ N	102°6′35″ E	0	7	229	21.7	740	7.96	4876	4297	1525	566	7	0.02	3331	2466.3	1128	0	0.7062	1.7	−5.93	0.03	3.62	8.22	−5	Degas
Yuantong	27°4′3″ N	102°7′16″ E	0	61	10	22.1	1543	5.79	8659	16,079	517	771	13	0.02	27,234	981.1	352	78	0.7069	0.3	−6.49	0.02	3.63	8.24	−11,077	Degas
Wanniangou	27°6′50″ N	102°8′46″ E	0	160	3260	24.6	873	7.99	4704	6553	1132	502	9	0.02	10,908	1681.5	2419	79	0.7066	2.9	−5.48	0.06	3.76	8.5	−9812	Degas
Anning	26°48′3″ N	101°59′14″ E	0	136	480	22	876	8.16	5686	4907	1636	550	11	0.02	10,338	5205.6	792	79	0.7085	5.4	−14.2	0.02	3.54	8.06	−5863	Degas
Majiatian	26°33′53″ N	101°45′14″ E	0	427	1600	25.8	1862	7.72	12,591	13,363	5862	625	25	0.02	28,049	2652.1	5494	84	0.7056	6.2	−11.8	0.03	3.66	8.3	−78,490	Degas
Hongfa	26°35′44″ N	101°56′54″ E	0	4	3	24.9	591	7.3	3569	2691	1508	106	7	0.02	1965	5060.8	863	79	0.7098	4.6	−8.92	0.01	3.47	7.92	47	Absorb
Jintai	26°36′57″ N	101°56′7″ E	0	3	3	22.7	1547	7.41	6926	2653	1662	72	8	0.02	3328	4305.1	2027	78	0.7116	4.1	−10.38	0.03	3.52	8.01	−6	Degas
Xinlongmang	26°38′48″ N	101°57′34″ E	0	39	702	24	2059	8.13	17,484	15,902	3919	1916	28	0.02	34,351	3174	2401	77	0.7122	5.2	−11.75	0.03	3.59	8.16	−8110	Degas
Hongxin	26°36′8″ N	101°56′12″ E	11	303	919	23.4	936	8.2	7964	5499	1090	130	19	0.02	12,591	3641.1	612	78	0.7116	6.8	−9.32	0.02	3.62	8.21	−22,011	Degas
Yili	26°36′15″ N	101°56′36″ E	101	7	116	23.3	516	7.66	3494	2750	560	34	4	0.02	1798	4415.5	263	78	0.7083	3.3	−11.31	0.02	3.55	8.06	66	Absorb
Ertan	26°37′0″ N	101°56′16″ E	105	4	2	22.6	627	7.4	4606	3153	909	58	5	0.02	2356	4535.7	617	82	0.7083	5	−12.04	0.03	3.75	8.48	24	Absorb
Liyu	26°44′2″ N	101°58′55″ E	290	263	84	23.9	1654	7.35	15,276	11,610	1392	1818	26	0.02	25,691	8390	444	78	0.7139	3.1	−6.31	0.01	3.55	8.07	−32,956	Degas
Zhonghe	27°2′39″ N	102°4′43″ E	365	112	380	19.5	698	7.66	4179	3928	1051	327	5	0.02	3010	2395.4	1015	91	0.7067	1.2	−9.9	0.04	−0.56	−0.15	−277	Degas
Hengtong	27°0′40″ N	102°10′41″ E	365	15	100	19.5	240	7.92	1381	1186	354	60	3	0.02	670	1928.2	166	111	0.7061	3.5	−12.78	0.03	3.73	8.45	134	Absorb
Fengyuan	26°37′59″ N	101°47 52″ E	422	442	1190	22.9	934	7.62	5666	5359	2574	131	9	0.02	12,519	2333.6	1071	80	0.7056	1.1	−10.42	0.04	3.69	8.35	−34,023	Degas
Qianfan	26°36′24″ N	101°55′36″ E	498	6	57	22.3	466	7.3	2716	2231	1044	41	6	0.02	1028	6803.2	218	124	0.7062	4.8	−13.62	0.06	3.54	8.05	145	Absorb
Zhongtian	26°35′5″ N	101°56′13″ E	498	8	2	20.4	499	7.65	3556	2593	492	15	5	0.02	1592	5111.8	226	79	0.7085	5.3	−12.66	0.03	3.64	8.26	106	Absorb
Tianlong	26°34′57″ N	101°56′23″ E	498	5	33	22	343	7.14	2178	1632	397	42	3	0.02	1630	5813.7	157	77	0.7119	5.7	−12.66	0.04	3.41	7.79	80	Absorb
Xiaoshuijin	26°37′25″ N	101°56′38″ E	498	112	68	20.4	799	7.4	5234	5265	1426	81	13	0.04	10,357	2790.6	662	79	0.7135	3.6	−7.94	0.04	3.17	7.32	−6711	Degas
Heigutian	26°40′13″ N	101°58′37″ E	498	7	61	24.2	1419	7.73	9654	8063	1920	1000	26	0.02	16,806	6136.4	668	79	0.7103	4.9	−17.19	0.05	3.32	7.6	−565	Degas
Baicao	26°41′54″ N	102°1′46″ E	696	272	95	16.4	547	7.85	3536	3573	370	116	11	0.02	2370	3078.1	179	0	0.7088	3.9	−9.95	0.02	3.71	8.4	387	Absorb
Sandaoguai	26°34′59″ N	102°2′47″ E	696	12	188	18	541	7.34	4049	3102	208	132	9	0.02	2110	3500.4	240	0	0.7072	3.8	−7.24	0.03	3.32	7.6	62	Absorb
Yuanda	26°38′32″ N	101°57′49″ E	730	1.3	-	22.6	719	7.16	5564	3582	1278	61	14	0.02	3067	4630	590	78	0.7074	3.8	−11.22	0.02	3.35	7.65	1	Absorb
Jinlong	26°35′57″ N	101°56′30″ E	731	5	2	24	965	7.44	8331	3550	2440	217	9	0.02	13,297	2809.1	743	78	0.7098	2.6	−10.24	0.02	3.54	8.05	−398	Degas
Xinmao	26°40′59″ N	101°56′1″ E	731	9.6		21.4	902	7.65	6819	6051	1288	69	11	0.02	11,150	4851.2	616	80	0.7084	2.2	−10.7	0.01	3.7	8.38	−560	Degas
Bochuang	26°32′40″ N	101°56′4″ E	733	0.7	-	18.6	282	7.69	1754	1355	306	39	3	0.02	863	5247.8	196	77	0.7112	5.9	−10.69	0.05	3.58	8.14	14	Absorb
Xiushuihe	26°35′26″ N	102°3′27″ E	1280	16	37	17.5	451	7.15	3556	3154	300	63	7	0.04	1592	3711	177	81	0.7083	3.9	−12.85	0.03	3.42	7.79	140	Absorb
Tianrun	26°38′15″ N	101°57′26″ E	1460	1	-	23.6	480	7.86	2574	2539	1111	8	6	0.02	2748	6902.2	238	77	0.7113	5	−12.26	0.03	3.69	8.36	11	Absorb
Detian	26°43′52″ N	101°59′2″ E	1850	9	300	22.1	351	7.6	1174	2989	546	47	1	0.02	621	8162.8	184	80	0.7162	11.3	−12.06	0.04	3.25	7.48	291	Absorb

NBS 987 is determined as 0.71026. The occupied land of mines was identified [42,43].

). For instance, the Detian tailings pond has been inactive for approximately five years (longest closure around 1850 days), and the overlying land has been converted into a weedy field.

Filtrate samples from 30 tailings ponds arising from V-Ti-Fe mine mineral dressing were collected to determine the hydrogeochemical properties, with one sample per pond to assess their hydrogeochemical properties. There is a lower amount of seepage water from the tailings ponds owing to the high permeability of the tailings ponds and the valley’s hot and dry environment. To accurately reflect the chemical weathering processes within tailings ponds, filtrate sampling sites were strategically positioned in the starter dyke. This location ensures extended water-tailings contact, providing valuable insights for research purposes. For comparative purposes, background samples were collected at the upstream in the Wanniangou tailings pond, situated in an isolated valley with dense vegetation. This site, free from mining and agricultural disturbances, represents a natural weathering process. Meanwhile, the contemporaneous hydrogeochemical characteristics of the local river system (Anning River Basin) have been extensively documented in the existing literature [45].

3.2. Analysis

Tailings leachate samples were filtered (syringe, nylon needle filter, 0.45 μm) and temporarily stored in polyethylene bottles with air-tight caps at 4 °C to minimize CO₂ leakage and biological activity. The temperature, electric conductance, and Total Dissolved Solids (TDS) were measured in the field work. The major cationic concentrations of the filtrates (e.g., Ca²⁺, Mg²⁺, Na⁺, K⁺, and Sr²⁺) were measured using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) with a precision of 5% (Chengdu University of Technology, China). Anionic concentrations (Cl⁻, NO₃⁻, and SO₄²⁻) were determined using a Dionex DX-120 Ionic Chromatograph (IC, Thermo Scientific^TM, Waltham, MA, USA) with a precision of 5% (Chengdu University of Technology, China). The alkalinity of the leachate was determined in the field shortly after sampling, performing the Gran titration method [46]. The Dissolved Inorganic Carbon (DIC), also known as ΣCO₂, is composed of four dominant species: dissolved carbon dioxide (CO₂), carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻) [47]. Within a neutral environment, alkalinity values were observed to be nearly equivalent to those of DIC, with a variation of approximately 5%.

The δ¹³C_DIC-VPDB values of the leachate samples were determined using a Finnigan DeltaPlus XL mass spectrometer in the continuous flow mode with a gas bench device and an A200s auto-sampler (Thermo Finnigan LLC, San Jose, CA, USA). The analytical precision was 0.2‰, estimated from internal standards (Chengdu University of Technology, China). The δ³⁴S_V-CDT values were measured using an Elemental Analyzer and Isotope Ratio Mass Spectrometer (EA-IRMS) at the Iso-Analytical Laboratory of the Beijing Research Institute of Uranium Geology (BRIUG), China. The resulting SO₂ gas is passed through a GC column and measured in the mass spectrometer. Analytical precision calculated from the standards is better than ±0.3‰ (1σ). The water sample ⁸⁷Sr/⁸⁶Sr isotopic compositions were measured using a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) on a Nu Plasma Instrument after chromatographic purification using a cationic exchange resin. Analyses were referenced against NBS SRM 987 (0.710250), with long-term averages of 0.710246 and 0.710240 and 2–σ internal precisions of 0.000016 and 0.000014 on the Sector 54 and IsoProbe, respectively (Chengdu University of Technology, China).

4. Results

4.1. Major Ions

The tailings seepage exhibited a temperature range of 16.4 °C to 25.8 °C, averaging 21.9 °C. Collectively, the water was circumneutral, ranging from pH 7.1 to 8.2 (arithmetic mean 7.6 ± 2.3). However, the pH was significantly lower (5.8) in the Yuantong tailings pond. The TDS in the filtrates exhibited significant variability, ranging from 240 to 2059 mg/L (arithmetic mean 847 ± 481 mg/L). Correspondingly, the ionic concentrations in the filtrates varied greatly among tailings ponds. The cationic concentrations in the filtrates were in the order of Ca²⁺ (1174–17,484 μmol/L, arithmetic mean 5823 ± 3875 μmol/L) > Mg²⁺ (1186–16,079 μmol/L, arithmetic mean 5170 ± 4003 μmol/L) > Na⁺ (208–5862 μmol/L, arithmetic mean 1344 ± 1172 μmol/L) > K⁺ (8–1916 μmol/L, arithmetic mean 331 ± 489 μmol/L). The Ca²⁺ and Mg²⁺ accounted for 79%–95% (arithmetic mean 87%) of the total cations in the filtrates, and the mean [Mg²⁺/Ca²⁺] molar ratio was 0.92, which revealed that the cationic budgets in the filtrates were alternately liberated from dolomite and/or Mg-silicates [48,49]. Although dissolved Fe is present in significant quantities within the solid tailings waste, its occurrence in the filtrates was found to be minimal [37]. The SO₄²⁻ showed a high concentration in the filtrates (621 to 34,351 μmol/L, arithmetic mean 8616 ± 9429 μmol/L), and other anions including HCO₃⁻ (981 to 8390 μmol/L, arithmetic mean 4267 ± 1862 μmol/L), Cl⁻ (157 to 5494 μmol/L, arithmetic mean 869 ± 1075 μmol/L), and NO₃⁻ (0 to 124 μmol/L, arithmetic mean 75 ± 27 μmol/L). The [HCO₃⁻/SO₄²⁻] molar ratio ranged from 0.04 to 13.1, with a mean value of 1.9. The hydrogeochemistry of the rivulet was lower than that of the tailings filtrates. The TDS is 166 mg/L, along with the lower level of cations (mean Ca²⁺, Mg²⁺, Na⁺, and K⁺ were 1057, 673, 130 and 54 μmol/L, respectively), and anions (mean HCO₃⁻, SO₄²⁻, Cl⁻, and NO₃⁻ were 3328, 115, 66, and 21 μmol/L, respectively). The HCO₃⁻ concentration was significantly higher than SO₄²⁻, and the molar ratio [HCO₃⁻/SO₄²⁻] was ~28.6 (Table 1).

The dissolved ions in the tailings filtrates exhibited significant interactions. The correlation between SO₄²⁻ and cations (e.g., Ca²⁺, Mg²⁺, Na⁺, and K⁺) was positive, and the correlation coefficient sequence was Mg²⁺ (0.96) > Ca²⁺ (0.92) > K⁺ (0.82) > Na⁺ (0.68). These synergistic variations suggest that sulphide oxidation in the tailings is likely to be the major factor affecting the chemical weathering of the tailings (

Table 2. Pearson correlation coefficients.

	Cessation	Occupied Land	Volume	Temperature	TDS	pH	Ca2+	Mg2+	Na+	K+	Sr2+	Fe	SO42−	HCO3−	Cl−	NO3−
Closure	1.00
Occupied land	−0.26	1.00
Volume	−0.26	0.41	1.00
Temperature	−0.43	0.06	0.39	1.00
TDS	−0.46	0.31	0.23	0.54	1.00
pH	−0.15	0.32	0.35	0.05	−0.04	1.00
Ca2+	−0.39	0.32	0.17	0.50	0.93	0.07	1.00
Mg2+	−0.35	0.31	0.28	0.41	0.86	−0.20	0.85	1.00
Na+	−0.34	0.41	0.38	0.60	0.72	0.27	0.69	0.58	1.00
K+	−0.30	0.14	0.15	0.39	0.78	0.07	0.87	0.81	0.46	1.00
Sr2+	−0.35	0.47	0.19	0.38	0.84	0.19	0.90	0.79	0.63	0.79	1.00
Fe	0.19	0.25	−0.15	−0.40	−0.11	−0.02	−0.08	−0.07	−0.09	−0.16	0.08	1.00
SO42−	−0.37	0.39	0.30	0.51	0.90	−0.07	0.92	0.96	0.68	0.82	0.86	−0.06	1.00
HCO3−	0.34	−0.10	−0.36	0.15	−0.10	0.06	−0.01	−0.19	−0.17	0.11	0.05	−0.03	−0.16	1.00
Cl−	−0.42	0.42	0.61	0.51	0.64	0.26	0.52	0.51	0.87	0.31	0.48	−0.07	0.53	−0.30	1.00
NO3−	−0.07	0.04	0.06	0.37	0.07	−0.07	0.05	0.04	0.14	−0.02	0.01	0.14	0.12	0.22	0.08	1

). However, SO₄²⁻ and HCO₃⁻ were weakly negatively correlated (−0.16). Analysis revealed that NO₃⁻ concentrations remained relatively stable across the tailings ponds. Furthermore, no statistically significant correlation was observed between NO₃⁻ and any of the other measured parameters (Table 2, Figure 2). A weak positive correlation between NO₃⁻ concentrations and temperature (r = 0.37) was identified. This finding suggests that external inputs, rather than the chemical weathering processes within the tailings waste, likely constitute the primary source of NO₃⁻.

4.2. Temporal Weathering Decline

The weathering rate of the tailings particles was significantly accelerated in mine tailings where rock crushing occurred (

Table 3. Weathering loads of mine waste and river basins.

Sampling Site Description	Mean TDS (mg/L)	R	References
V-Ti-Fe mine tailings filtrates, Panxi region, China	847.4	1	Present study
Background, Panxi region, China	166.3	5.1	Present study
Emeishan basalt, Yunnan province, China	43.6	19.4	[54]
Anning River, Panxi region, China	95.9	8.8	[45]
Yangtze River, China	224.4	3.8	[51]
Global river average	69.0	12.3	[50]

R equal to the TDS of V-Ti-Fe mine tailings filtrates divided by other waters.

). The particle size of the Panxi V-Ti-Fe tailings waste ranged from 0.01 μm to 551.94 μm (arithmetic mean 63.18 μm) [37]. Consequently, the TDS of tailings filtrates are of much greater magnitude (8.8-, 3.8-, and 12.3-fold) than those of the Anning, Changjiang, and global average rivers, respectively [45,50,51,52,53], and 5.1-fold upstream and 19.4-fold measured upstream of the Emeishan basalt rocks [54].

Nevertheless, the correlation coefficient between the closure time and the ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, Sr²⁺, SO₄²⁻_, and Cl⁻) exhibited a moderately negative correlation (less than −0.30; Table 2). Meanwhile, the TDS and most ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄²⁻ and Cl⁻, excluding NO₃⁻ and HCO₃⁻) in the filtrates showed a decline from the operating to suspended tailings ponds (Figure 2). Average filtrate TDS decreased from 1228 mg/L during operation to 779, 694, and finally 593 mg/L for one, two, and more than three years of closure in the tailings ponds, respectively. Likewise, average SO₄²⁻ concentrations declined from 14,514, 7686, and 6052 μmol/L to 4763 μmol/L, respectively (Table 1).

4.3. Isotopic Geochemistry

In Table 1, the Sr²⁺ concentrations in the tailings pond filtrates range from 1 to 28 μmol/L, averaging 11 μmol/L, which is significantly higher than the Anning River (mean 1.1 μmol/L) [14]. The ⁸⁷Sr/⁸⁶Sr ratios ranged from 0.7056 (Majiatian tailings pond, operating) to 0.7162 (Detian tailings pond, closed), with a mean value of 0.7091. This variation was comparable to the nearby Yalong River system, the mainstem of the Anning River, where ⁸⁷Sr/⁸⁶Sr ratios ranged from 0.7074 to 0.7198 [14].

The δ¹³C isotopic ratios ranged from −17.19 per-mil (Heigutian tailings pond, closed) to −5.48 per-mil (VPDB, Wanniangou tailings pond, operating), with a mean value of −10.72. Additionally, the δ³⁴S_V-CDT of the leachates predominantly ranged from 0.3 per-mil to 6.8 per-mil (mean 4.0 per-mil), except for the Detian tailings pond, which had a higher δ³⁴S_V-CDT (11.3 per-mil), similar to that of the Emeishan basalt [55].

5. Discussion

5.1. Sulphide Oxidation

5.1.1. Provenance of SO₄²⁻

The predominance of minor pyrrhotite (FeS1-1.18) and pentlandite (Fe(Ni,Fe)₈S₈) in non-oxidized Panxi V-Ti-Fe mine tailings (Fe(Ni,Fe)₈S₈) [37,56], coupled with the exclusive use of gravity and magnetic separation in mineral dressing [37], suggests that sulphide oxidation plays a crucial role in controlling the chemical weathering of these tailings. This conclusion is further supported by additional observations:

(1) An antagonistic relationship exists between SO₄²⁻ and HCO₃⁻. Generally, the oxidation of sulphide generates sulphuric acid, and the H⁺ ions can be neutralised by HCO₃⁻, leading to a decrease in HCO₃⁻ concentration in the filtrates. In Table 2, SO₄²⁻ in the filtrates shows a synergistic variation with cations but a negative correlation with HCO₃⁻.

(2) Sulphide-trapped minerals (olivine, pyroxenes, ilmenite, and garnet) or rocks (basalt glasses, peridotites, eclogites, and kimberlites) exhibit a wide range of δ³⁴S values, from −4.9 ± 1 per-mil to +8 ± 1 per-mil [57]. Concurrently, basaltic liquid δ³⁴S values are estimated to range from +0.3 to +1.6 per-mil [58]. The V-Ti-Fe mine in the Panxi region is associated with the Emeishan basic magma intrusion [59], and the δ³⁴S of the Emeishan basaltic rocks range from +0.4 to +7.0 per-mil [55]. Notably, the δ³⁴S value of the V-Ti-Fe mine waste filtrates ranges from +0.3 to +6.8 per-mil (mean 4.0 per-mil, except for the Detian tailings pond), revealing that the sulphide in the V-Ti-Fe mine waste and the Emeishan basalt is quite homogeneous. These findings collectively support the hypothesis that sulphide oxidation significantly influences the chemical weathering of tailings wastes.

5.1.2. Sulphide Depletion in the Tailings Ponds

The observed enrichment of δ³⁴S in the filtrates provides evidence for a substantial depletion of sulphide oxidation within the tailings pond environment. The δ³⁴S of SO₄²⁻ in water represents a mixture of S derived from rock, soil, and atmosphere. The heavier δ³⁴S in the water (ranging from +4 to +14 per-mil) results from SO₄²⁻ reduction or bacterial S reduction [47,60]. Alternatively, heavier δ³⁴S can also be attributed to the sulphate-derived SO₄²⁻ dissolution [61]. In terms of the tailings filtrates, the δ³⁴S values of most ceased ponds’ filtrates were identical to those of the operating tailings ponds, with a conservative δ³⁴S value of less than 6.8 per-mil, demonstrating the sulphide oxidation of mantle-derived ultramafic and mafic rock [55,57,58].

The Detian mine tailings, inactive for a period of five years, exhibit a significantly elevated δ³⁴S value of 11.3‰. This finding suggests that the observed SO₄²⁻ may originate from either sulphate reduction processes or other sources of sulphate within the environment. The scarcity of organic matter (vegetation) within the tailings limits the availability of oxygen-consuming organic decay, making sulphate reduction unlikely. On the other hand, the SO₄²⁻ in the filtrates with higher δ³⁴S may indicate a depletion of sulphide in the tailings ponds and an elevated portion of the upstream-derived S mixture. For example, the Detian pond filtrates disclose the lowest SO₄²⁻ concentration among the ponds (621 μmol/L) with approximately one fifth coming from an external input in the tailings pond (~115 μmol/L of SO₄²⁻ flux in the upstream and ~122 μmol/L in the Anning River [45]). Meanwhile, the diagrams of the molar ratios [HCO₃⁻/Na⁺] and [Ca²⁺/Na⁺] depict the trend of end member evaporation (Figure 3a). The Anning River has a high proportion of sulphate from gypsum inputs (63%) [16]. Based on these observations, we infer that the Detian tailings filtrates could mix a higher portion of external SO₄²⁻ (e.g., sulphate) than other ponds.

5.2. Carbonate Weathering

5.2.1. Provenance of Carbonates

The ⁸⁷Sr/⁸⁶Sr ratios of the filtrates (mean 0.7091) were consistent with modern marine carbonates (0.708–0.709) [62] and distinctly higher than the Panxi intrusion (0.7043–0.7055) [63,64]. This suggests that the Sr isotopes were derived from carbonates rather than magmatic silicates. The genesis of carbonates in the Panxi V-Ti-Fe deposit is linked to the intrusion of Emeishan mafic magma into dolomites from Neoproterozoic Sinian formations in the Sichuan Basin [65,66,67,68]. Large contact aureoles, mostly composed of brucite marbles, calc-silicate rocks, and skarns, develop at the contact of the intrusions [67]. Consequently, calcite (~4.2 vol. %) and dolomite (~1.5 vol. %) have been identified as constituents of the tailings waste [37]. Sulphuric acid preferentially dissolves carbonates owing to their dissolution kinetic potential [69]. Calcite dissolution is directly proportional to H⁺ activity. Dolomite dissolution is relatively inert compared to calcite dissolution, controlled by parallel reactions involving H⁺, H₂O, and H₂CO₃ [70,71,72]. Accordingly, the dissolution of Ca-Mg-Fe-carbonate minerals occurs in the sequence of calcite, dolomite, ankerite, and siderite [73,74].

In this study, the dolomite dissolution was likely the main contributor to the filtrates. The weathering reaction within near-neutral pH conditions for the tailings waste will proceed most readily for calcite and dolomite and least readily for magnesite [75], which is nearly equivalent to the tailings filtrates in the present study (pH range 5.8–8.2). On the other hand, a [Mg²⁺/Ca²⁺] ratio ranging from 0.01 to 0.26 represents limestone dissolution, and a ratio greater than ~0.80 demonstrates dolomite dissolution [48,49]. The average [Mg²⁺/Ca²⁺] ratio in the filtrates is 0.92 (ranging from 0.38 to 2.55, 77% are larger than 0.8), indicating dolomite dominates the dissolution in the tailings. With rare exceptions, the [Mg²⁺/Ca²⁺] ratios of the Jintai and Jinlong tailings ponds were 0.38 and 0.42, respectively, suggesting both calcite and dolomite dissolution occurred in the operating tailings pond. These two ponds, characterized by elevated TDS values of approximately 1256 mg/L, demonstrated a higher dissolution load compared to the average TDS of approximately 818 mg/L observed in the remaining tailings ponds.

5.2.2. Deacidification of Tailings Ponds

Residual carbonates in the tailings ponds are vital for maintaining a neutral hydrogeochemical environment. In general, one mole of H₂SO₄ is consumed in an acid–base reaction to produce two moles of HCO₃⁻. An [HCO₃⁻/SO₄²⁻] molar ratio larger than two reveals that excessive bicarbonate can buffer the sulphuric acid in the filtrates.

In this study, the average [HCO₃⁻/SO₄²⁻] ratio ranged from 0.04 to 13.1, with 10 of 30 tailings ponds (~33.3%) having a ratio greater than two (e.g., Hongfa, Tianlong, and Detian). In particular, the [HCO₃⁻/SO₄²⁻] ratios of the ceased tailings ponds averaged ~2.5, proving sufficient bicarbonate present to buffer H₂SO₄. The [HCO₃⁻/SO₄²⁻] ratio of the operating tailings ponds was approximately 0.66, and the pH of the filtrates was close to neutral. This is attributed to CO₂ emissions (see Section 5.4). Therefore, such near-natural pH conditions (deacidification of H₂SO₄) are environmentally friendly to the ecosystem as they retard toxic metals [76,77]. In particular, the pH value of the Yuantong tailings pond (operating) was 5.79, which was below the average level of 7.6. The [HCO₃⁻/SO₄²⁻]_Yuantong ratio of 0.04 is significantly lower than two. These findings indicate that the pond is acidic and cannot neutralise acids. Engineering treatment is necessary to reduce the acidic load and to monitor environmental degradation.

5.3. Silicate Weathering

In this study, we assumed that the remaining cations were sourced from silicate weathering rather than from sulphate oxidation or carbonate dissolution. The reason is (1) the molar ratios of [HCO₃⁻/Na⁺] to [Ca²⁺/Na⁺] and [Mg²⁺/Na⁺] to [Ca²⁺/Na⁺] indicated that the weathering process involves a combination of carbonate and silicate rocks (Figure 3a,b). (2) The K⁺ in river water is primarily derived from silicate weathering [78]. In this study, the K⁺ is relatively abundant in the filtrates (mean 330.8 μmol/L), which compares with the upstream (mean 54.0 μmol/L) and Anning River (mean 52.19 μmol/L) [45]. Hence, elevated K⁺ concentrations in the filtrates may be attributed to silicate weathering, given the presence of hornblende in the tailings [37]. (3) A molar ratio of [Na⁺/Cl⁻] equal to one suggests anthropogenic input. Excessive Na⁺ in water is linked to the chemical weathering of silicates [79]. The [Na⁺/Cl⁻] ratio of filtrates ranged from 0.47 to 4.79 (mean 2.06, the ratio of three tailings ponds less than one, including Wanniangou, Jintai, and Sandaoguai tailings ponds). This evidence suggests that silicate weathering contributes to the water–rock interactions within the tailings environment.

The solute flux of cations derived from silicate weathering was determined by correcting the background [13]. It was assumed that K⁺ originated from both external and silicate sources. The regional silicate Na⁺/K⁺ ratio was ~1.2 [14]. The background K⁺ concentration in this study was 54.0 μmol/L, similar to the average concentration in the Anning River (52.43 μmol/L) [45]. Once the silicate weathering of Na⁺ was calculated, it was possible to determine the other cations produced from the ratios of these cations to Na⁺ in the silicate rocks. The [Ca²⁺/Na⁺] ratio of the Emeishan basalt was reported as 1.87 [80], but this may be due to a mixture of carbonate and silicate dissolution. In contrast, the representative [Ca²⁺/Na⁺] ratio of silicate weathering was equal to 0.55, South Han River, South Korea [15] and 0.5 in the Skeena and Nass Rivers, Canada [13]. Therefore, 0.5 was the representative ratio for silicate weathering of V-Ti-Fe mine tailings. For the [Mg²⁺/Na⁺] ratio of silicate weathering, 0.4 was adopted as a conservative estimate [12], representing volcanic and volcanic-derived rock weathering (associated with basalts, andesites, and granodiorites) and concurrent with Emeishan mafic rock weathering [80].

According to the extrapolation described above, it is estimated that ~15.7% (ranging from 3.4% to 45.2%) of the cationic flux was sourced from exotic inputs, likely linked to the Anning River water. Atmospheric inputs were excluded from consideration as the sampling was conducted during sunny days without precipitation, and atmospheric input calculated into river inputs. Although sulphuric acid preferentially dissolves dolomite, silicate weathering is also very active in pristine tailings. Approximately 5.1% (ranging from 0.2% to 19.2%) of the cationic flux originated from silicate weathering. This contribution varied between active and ceased tailings ponds, with active ponds contributing approximately 8.7% and ceased ponds contributing approximately 3.6%. Carbonate weathering accounted for approximately 79.2% (from 53.9% to 90.1%, Figure 4) of the cationic input to the filtrates, consistent with the discussion in Section 5.2. For instance, in Figure 3 and Figure 4, the characteristics of Baicao tailings pond is consistent, with high Ca²⁺, Mg²⁺ and bicarbonate levels, indicating weathering of carbonate rocks.

The accumulation of secondary minerals prevents reactivity between primary minerals and water, consequently decelerating the dissolution reactions involving primary minerals [81,82]. The acidity of the tailings leachate is typically circumneutral, and the oxygenic oxidation of Fe(II) ions is accelerated, leading to the formation of Fe(III) precipitates (e.g., hydroxide coating) [83]. For instance, secondary melanterite, rozenite, gypsum, jarosite, and goethite can be reprecipitated at a 1 m depth in the tailings pond [84]. In Table 1, the SI of the FeOOH (mean 3.42) and Fe₂O₃ (7.82) are significantly larger than zero, indicating the dissolved iron is prone to precipitation in the tailings pond. As a result, the multi-layered phases passivated the tailings particles against further reactions and dissolution. Conversely, a modest increase in HCO₃⁻ concentration was observed within the filtrates, accompanied by a moderate positive correlation (0.34) between HCO₃⁻ and closure time. These observations suggest that the relative contribution of carbonic weathering processes may have increased with the decreased availability of sulfuric acid within the tailings.

5.4. Transitions of CO₂ Emissions to Sequestration

5.4.1. CO₂ Emissions

Equations (1) and (2) depict the complete neutralisation reaction of sulphuric-carbonate weathering (SCW). Importantly, this process leads to the production of one mole of H₂CO₃ per mole of sulfuric acid, thereby contributing to CO₂ emissions. This results in a [HCO₃⁻]/[Ca²⁺+Mg²⁺] ratio of zero and a [SO₄²⁻]/[Ca²⁺+Mg²⁺] ratio of one.

$$2{\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4+{2(\mathrm{C}\mathrm{a},\mathrm{M}\mathrm{g})\mathrm{C}\mathrm{O}}_3\to {\mathrm{C}\mathrm{a}}^{2+}+{\mathrm{M}\mathrm{g}}^{2+}+{2\mathrm{H}}_2{\mathrm{C}\mathrm{O}}_3+2{\mathrm{S}\mathrm{O}}_4^{2-}$$

(1)

$${\mathrm{H}}_2{\mathrm{C}\mathrm{O}}_3\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{C}\mathrm{O}}_2\uparrow$$

(2)

In neutral and unsaturated environments, DIC predominantly exists in the form of bicarbonate, and the ratios of [HCO₃⁻]/[Ca²⁺+Mg²⁺] and [SO₄²⁻]/[Ca²⁺+Mg²⁺] approach ~1 and ~0.5, respectively (Equation (3)).

$${\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4+{2(\mathrm{C}\mathrm{a},\mathrm{M}\mathrm{g})\mathrm{C}\mathrm{O}}_3\to {\mathrm{C}\mathrm{a}}^{2+}+{\mathrm{M}\mathrm{g}}^{2+}+{2\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{S}\mathrm{O}}_4^{2-}$$

(3)

Under the condition of sulphide depletion, carbonic-carbonate weathering (CCW) leads the water–rock interaction, characterised by [HCO₃⁻]/[Ca²⁺+Mg²⁺] and [SO₄²⁻]/[Ca²⁺+Mg²⁺] ratios of two and zero, respectively (Equation (4)) [79].

$${{(\mathrm{C}\mathrm{a},\mathrm{M}\mathrm{g})\mathrm{C}\mathrm{O}}_3+\mathrm{H}}_2{\mathrm{C}\mathrm{O}}_3\to 0.5{\mathrm{C}\mathrm{a}}^{2+}+{0.5\mathrm{M}\mathrm{g}}^{2+}+{2\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$$

(4)

In the present study, the initial molar ratio of [HCO₃⁻]/[Ca²⁺+Mg²⁺] ranges from 0.04 to 1.96 (mean 0.60), and it declines to 0.44 (mean) after correcting external input [45]. The [HCO₃⁻]/[Ca²⁺+Mg²⁺] ratio of operating tailings ponds is 0.19 (mean), and it elevates to 0.55 (mean) for the closed tailings ponds. Conversely, the initial [SO₄²⁻]/[Ca²⁺+Mg²⁺] ratio ranges from 0.15 to 1.14 (mean 0.61), and it elevates to 0.66 (mean) after correcting surface water input. In particular, the [SO₄²⁻]/[Ca²⁺+Mg²⁺] ratio of 13 tailings ponds ranges from 0.9 to 1.1 (corrected ratio, average 1.06), which reflects a complete neutralisation and the presence of CO₂ emissions (Equations (1) and (2)). For instance, the [SO₄²⁻]/[Ca²⁺+Mg²⁺] ratio in the Wanniangou tailings pond was 1.05. The HCO₃⁻ concentration declined from 3328 μmol/L (background water) to 1681.5 μmol/L (filtrates), showing a net CO₂ release in the filtrates.

Validation of the δ¹³C values reveals a range of −9.6 per-mil to 1.3 per-mil, with a mean of −2.5 per-mil, for the Panxi V-Ti-Fe ore [66]. These values coincide with those of the surrounding carbonate bodies (ranging from 1.58 to 2.63 per-mil) [66,85]. In the filtrates, the δ¹³C_DIC appears to span a wider range of −17.19 to −5.48, which is parallel to most groundwater and surface water (from −15 to −5 per-mil) [86]. Therefore, a mixture of upstream and CO₂-emissions modified carbon isotopes was observed in the filtrates, and the sulphide-derived acid caused the release of dissolved carbon.

5.4.2. Sequestration of CO₂ in Tailings Ponds

The SO₄²⁻ budget in the filtrates are equivalent to end-members of SCW, sulphide-silicate weathering (SSW), and the exotic sulphates input (ES, Equation (5)):

$${\left[{SO}_4\right]}_{Filtrate}={\left[{SO}_4\right]}_{scw}+{\left[{SO}_4\right]}_{SSW}+{\left[{SO}_4\right]}_{ES}$$

(5)

The CO₂ emissions budget is the sum of the DIC of SCW and ES minus the DIC in the filtrates (Table 1, HCO₃⁻ ≈ DIC in a neutral pH ambient):

$${\left[{CO}_2\right]}_{Degass}=2\times {\left[{SO}_4\right]}_{SCW}+{\left[{HCO}_3\right]}_{ES}{-\left[{HCO}_3\right]}_{Filtrates}$$

(6)

The annual CO₂ emissions (F) can be expressed as:

$$F=\sum h\times s\times c$$

(7)

where h is the local annual meteoric precipitation, 849.4 mm/year [41]; s is the land occupation of the tailings ponds (Table 1, 298.2 km²); and c is the ${\left[{CO}_2\right]}_{Degass}$ (Equation (6), μmol/L). The quantity of CO₂ emissions from the tailings ponds can be calculated from Equations (5)–(7). Our analysis indicates that 16 of the 30 tailings ponds, encompassing eight operating ponds and eight ceased ponds, are likely to emit CO₂, accounting for ~238,743 tons/year. The remaining 14 tailings ponds, comprising one operating pond (Hongfa) and 13 ceased ponds (e.g., Detian and Qianfan), demonstrated a capacity to absorb atmospheric CO₂ at an estimated rate of approximately 1508 tons per year. The net CO₂ emissions from all tailings ponds were determined to be approximately 237,235 tons per year (Table 1).

5.4.3. CO₂ Transition from Emissions to Sequestration

Table 1 shows that tailings ponds exhibit CO₂ absorption with a closure time ranging from 101 to 1850 days, averaging 731 days (excluding the Hongfa tailings pond). This suggests that V-Ti-Fe tailings ponds may begin to act as net absorbers of atmospheric CO₂ after approximately two years of closure. Nevertheless, this is an arbitrary estimate, as the CO₂ transition from emission to sequestration largely depends on the geochemical conditions of the tailings waste. Our analysis revealed key factors influencing CO₂ emissions and absorption in the tailings ponds (Figure 5):

TDS contents: TDS values ranging from 698 to 2059 mg/L were generally associated with CO₂ emissions, while CO₂ absorption was observed within the TDS range of 240 to 719 mg/L.

Sulfate (SO₄²⁻) concentration: Higher SO₄²⁻ concentrations (3010 to 34,351 μmol/L) were associated with CO₂ emissions, while CO₂ absorption was observed under conditions of decelerated sulfide oxidation, with SO₄²⁻ concentrations ranging from 621 to 3068 μmol/L. Notably, SO₄²⁻ loads below approximately 3000 μmol/L were generally associated with CO₂ absorption.

[HCO₃⁻]/[SO₄²⁻] ratio: An initial molar [HCO₃⁻]/[SO₄²⁻] ratio exceeding 1.299 was indicative of CO₂ absorption, whereas ratios below 1.294 were associated with CO₂ emissions.

Therefore, a TDS of less than ~700 mg/L could help determine the CO₂ absorption of V-Ti-Fe mine waste. We believe a portable TDS meter is an economical and simple way to judge the CO₂ transition from emissions to sequestration during fieldwork. Overall, the reference values (SO₄²⁻ ≈ 3000 μmol/L, [HCO₃⁻/SO₄²⁻] ≈ 1.299, and TDS ≈ 700 mg/L) represent the transition episodes from CO₂ emissions to sequestration.

6. Conclusions

The chemical weathering of ultramafic and mafic mine tailings was hypothesised to facilitate the transition from CO₂ emissions to absorption over a shorter timescale. The study revealed the following key findings: (1) the dissolution load of mine tailings depended significantly on sulphide oxidation and the closure time of mine tailings. The depletion sequence of carbonate and sulphide in the V-Ti-Fe mine tailings waste could determine the deacidification and alkalinity of the tailings ponds, and further affect the remobilisation of ions within tailings ponds. (2) The geochemical indices around the critical value could represent the transition interval from CO₂ emissions to sequestration (closure time ≈ 2 years, SO₄²⁻ ≈ 3000 μmol/L, initial [HCO₃⁻/SO₄²⁻] ≈ 1.29, and TDS ≈ 700 mg/L). (3) A portable TDS meter (TDS < 700 mg/L) is an economical and simple reference tool with which to infer the CO₂ absorption of V-Ti-Fe mine waste.

Intensive sulphide oxidation often occurs at fresh interfaces (e.g., fresh mine tailings). Currently, the anthropogenic transformation of the Earth’s surface (e.g., agriculture, construction, and mining) has added new interfaces for sulphate release, and the impact of these activities should be considered in further studies. Meanwhile, this study presents a basis for shortened monitoring timescales of weathering processes of ultramafic and mafic rocks. Although this study was conducted on the V-Ti-Fe mine waste and specific environments of the Panxi Valley, China, it highlights the further opportunities to examine anthropogenic materials as proxies for shortening monitoring timescales.