This study was conducted to determine the uptake of trace elements by the emergent wetland plant species Eleocharis equisetina at the historic Jumna tin processing plant, tropical Australia. The perennial emergent sedge was found growing in acid waters (pH 2.45) and metal-rich tailings (SnAsCuPbZn). E. equisetina displayed a pronounced acid tolerance and tendency to exclude environmentally significant elements (Al, As, Cd, Ce, Co, Cu, Fe, La, Ni, Pb, Se, Th, U, Y, Zn) from its above-substrate biomass. This study demonstrates that geobotanical and biogeochemical examinations of wetland plants at abandoned mined lands of tropical areas can reveal pioneering, metal-excluding macrophytes. Such aquatic macrophytes are of potential use in the remediation of acid mine waters and sulfidic tailings and the reclamation of disturbed acid sulfate soils in subtropical and tropical regions.

Sundarban along with its networks of rivers, creeks and magnificent mangroves form a unique ecosystem. Acid sulphate soils have developed in this ecosystem under anoxic reducing conditions. In the present study, we have investigated the distribution of acid sulphate soils along with its elemental characterization and possible sources in four reclaimed islands of Indian Sundarban like Maushuni (I1), Canning (I2), Bally (I3) and Kumirmari (I4). Elements show moderate to strong correlation with each other (P < 0.01; P < 0.05). Except Si, Ca and Pb, a higher enrichment factor was observed for K, Cr, Mn, Fe, Ni, Cu and Zn. Geo-accumulation index values of all sampling locations reveal that Cr, Fe, Cu and Zn are in Igₑₒ class 1. The pollution load index value of the reclaimed islands of Indian Sundarban varies between 1.31 and 1.48. The observation of this study could help to strategize policies to mitigate and manage acid sulphate soils in Indian Sundarban.

The principles of limestone drain systems that are commonly used to passively remediate acid rock drainage have been adapted and modified for remediation of acidic and metal-rich drainage that is produced from broad scale agricultural land use of regions underlain by Acid Sulfate Soils (ASS). The acidic drainage water from sugar cane fields in an ASS catchment was collected from an open drain, filtered to reduce the transport of large particulates, and passed vertically through a polyethylene tank, which was filled with limestone aggregates (<75 mm). This Closed Tank Reactor (CTR) uses the principles of oxic and anoxic limestone drain systems that are designed to increase the partial pressure of carbon dioxide and thereby the alkalinity produced from the dissolution of limestone by metal-laden influent. During a non-continuous 70 day monitoring period, the discharge from the CTR had higher pH, lower acidity and lower metal concentrations compared to the inflow. Under average flow conditions (9 lpm), similar proportions of incoming dissolved aluminium and iron (61% and 56% respectively) were retained within the CTR. Two perforated pipes in the base of the CTR were used to flush precipitates from the system under rapid flow conditions (>50 lpm). The flushing was effective in removing approximately 10% of accumulated iron but only about 0.3% of accumulated aluminium from the CTR. Accumulation of aluminium inside the CTR is likely to present operational problems in attempts to apply such technology to many coastal acid sulfate soil drains.

The effects of soil processes, related to the oxidation of sulphide sediments, on heavy metal concentrations in the soil and soil solution were investigated in a Norway spruce stand on a fine-textured, acidic soil rich in sulphates located on the isostatic land-uplift western coast of Finland. The age of the soil is ca. 300–400 years, and the soil texture is silt and till. The chemical properties of the soil and soil solution clearly reflected the formation of acid sulphate (AS) soil. Compared to background reference values for podzolic coniferous forest soil, the pH of the soil solution in the mineral soil (20–40 cm depth) was very low, and the Al, Fe and S concentrations extremely high. The Zn and Ni concentrations in the soil solution were also strongly elevated, and similar to the concentrations reported close to anthropogenic heavy-metal emission sources. The concentrations of Cd and Cu were also frequently elevated. In contrast, the acidity and metal concentrations of the soil solution sampled in the organic layer were not elevated. Similarly, exchangeable Zn and Ni concentrations were also elevated in the mineral soil, but not in the organic layer. Because Norway spruce has a very superficial rooting system and the zone with exceptionally high metal concentrations did not extend up to the topmost soil layers, sulphide-oxidation derived soil acidification is not likely to pose a serious threat to forest ecosystems growing on this type of site. Despite the elevated concentrations of protons and many metals in the mineral soil and soil solution (20–40 cm), the nutrient status of the spruce stand was satisfactory and the general health of the stand has been reported to be relatively good.

Due to the episodic nature of rainfall and the high dissolved metal concentrations in the acid sulfate soil catchment of Clothiers Creek (NSW, Australia), active treatment was considered more appropriate than passive treatment. Alkaline reagents were added to oxidised shallow drains to remove acidities ranging from 438 to 1,837 mg/L CaCO₃. A fine limestone slurry was produced from the pounding together of limestone rock fragments within a rotating drum and, on addition to drain waters, was found to remove acidity to varying degrees of effectiveness (from 12 to 100%). The efficiency decreased as the pH of the water approached neutrality due to calcite saturation and the slow reaction rate of limestone at high pH. Hydrated lime powder was also mixed with drain water in the rotating drum though most mixing occurred once the slurry entered the drain where efficiencies ranging from 67 to 89% were observed. A powdered mixture of MgCO₃ and CaCO₃ was only 11% effective in treatment of the drainage water due to the slow rate of reaction of MgCO₃. Whilst the active treatment system was capable of treating a large acidity flux (particularly using hydrated lime) it required regular addition of reagent and the dosing of hydrated lime was particularly difficult to control. Future designs of this active treatment system should be automated to prevent adverse aquatic impacts due to overdosing.

There is currently no comprehensive acid sulfate soil (ASS) hazard mapping in Colombia. This study aims to create reliable prediction surfaces to estimate the types and subtypes of inland ASS, spatial variability of acidity and salinity, and acidification and salinization hazards. We used a combination of factor analysis, geostatistical tools with ordinary and indicator kriging, and Geographic Information System utilities to design a spatial prediction model. The studied variables were soil reaction, redox potential, and electrical conductivity at the A and B horizons obtained from a detailed systematic sampling in a sector of the Sinú River floodplain, Colombia. Two factors were identified; the first allowed us to delimit the homogeneous behavior of acidity as well as the types and subtypes of inland ASS precisely while the second facilitated the identification of ASS subtypes. Our findings indicate that 82% of the area reports very high and high to moderate acidification hazards in active ASS. A high salinization hazard exists in 26% of active ASS and 74% of both active and post-active ASS. These findings suggest serious acidification and salinization hazards and the need for urgent appropriate economic and environmental management. The approach applied here can be implemented at a field scale to improve understanding of the activity and behavior of ASS based on the acidity and salinity, which can facilitate a more reliable mapping of acidification and salinization hazards.

A 96-h laboratory experiment was conducted to assess the potential of biochar as a water protection tool for acid sulfate soil runoff. Acid sulfate soils pose a risk to water bodies due to acid, metal-rich runoff, especially in drained peatland forests. New water protection methods, such as adsorption with biochar, are needed. We investigated the capability of spruce and birch biochar to adsorb metals and reduce acidity in the water. Water from an acid sulfate site was stirred with biochar, biochar with lime, and biochar with ash. We determined water Al, S, Fe, Cu, Co, Cd, Ni, and Zn concentrations periodically, as well as pH and total organic carbon at the beginning and the end of the experiment. The studied substances are considered the most abundant and environmentally harmful elements in the acid sulfate soils in Finland. Biochar surface characteristics were analyzed with FTIR spectroscopy. Concentration changes were used to parametrize adsorption kinetics models. Biochar adsorbed metals and increased pH, but lime and ash additives did not always improve the adsorption. Spruce biochar and ash addition had generally higher adsorption than birch biochar and lime addition. The adsorption was dominated by Al and Fe at lower pH, while increasing pH improved the adsorption of Cd and Zn. The results show that biochar can increase the water pH, as well as adsorb Al, Fe, Co, Cd, Ni, and Zn. Further work could include an actual-scale biochar reactor in a laboratory and field conditions.

Aluminum (Al) toxicity in acidic soils is a major problem in rice crop production, especially in the acid sulfate soil (pH < 4.0). Selecting Al-tolerant varieties of rice with low toxicity is one of the most appropriate strategies to overcome this problem. In the present study, we investigated the Al content in different rice genotypes, IR64 (high yielding), RD35 (local acidic-tolerant), and Azucena (AZU, positive-check Al-tolerant), and their physiological and morphological adaptations under a wide range Al (10, 25, 50 mM [Al₂(SO₄)₃]) treatments in the greenhouse conditions. Under 50-mM Al treatment, Al levels in the root tissues of rice seedlings cvs. AZU and IR64 were increased by 2.74- and 2.10-fold over control. Interestingly, Al contents in the roots of cv. RD35 were also exhibited by 2.04-fold over control. Similarly, Al contents in the leaves trend to increase in relation to a degree of Al treatments, leading to increase leaf temperature, chlorophyll degradation, limited CO₂ assimilation, and negative effect on root traits under 50 mM Al were evidently observed. Therefore, leaf temperature was considered a sensitive parameter regulated by high concentration of Al (50 mM), leading to increase in crop water stress index (CWSI > 0.6) and decrease in stomata conductance. Net photosynthetic rate (Pₙ) and transpiration rate (E) in rice seedlings of cv. RD35 subjected to 50 mM Al were significantly dropped by 74.76% and 47.71% over the control, respectively, resulting in reduced growth performances in terms of root length (26.57% reduction) and shoot fresh weight (46.15% reduction). An enrichment of Al in the root tissues without toxicity in rice cv. AZU may further help in discovering the Al homeostasis. In summary, Al enrichment in rice genotypes grown under Al-treatments was evidently observed in the root, leading to the limited root growth, root length, and root dry weight, especially in cv. RD35. Al restriction in the root tissues of cv. AZU (Al-tolerant) may play a key role as defense mechanisms to avoid translocation to other organs and the stomata closure was an alternative key factor to limit H₂O transpiration.

An open limestone channel (OLC) was constructed within an existing drain to treat the acidic and metal-rich drainage waters generated from an acid sulfate soil (ASS) catchment. The OLC was constructed downstream of a catchment pump and it consisted of a series of ponds and limestone sections. The accumulation of sediment over the limestone, preventing contact of limestone with acidic water, was the greatest problem impacting the OLC in its first year of operation. The continuous or sporadic operation of the catchment pump (at 120 l/s) was not sufficient to flush sediment from the limestone. The accumulation of large amounts of sediment onto the limestone reduced the amount of alkalinity and calcium released into solution. However, if the sediment is removed by agitating the limestone then an equivalent or greater amount of alkalinity may be added to solution and more metals removed from solution compared to fresh limestone. The coating on the limestone had a high concentration of manganese oxides in addition to slightly lower concentrations of aluminium and iron. Removal of these metals from the water was due to the increase in pH produced by limestone dissolution in addition to sorption reactions of the existing coating which had natural microbial activity.

Drainage water from acid sulfate soils with sulfuric material has high concentrations of protons and dissolved metals which can have detrimental effects on the surrounding ecosystems. Liming is expensive; therefore, alternative methods are needed. Organic materials such as plant residues, compost or biochars can bind protons and metals but have not been evaluated with respect to remediation of acid drainage water from acid sulfate soils. In this study, eight organic materials (compost, two straws and five biochars differing in feed stock and production temperature) were placed in small PVC cores at 1.5 g C/core and synthetic acid drainage water (pH 3, 28 mg Fe/l and 2 mg Al/l, properties based on long-term averages of drainage water from sulfuric acid sulfate soils) was applied in four leaching events. Mallee biochar produced at 550 °C and wheat biochar produced at 450 °C had high retention capacity for protons, Fe and Al. Retention was low in compost and wheat straw. Retention of protons was positively correlated with organic C concentration of the materials. Retention of Fe and Al was correlated with percentage alkyl, aryl and ketone groups. Other properties such as release of native Fe and Al and amount of material per core could explain differences in ability of organic materials to retain protons, Fe and Al. We conclude that some organic materials such as mallee biochar produced at 550 °C and wheat biochar produced at 450 °C could be used to remediate acidic drainage water.