Wetland treatment at extremes of pH: a review

A comprehensive review on the treatment of pesticide-contaminated wastewater with special emphasis on organophosphate pesticides using constructed wetlands

Pesticides pose a significant threat to aquatic ecosystems due to their persistent nature and adverse effects on biota. The increased detection of pesticides in various water bodies has prompted research into their toxicological impacts and potential remediation strategies. However, addressing this issue requires the establishment of robust regulatory frameworks to determine safe thresholds for pesticide concentrations in water and the development of effective treatment methods. This assessment underscores the complex ecological risks associated with organophosphate pesticides (OPPs) and emphasizes the urgent need for strategic management and regulatory measures. This study presents a detailed examination of the global prevalence of OPPs and their potential adverse effects on aquatic and human life. A comprehensive risk assessment identifies azinphos-methyl, chlorpyrifos, and profenfos as posing considerable ecological hazard to fathead minnow, daphnia magna, and T. pyriformis. Additionally, this review explores the potential efficacy of constructed wetlands (CWs) as a sustainable approach for mitigating wastewater contamination by diverse pesticide compounds. Furthermore, the review assess the effectiveness of CWs for treating wastewater contaminated with pesticides by critically analyzing the removal mechanism and key factors. The study suggests that the optimal pH range for CWs is 6-8, with higher temperatures promoting microbial breakdown and lower temperatures enhancing pollutant removal through adsorption and sedimentation. The importance of wetland vegetation in promoting sorption, absorption, and degradation processes is emphasized. The study emphasizes the importance of hydraulic retention time (HRT) in designing, operating, and maintaining CWs for pesticide-contaminated water treatment. The removal efficiency of CWs ranges from 38% to 100%, depending on factors like pesticide type, substrate materials, reactor setup, and operating conditions.

Evaluating drained water quality in a pilot worm-sludge treatment reed bed planted with Arundo donnas in the Mediterranean climate

This study evaluated the impact of incorporating earthworms (Eisenia fetida) on the drained water quality from a sludge treatment reed bed. The experiment encompassed four setups of treatment beds in two replicates: planted with Arundo donax and addition of earthworms, planted without earthworms, unplanted with earthworms, and treatment bed without plants nor earthworms as control. The units were fed every two weeks with mixed sewage sludge, a blend of primary and secondary sludge over 24 cycles. The mixed sewage sludge had mean dry and volatile solid contents of 24.71 g.DS.L-1 (± 13.67) and 19.14 g.VS.L-1 (± 10.29) resulting a sludge loading rate of 43.59 kg.DS.m-2.year-1 (± 14.49). The inclusion of earthworms in the planted unit reduced release masses of total suspended solids, chemical oxygen demand, nitrate and phosphorous by 43, 45, 75 and 45 % compared to the planted unit. Plant biomass production increased by 43 % with the earthworm presence. The removal efficiency of the units improved after a ramp-up phase (after six months feeding) of which the concentration of TSS, COD and Escherichia coli met limits for water reuse while nitrogen components and phosphorous surpassed the limits. The planted unit with earthworms removed 99 and 99 % of TSS and COD, respectively. Overall, water loss namely through evapotranspiration and earthworm hydration need, positively correlated with pollutant concentration, and earthworm-planted unit had 46 % higher water loss compared to control unit.

In Situ Use of Mining Substrates for Wetland Construction: Results of a Pilot Experiment

This paper evaluates an experimental wetland as part of a pilot soil reclamation project in a mining area. The wetland was constructed using materials of mining origin from the area; most reactive materials of acid pH were stabilised using limestone filler. The study selected macrophytes that are tolerant to potentially toxic elements (PTEs) and resistant to salinity, namely Phragmites australis, Juncus effusus, and Iris pseudacorus. These macrophytes were then placed in pots containing substrates composed of different mixtures of topsoil, peat, and mining waste (black or yellow sand). A thorough analysis of the physicochemical and mineralogical characteristics of the materials included studies of PTE mobilisation. This study emphasises the significance of the rhizosphere in directing the transfer of PTEs to the plant and the correlation between the substrate and the development of plant defence mechanisms, such as the formation of Fe-plates. Scanning electron microscopy was used to highlight these aspects and validate the results of the analytical determinations. These wetlands can be proposed as a phytoremediation strategy for areas affected by mining and maritime influence. They are easy to construct and remain stable, providing important ecosystem services such as the natural attenuation of acid mine drainage, support for vegetation development and fauna, and a clean ecosystem.

Wetland Destruction in a Headwater River Leads to Disturbing Decline of In-stream Nitrogen Removal

Wetlands have long been recognized as efficient nitrogen (N) processing systems. While widespread interest is in constructing wetlands to mitigate N pollution, there is a dearth of information about the environmental consequences following wetland dismantlement. This study elucidated the changing trajectories of water quality and N removal capacity in a headwater river that initially contained a series of constructed wetlands but later underwent wetland destruction. An estimated 17% surge in total N concentration has been reported since the wetlands' destruction. This adverse trend is primarily attributed to a weakened in-stream N removal capacity, which was reduced to a mere 25% of the levels observed when the wetlands were operational. Further analysis confirms that the presence of wetlands actively shapes desirable environmental settings for N processing. In stark contrast, wetland destruction leads to unfavorable environmental conditions, which not only restrain in-stream anaerobic metabolisms but also trigger algal proliferation and biological N fixation. Collectively, this research provides compelling evidence of the detrimental consequences associated with wetland destruction, emphasizing the need for remedial strategies to mitigate these negative effects.

A review: The formation, prevention, and remediation of acid mine drainage

In abandoned open-pit coal mines, surface water and groundwater form acidic waters with high concentrations of metal ions due to chemical interactions with ores such as pyrite, and the formation of acid mine drainage (AMD) is one of the major sources of pollution of world concern. For this reason, this paper reviews the formation mechanisms and influencing factors of AMD. It also describes the prediction, prevention, and remediation techniques for AMD, identifying key research gaps. It also discusses the current challenges and shortcomings faced globally in the management of AMD. The formation of AMD is mainly caused by the oxidation of pyrite in mines, but it is mainly influenced by history, climate, topography, and hydrogeology, making the formation mechanism of AMD extremely complex. Currently, the remediation technologies for AMD mainly include active treatment and passive treatment, which can effectively neutralize acidic wastewater. However, the prediction technology for AMD is blank, and the source treatment technology such as passivation and microencapsulation only stays in the experimental stage. This leads to the high cost of treatment technologies at this stage and the inability to identify potential risks in mines. Overall, this review provides remediation tools for AMD from predicting root causes to treatment. Geophysical technology is an effective method for predicting the motion path and pollution surface of AMD in the future, and resource recovery for AMD is a key point that must be paid attention to in the future. Finally, integrated treatment technologies that deserve further exploration need to be emphasized.

Comparison study on enhancement of phosphorus recovery from low-strength wastewater treated with different magnesium-based electrochemical constructed wetland

Phosphorus (P) recovery from wastewaters treated with constructed wetlands (CWs) could alleviate the current global P crisis but has not received sufficient attention. In this study, P transformation in different magnesium-based electrochemical CWs, including micro-electrolysis CW (M-CW), primary battery CW (P-CW), and electrolysis CW (E-CW), was thoroughly examined. The results revealed that the P removal efficiency was 53.0%, 75.8%, and 61.9% in the M-CW, E-CW, and P-CW, respectively. P mass balance analysis showed that P electrode deposition was the main reason for the higher P removal in the E-CW and P-CW. Significant differences were found between the E-CW and P-CW, P was distributed primarily on the magnesium plate in the P-CW but was distributed on the carbon plate in the E-CW. The E-CW had excellent P recovery capacity, and struvite was the major P recovery product. More intense magnesium plate corrosion and alkaline environment increased struvite precipitation in the E-CW, with the proportion of 61.6%. The results of functional microbial community analysis revealed that the abundance of electroactive bacteria was positively correlated with the deposition of struvite. This study provided an essential reference for the targeted electrochemical regulation of electric field processes and microorganisms in CWs to enhance P recovery.

Subsurface flow constructed wetlands for treating of simulated cadmium ions-wastewater with presence of Canna indica and Typha domingensis

The presence of toxic cadmium ions in the wastewater resulted from industrial sector forms the critical issue for public health and ecosystem. This study determines the ability of four vertical subsurface flow constructed wetlands units in the treatment of simulated wastewater laden with cadmium ions. This was achieved through using sewage sludge byproduct as alternative for the traditional sand to be substrate for aforementioned units in order to satisfy the sustainable concepts; however, Canna indica and Typha domingensis can apply to enhance the cadmium removal. The performance of constructed wetlands has been evaluated through monitoring of the pH, dissolved oxygen (DO), temperature, and concentrations of cadmium (Cd) in the effluents for retention time (0.5-120 h) and metal concentration (5-40 mg/L). The results demonstrated that the Cd removal percentage was exceeded 82% beyond 5 days and for concentration of 5 mg/L; however, this percentage was decreased with smaller retention time and higher metal concentration. The Grau second-order kinetic model accurately simulated the measurements of effluent Cd concentrations as a function of retention times. The FT-IR analysis indicated the existence of certain functional groups capable of enhancing the Cd removal. The treated wastewater's pH, DO, temperature, total dissolved solids (TDS), and electrical conductivity (EC) all meet the requirements for irrigation water.

Development of a novel sizing approach for passive mine water treatment systems based on ferric iron sedimentation kinetics

The removal of dissolved and particulate iron (Fe) from contaminated mine drainage is an omnipresent challenge in, and legacy of, the mining industry worldwide. The sizing of settling ponds and surface-flow wetlands for passive Fe removal from circumneutral, ferruginous mine water is based either on a linear (concentration-independent) area-adjusted removal rate or flat assignment of an experience-based retention time, neither of which reflects the underlying Fe removal kinetics. In this study, we evaluated the Fe removal performance of a pilot-scale passive system operating in three identical, parallel lines for treatment of mining-influenced, ferruginous seepage water to determine and parameterise a robust, application-orientated model approach for sizing of settling ponds and surface-flow wetlands, each. By systematically varying flow rates (and thus residence time), we were able to demonstrate that the sedimentation-driven removal of particulate hydrous ferric oxides in settling ponds may be approximated by a simplified first-order approach at low to moderate Fe levels. The first-order coefficient was found in the order of 2.1(±0.7) × 10^-2 h^-1, which corresponds well with previous laboratory studies. The sedimentation kinetics may be combined with the preceding Fe(II) oxidation kinetics to estimate the required residence time for pre-treatment of ferruginous mine water in settling ponds. In contrast, Fe removal in surface-flow wetlands is more complex due to the phytologic component, which is why we advanced the established area-adjusted Fe removal approach by parameterising the underlying concentration-dependency for polishing of pre-treated mine water. The quantitative results of this study provide a novel, conservative approach for customised sizing of settling ponds and wetlands in integrated passive mine water treatment systems.

Influence of sediment quality and microbial community on the functioning capacity of a constructed wetland treating alkaline leachate after 5.5 years in operation

Constructed wetlands (CWs) have been demonstrated as a cost-effective alternative to chemical treatment systems for mine waters, with the microbial communities attributed to promoting carbonation and aiding pH neutralization. However, few data are available for the long-term use of CWs treating alkaline leachates nor the activity of microbes within them. To investigate the feasibility of CW to buffer alkaline pH, a pilot-scale wetland was implemented in 2015 to treat alkaline bauxite residue leachate. After 5.5 years, samples of supernatant water and sediment were taken at 0.5 m increments along the 11 m long wetland. Waters were analysed for pH, EC and metal(loid) content, while sediment was subjected to physico-chemical assessment and element fractionation. Microbial biomass and community were assessed by phospholipid fatty acid analysis (PLFA) and functionality by the Rapid Automated Bacterial Impedance Technique (RABIT). Evidence presented demonstrates that the CW operating for 66 months effectively treats bauxite residue leachate, with reduced influent pH from 11.5 to 7.8. Trace element analysis revealed effective reduction in Al (94.9 %), As (86.7 %) and V (57.6 %) with substrate analysis revealing a frontloading of elevated pH and trace element content in the first 5 m of the wetland. Sediment Al, As and V were present mostly (>94 % of total) in recalcitrant forms. Sediment Na was mostly soluble (48-62 %), but soils were not sodic (ESP < 15 %). Investigations into the microbial community revealed greatest biomass was in the first 5 m of the wetland, where pH, EC and metal contents were greatest. Microbial respiration using endemic Phragmites australis as a substrate demonstrates an ability to cycle recalcitrant carbon sources within a CW system. These novel microbial findings highlight the need for further investigation into the microbial communities in alkaline CWs.

Cellulose-Based Fluorescent Material for Extreme pH Sensing and Smart Printing Applications

Environment-responsive fluorescence materials are being widely investigated for instrument-free determination of various environmental factors. However, developing an eco-friendly cellulose-based fluorescent pH sensor for sensing extreme acidity and alkalinity is still challenging. Herein, a highly fluorescent and multifunctional material is developed from biopolymer-based cellulose acetate. A biopolymer-based structure containing responsive functional groups such as -C═O and -NH is constructed by chemically bonding 5-amino-2,3-dihydrophthalazine-1,4-dione (luminol) onto cellulose acetate using 4,4'-diphenylmethane diisocyanate (MDI) as a cross-linking agent. The prepared material (Lum-MDI-CA) is characterized by UV-vis, Fourier transform infrared, ¹H NMR, ¹³C NMR spectroscopies, and fluorescence techniques. The material exhibits excellent aqua blue fluorescence and demonstrates extreme pH sensing applications. Interesting results are further revealed after adding a pH-unresponsive dye such as MTPP as the reference to develop the ratiometric method. The ratiometric system clearly differentiates the extreme acidic pH 1 from pH 2 and extreme alkaline pH 12, 13, and 14 by visual and fluorescence color change response under a narrow pH range. In addition, the material is fabricated into transparent flexible fluorescent films which demonstrate an outstanding UV shielding, security printing, and haze properties for smart food packaging and printing applications.

Use of lignocellulosic waste materials in the passive treatment of highly alkaline wastewater contaminated with sulfates and metals - From a laboratory study to pilot scale

Passive wastewater treatment systems are an alternative to costly and ineffective chemical wastewater treatment methods. Lignocellulosic waste materials (LWM) are often used in passive wastewater treatment systems as a cheap and accessible source of nutrients. LWM, such as spent mushroom compost and woodchips, have been implemented for the successful management of mildly alkaline effluents, which constitute a large fraction of industrial wastewater. The objective of the study was to provide an extensive study of the parameters in four types of commonly used LWM (raw and composted sawdust, spent mushroom compost and woodchips), which can be used in the planning of a passive wastewater treatment plant. LWM were shown to remove up to 90% Zn²⁺ and Pb²⁺ from a model solution and neutralize wastewater. Moreover, the LWM were inhabited by a physiologically diverse microbial consortium containing sulfate-reducing and cellulolytic microbes, which can influence the treatment process. Another purpose of this study was to construct a pilot wastewater treatment plant based on the use of LWM and gravel and to present its ability to effectively treat extremely alkaline flotation wastewater (pH = 12) originating from a lead and zinc mine located in Montenegro. The treated wastewater had a unique, but challenging chemical composition for passive treatment, as it was heavily contaminated with sulfates (∼1200 mg/L) and lead (∼1 g/L). The removal within the developed installation reached a rate of 66%, while the treated effluent, after initial neutralization, was maintained at a pH of approximately 7. Lead and zinc concentrations after treatment were also kept at levels required by Montenegrin law for wastewater disposal.

Basic oxygen furnace sludge to treat industrial arsenic- and sulfate-rich acid mine drainage

In this study, four systems (S1, S2, S3, and S4) were evaluated to determine whether basic oxygen furnace sludge (BOFS), mainly composed of Fe (84%, mostly as elemental Fe and FeO), Ca (3%, as CaCO₃), and Si (1%), is capable of removing As-spiked, Mn, Mg, and sulfate from an industrial acid mine drainage (AMDi) collected in a gold mine in Minas Gerais, Brazil. In the S1 system (BOFS/deionized water pH 2.5), the stability of the residue was evaluated for 408 h under agitation. The results showed that only Ca and Mg were solubilized, and the pH increased from 2.5 up to 11.4 within the initial 24 h and kept still until the end of the experiment (408 h). The S2 system (BOFS/AMDi) achieved 100% removal of As and Mn, and 70% removal of sulfate after 648 h. In the first 30 min, the pH increased from 2.5 to 10, which was maintained until the end of the experiment. The removal of As, Mn, and sulfate in the presence of hydrogen peroxide (S3 and S4 systems - BOFS/AMDi/H₂O₂) was similar to that in the S2 system, which contained only BOFS. The formation of iron oxides was not accelerated by H₂O₂. As regards the removal of arsenic and sulfate species, the formation of incipient calcium arsenate and calcium sulfate dehydrated was indicated by X-ray diffraction analysis and PHREEQC modeling. Dissolved manganese and magnesium precipitated as oxides, according to the geochemical modeling. After contact with AMDi, the raw BOFS, initially classified as hazardous waste, became a non-inert waste, which implies simplified, less costly disposal. Except for sulfate, the concentrations of all the other elements were below the maximum permitted levels.

Removal of Phosphorus from Hypolimnetic Lake Water by Reactive Filter Material in a Recirculating System—Laboratory Trial

A toolbox of methods must be available for the remediation of lakes and water bodies suffering from eutrophication. One method suggested is hypolimnetic withdrawal based on a closed-circuit system. Prior to the start of a pilot-scale test at Lake Hönsan, Sweden, a laboratory trial with containers filled with water and bottom sediment from this lake was performed. A peristaltic pump distributed equal bottom water volume to four columns, two filled with glass beads and two with the filter material Polonite, and then back to the surface of the containers. The reactive filter medium (RFM) removed phosphate (PO4-P) efficiently (98.6%), despite the relatively low influent concentration (390 µg L−1). The control column filled with glass beads, removed 2.9% of the PO4-P. The anoxic sediment, containing 2.47 mg P g−1, released PO4-P, which was indicated by the increased concentration in near-bottom water. The redirected water after RFM filtration had high pH (x¯=11.1); however, an equalization took place in the water mass to a lower but still increased pH value (x¯=8.7) compared to the control (x¯=7.02). This article reports the pros and cons of a full-scale system using the proposed method.

A review on the removal of heavy metals and metalloids by constructed wetlands: bibliometric, removal pathways, and key factors

Heavy metals and metalloids (HMMs) pose a serious threat to both environmental and human health. The unique characteristics and environmental toxicity of HMMs make their removal from the environment a major challenge. Constructed wetlands (CWs) are increasingly being used as an eco-friendly system for the removal of HMMs from aqueous environments. In this review, bibliometric analysis was performed using the Scopus database using VOSviewer software to assess the developing use of CWs in recent years. Heavy metal and metalloid (HMM) removal pathways were reviewed (such as precipitation, co-precipitation, adsorption and ion exchange, plant action and microbial action) along with the impact of key factors (pH, chemical oxygen demand, dissolved oxygen, HMM concentration, and temperature). This review aimed to establish the connections between published results, to help effectively optimize the use of CWs for the removal of HMMs and identify the most critical factors for their effective removal. Important aspects that require further research include assessing the synergistic toxicity between different pollutants and combining the use of CWs with other technologies to optimize pollutant remediation efficiency.

Sustainable ecological engineering systems for the treatment of domestic wastewater using emerging, floating and submerged macrophytes

Water scarcity is perceived as a global systemic risk since there is an inexorable rise in water demand. An ecological engineering system mimics a natural ecosystem by balancing the trophic conditions for effective treatment of wastewater in a sequential manner. The present study was designed using emergent, floating and submerged macrophytic plants in a systematic approach. The study was evaluated with several components such as plants (water hyacinth, water lettuce and water thymes), aeration (supply of oxygen), and physical adsorption (activated carbon). Domestic wastewater collected from the local effluent treatment plant was treated individually and by combining all the components. Diverse experimental setups viz., lake sediment (control reactor), aeration, activated carbon blocks, water hyacinth, water lettuce, and water thymes were individually studied. Further the above components were combined, such as lake sediment + aeration + activated carbon blocks with plants like water hyacinth, water lettuce, and water thymes. The study inferred along with phytoremediation, and the external factors enhanced the treatment performances. Water hyacinth documented enhanced chemical oxygen demand removal efficiency of 85.71%, followed by water lettuce (80%), and water thymes (77.14%) along with the plants, both aeration, and activated carbon had stimulated the wastewater treatment. The highest removal efficiency of nitrate (70.23%), phosphate (63.64%), and sulphate (61.16%) were observed in water hyacinth due to its thick roots, and fibrous tissues reported effective treatment. The study hypothesized that these processes could be an effective strategy to restore the lakes and regulate the environmental flow. The study infers that an ecological engineering system symbiotically enables to self-organize the ecosystem within the boundary.

Enriched Co-Treatment of Pharmaceutical and Acidic Metal-Containing Wastewater with Nano Zero-Valent Iron

Among traditional hazardous waste sources, pharmaceutical-containing wastewater and acidic mine drainage need treatment to preserve the expected water supply quality. A nano zero-valent iron (nZVI)-enriched treatment of these two streams is evaluated for simultaneous removal of various heavy metal ions, organic pollutants, sulfates, the efficiency of the treatment system, and separation of reaction products in the fluidized-bed reactor. The reactor packed with silica sand was inoculated with sludge from an anaerobic digester, then 1–3 g/L of nZVI slurry added to cotreat a hospital feed and acid mine wastewater at 5:2 v/v. The biotreatment process is monitored through an oxidation–reduction potential (Eh) for 90 days. The removal pathway for the nZVI used co-precipitation, sorption, and reduction. The removal load for Zn and Mn was approximately 198 mg Zn/g Fe and 207 mg Mn/g Fe, correspondingly; achieving sulfate (removal efficiency of 94% and organic matter i.e., chemical oxygen demand (COD), biological oxygen demand (BOD), dissolved organic carbon (DOC), total dissolved nitrogen (TDN) reduced significantly, but ibuprofen and naproxen achieved 31% and 27% removal, respectively. This enriched cotreatment system exhibited a high reducing condition in the reactor, as confirmed by Eh; hence, the nZVI was dosed only a few times in biotreatment duration, demonstrating a cost-effective system.

Quantifying climate, streamflow, and watershed control on water quality across Southeastern US watersheds

Identification of the key variables that influence spatial variation in stream water quality is crucial for designing sustainable water management strategies. In this study, we investigated the key variables that influence the spatial variability of stream of water quality, across multiple watersheds. This study uses water quality data collected over 19 years for 59 watersheds located in the Southeast Atlantic region of the United States, which includes the states of North Carolina, South Carolina, and Georgia. A conceptual modeling framework was developed to understand the linkage between the long-term mean water quality constituents (Total nitrogen, Total phosphorus, Turbidity, and pH) and the watershed characteristics (e.g., topography, land use/cover, soil type), streamflow data, and climatic variables (precipitation and temperature). The modeling results suggest that not only anthropogenic variables influence the mean water quality constituents, but other watershed characteristics, such as soil properties, have a significant impact. The natural watershed characteristics explain most of the spatial variability in the mean Turbidity and pH values in streams. The modeling results also suggest that once land use and soil properties are considered, watershed topography has a limited role to explain the variation in the mean water quality. Overall, the developed watershed models can be used to forecast stream water-quality responses to future land use, climate, soil, and land management changes within the study area.

Optimizing Constructed Wetlands for Safe Removal of Triclosan: A Box-Behnken Approach

Traditional constructed wetland designs typically result in variable efficiencies for trace organic contaminant removal. In this work, we used a Box-Behnken experimental design for optimizing the conditions of pH, nitrate concentration, and dissolved organic carbon (DOC) concentration that would maximize the rate of triclosan phototransformation while minimizing the accumulation of toxic byproducts. Triclosan is a frequently detected and toxic antimicrobial agent present in many consumer and industrial products. The results showed that high pH values (9.9) and low DOC concentration (11 mg/L^-) would maximize triclosan phototransformation rate while minimizing the accumulation of toxic byproducts. As long as DOC concentrations were larger than 33 mg/L, nitrate concentration did not show a significant effect on triclosan phototransformation rate. The major transformation products detected were 2,4-dichlorophenol and compounds with chemical formulas C₁₂H₈Cl₂O₂ and C₁₂H₉ClO₃, resulting from a chlorine loss or replacement by a OH group. In addition, 4-chlorocatechol was mainly detected during direct photolysis and 2,8-dichlorodibenzo-p-dioxin was only found during direct photolysis at pH 8. This study showed that wetland efficiency at removing triclosan can theoretically be increased by limiting DOC-contributing factors, e.g., emergent vegetation, and supporting pH-increase processes, e.g., via algae growth or by incorporating alkaline geomedia.

Neutralization of high pH and alkalinity effluent from recycled concrete aggregate by common subgrade soil

Use of recycled concrete aggregate (RCA) as highway basecourse material conserves virgin aggregate, reduces energy consumption and CO₂ emissions, and may also decrease costs during construction. However, concerns remain over possible negative environmental impacts associated with high pH (>11) effluent from RCA in contact with water. This study examines the reactive transport of high-pH and high-alkalinity water, modeled on RCA leachate, through model subgrade soils. By developing an understanding of the reactions controlling effluent neutralization, this study aims to quantify the change in pH from the discharge site through surrounding subgrade soils. Four types of subgrade soils with a range of mineral composition, Atterberg limits, and cation exchange capacities (CECs) are examined. They include a clayey sand (SC10), low-plasticity clays (M14, SC25), and a high-plasticity clay (CH38). Batch reaction experiments are used to develop kinetic parameters describing the neutralization of high-pH and -alkalinity leachate by clay minerals through mineral dissolution and reprecipitation. Given this information, a reactive transport model incorporating advection, diffusion, and reaction is used to model the change in pH as a function of distance traveled through model subgrade soils and is applied to laboratory-scale column experiments. The rate at which the high pH front travels is directly related to a soil's clay mineral content. Soils with high CECs effectively delay the propagation of hydroxide front by the dissolution of clay minerals. This study demonstrates that common subgrade soils with moderate clay content will effectively neutralize high pH leachate initially produced by RCA.

Removal of Al, Ga, As, V and Mo from alkaline wastewater using pilot-scale constructed wetlands

The study was initiated to evaluate constructed wetland technology as a method for treating alkaline (pH 8.0-8.6) drainage high in Al, Mo, V, As and Ga originating from bauxite residue storage areas. Pilot-scale horizontal flow constructed wetlands were operated over a 40-week period using three filter materials (granitic gravel, bauxite and alum water treatment sludge), and half of the wetlands were planted with Phragmites australis and the other half left unplanted. Gravel was the least effective medium for removing the target elements, while of the two active media, water treatment sludge was more effective than bauxite. Plants removed only small amounts of elements into their above- and below-ground dry matter (0.4-4.9% of that added). Nonetheless, the presence of plants greatly increased the effectiveness of all three media since their presence decreased effluent pH values by 0.5-1.3 pH units and that of the filter media by 0.4 pH units. Removal of elements followed the order Al > Ga > V > As > Mo. For planted wetlands, total elemental removal ranged from 18 to 98% for gravel, 80 to 99% for bauxite, and 93 to 99% for water treatment sludge. The lowest removal was for Mo (ranging from 18% for gravel to 93% for water treatment sludge) and the highest for Al (ranging from 98% in gravel to 99% in water treatment sludge). A sequential fractionation scheme for As, V and Mo on filter material at the end of the experiment showed that for bauxite and water treatment sludge, V and As were concentrated in the NaOH extractable fraction while Mo was concentrated in the less strongly adsorbed NaHCO₃ extractable fraction. It was concluded that a constructed wetland with water treatment sludge as an active filter material is an effective technology for removal of the target elements from the alkali drainage.

Constructed wetlands for steel slag leachate management: Partitioning of arsenic, chromium, and vanadium in waters, sediments, and plants

Constructed wetlands can treat highly alkaline leachate resulting from the weathering of steel slag before reuse (e.g. as aggregate) or during disposal in repositories and legacy sites. This study aimed to assess how metal(loid)s soluble at high pH, such as arsenic (As), chromium (Cr), and vanadium (V) are removed in constructed wetlands and how they accumulate in the sediments and the plants (Phragmites australis, common reed). The results show that reedbeds were very effective at removing calcium (98%), aluminium (81%), barium (98%), chromium (90%), gallium (80%), nickel (98%), and zinc (98%), and lowering pH and alkalinity. No statistical difference was found for As and V between leachate influent and wetland samples, showing that these metal(loid)s were not efficiently removed. As, Cr, and V were significantly higher in the reedbed sediments than in a reference site. However, sediment concentrations are not at levels that would pose a concern regarding reuse for agricultural purposes (average values of 39 ± 26 mg kg^-1 for As, 108 ± 15 mg kg^-1 for Cr, and 231 ± 34 mg kg^-1 for V). Also, there is no significant uptake of metals by the aboveground portions of the reeds compared to reference conditions. Results show statistically significant enrichment in metal(loid)s in rhizomes and also a seasonal effect on the Cr concentrations. The data suggest minimal risk of oxyanion-forming element uptake and cycling in wetlands receiving alkaline steel slag.

Critical Review: Biogeochemical Networking of Iron in Constructed Wetlands for Wastewater Treatment

Iron is present in all types of wastewater; however, besides acid mine drainage, where it is a major constituent of concern, it is usually neglected in other types of wastewaters. In all kinds of constructed wetlands, iron plays important role in removal of organics and phosphorus, and it has an impact on transformation of nitrogen, sulfur, and metals. The biogeochemistry of iron is well understood in natural wetlands, but knowledge about iron impact on microbiological and chemical transformations during wastewater treatment in constructed wetlands is very limited. So far, the sparse research in this area provides limited information on observed interactions with several varying parameters across the studies, making it difficult to draw fundamental and mechanistic conclusions. A critical review of the complex biogeochemical networking of iron in CWs is therefore necessary to fill the gap in knowledge on the role of iron and its biogeochemical multi-interactions in wastewater treatment processes of CWs. This review is the first with specific focus on iron, discussing its mitigation and retention in CWs with different configurations and operational strategies, and presenting both seasonal dynamics and the potential remobilization of Fe. It also comprehensively discusses the interactions of redox-controlled iron turnover with the biogeochemical processes of other elements, for example, carbon (C), nitrogen (N), phosphorus (P), sulfur (S), and heavy metals. The health response of wetland plants to both deficiency and toxicity of Fe in CWs designed with specific treatment targets has also been evaluated. Due to the complexity of various wastewater compositions and microredox gradients in the root rhizosphere in CWs, future research needs have also been identified.

Facile thermal activation of non-reactive cryptocrystalline magnesite and its application on the treatment of acid mine drainage

In this study, the authors report a facile thermal activation of non-reactive cryptocrystalline magnesite and explore its application on the treatment of acid mine drainage (AMD). The primary aim was to optimize the calcination-water interface reactive conditions. Parameters evaluated include calcination temperature, calcination time, AMD-calcination temperature interface, and AMD-calcination duration interface. PHREEQC geochemical modelling was also applied to substantiate obtained results. The results indicated that the formation of MgO and CaO increase with an increase in calcination temperature and time. The optimum temperature and calcination time were observed to be 800 °C and 30 min in the furnace. The pH was observed to increase with an increase in calcination temperature and time but reached equilibrium at 800 °C and 30 min respectively. Geochemical modelling validated the formation of gypsum with attenuation in Ca ions and predicted the formation of MgSO_4(aq). Metal species were observed to precipitate with an increase in pH. At 700 °C, Fe was completely removed, while Al, and Mn were completely removed from an aqueous system at 800 °C. This novel study invented the new calcination condition for non-reactive cryptocrystalline magnesite and proved its potential application in wastewater treatment. The calcination conditions were very short and therefore will save industries energy due to replacement of uneconomical and less environmental friendly pre-treatment options that lead to environmental degradation.

Bioremediation of Waste Water to Remove Heavy Metals Using the Spent Mushroom Substrate of Agaricus bisporus

The presence of heavy metals in waste water brings serious environmental pollution that threatens human health and the ecosystem. Bioremediation of heavy metals has received considerable and growing interest over the years. Thus, this paper presents the use of the Spent Mushroom Substrate (SMS) of Agaricus bisporus cultivation as a bioremediating agent to remove heavy metals that are present in industrial waters. These metals include chromium, lead, iron, cobalt, nickel, manganese, zinc, copper and aluminium. In particular, this study analyses the performance of SMS bioreactors with different groups of heavy metals at various concentrations. Between 80% and 98% of all contaminants that were analysed can be removed with 5 kg of SMS at hydraulic retention times of 10 and 100 days. The best removal efficiencies and longevities were achieved when removing iron (III), nickel and cobalt from contaminated water at a pH of 2.5. These results suggest that SMS can successfully treat waste water that has been contaminated with heavy metals.

Visual and Precise Detection of pH Values under Extreme Acidic and Strong Basic Environments by Cellulose-Based Superior Sensor

Precise determination of pH values with an instrument-free mode is challenging and practical in industry processes, life science, and environmental monitoring. Here, taking advantage of the intramolecular charge transfer process, we construct a cellulose-based superior pH sensor, Phen-MDI-CA, which contains a highly sensitive, pH-responsive, and extended conjugation structure by combining phenanthroline moiety as a chromophore with urea group as a bridge. The resultant Phen-MDI-CA can precisely and visually determine pH values under extreme acidic and strong basic environments, depending on the different colors. In visible light mode, Phen-MDI-CA can readily discriminate the pH values of 14.0, 13.0, 12.0, and 11.0 and distinguish pH 2.0 from 1.0 by the naked eye. In fluorescent mode, Phen-MDI-CA can recognize more accurate pH values of pH 11.6-13.2 at a 0.2-0.4 pH interval by the high-contrasting color change. After addition of pH-irresponsive dyes as the reference, the corresponding ratiometric systems exhibit different colors at a 0.2-0.4 pH interval during the pH values of 11.0-14.0 and 1.0-2.0. Benefiting from the excellent formability, the Phen-MDI-CA was readily fabricated into pH test strips by coating method. To our knowledge, this is the first study to monitor extreme acidity and strong basicity in such precise pH values with an instrument-free mode.

Competitive adsorption and desorption of arsenate, vanadate, and molybdate onto the low-cost adsorbent materials alum water treatment sludge and bauxite

When low-cost adsorbents are being used to remove contaminant ions (e.g. arsenate, vanadate, and molybdate) from wastewater, competitive adsorption/desorption are central processes determining their removal efficiency. Competitive adsorption of As, V, and Mo was investigated using equimolar oxyanion concentrations in single, binary, and tertiary combinations in adsorption isotherm and pH envelope studies while desorption of previously adsorbed oxyanions was examined in solutions containing single and binary oxyanion combinations. The low-cost adsorbent materials used were alum water treatment sludge (amorphous hydroxy-Al) and bauxite ore (crystalline Al oxides). Adsorption isotherm and pH envelope studies showed that Mo had only a small effect in decreasing adsorption of As and V but V and As had substantial and similar effects in reducing adsorption of the other. As had a greater effect than V in reducing adsorption of Mo and it was concluded that the affinity of oxyanions for the surfaces of water treatment sludge and bauxite followed the order As > V >> Mo. In 0.3 M NaCl electrolyte, desorption of previously adsorbed oxyanions amounted to 0.3-3.4% for V and As, and 11-20% for Mo. As had approximately four times greater effect than Mo in increasing desorption of V while V had about three times the effect of Mo in increasing desorption of As. Thus, the order of oxyanions in inducing desorption of the other oxyanions (i.e. As on V and As) was the same as that for adsorption selectivity: As > V >> Mo. Water treatment sludge was a more effective adsorbent than bauxite because it had a greater adsorption capacity for all three anions and, in addition, they were held more strongly so desorption in the background electrolyte was proportionately less. It was concluded that at similar molar concentrations, arsenate would tend to reduce adsorption of vanadate as well as displace vanadate already held on adsorbent surfaces while both anions will compete effectively with molybdate. The limiting factor for simultaneous removal of As, V, and Mo from multielement solutions by adsorption will therefore be the removal of Mo.

Phytotoxicity Evaluation of Wastewater from Rice Parboiling

The aim of this study was to evaluate the phytotoxicity of raw and treated effluents from a parboiling rice industry in the south of Brazil. The evaluation included 19 physicochemical parameters and a phytotoxicity test using lettuce and cucumber seeds as bioindicators. The germination index (GI) of both seeds was decreased when in contact with the raw effluent (p < 0.05) and with the treated effluent reduced the GI of cucumber seeds (p < 0.05) and the root length (RL) of lettuce seeds (p < 0.05). For lettuce seeds exposed to the treated effluent there was a negative correlation (p < 0.05) between RL and Mn (- 0.82) and between GI and Fe (- 0.68). For cucumber seeds in the raw effluent, the Fe showed negative correlation (p < 0.05) with seed germination (- 0.75), but, in the treated effluent, their GI presented positive correlation (0.61) with the ammoniacal-N (p < 0.05). The results indicated that both effluents showed phytotoxicity for lettuce and cucumber seeds.

Microbial Community Shifts in Response to Acid Mine Drainage Pollution Within a Natural Wetland Ecosystem

Natural wetlands are known to play an important role in pollutant remediation, such as remediating acid mine drainage (AMD) from abandoned mine sites. However, many aspects of the microbiological mechanisms underlying AMD remediation within wetlands are poorly understood, including the role and composition of associated microbial communities. We have utilized an AMD-polluted river-wetland system to perform rRNA sequence analysis of microbial communities that play a role in biogeochemical activities that are linked to water quality improvement. Next-generation sequencing of bacterial 16S rRNA gene amplicons from river and wetland sediment samples identified variation in bacterial community structure and diversity on the basis of dissolved and particulate metal concentrations, sediment metal concentrations and other water chemistry parameters (pH and conductivity), and wetland plant presence. Metabolic reconstruction analysis allowed prediction of relative abundance of microbial metabolic pathways and revealed differences between samples that cluster on the basis of the severity of AMD pollution. Global metabolic activity was predicted to be significantly higher in unpolluted and wetland sediments in contrast to polluted river sediments, indicating a metabolic stress response to AMD pollution. This is one of the first studies to explore microbial community structure dynamics within a natural wetland exposed to AMD and our findings indicate that wetland ecosystems play critical roles in maintaining diversity and metabolic structure of sediment microbial communities subject to high levels of acidity and metal pollution. Moreover, these microbial communities are predicted to be important for the remediation action of the wetland.

Floating Wetland Treatment of Acid Mine Drainage using Eichhornia crassipes (Water Hyacinth)

BACKGROUND

Acid mine drainage (AMD) is a major environmental impact associated with the mining industry. Elevated acidic conditions resulting from the discharge of AMD into the surrounding environment can cause heavy metals to dissolve and transport through water streams and accumulate in the aquatic environment, posing a risk to the health of living organisms. There have been several novel approaches in the remediation of AMD involving passive treatment techniques. The constructed treatment wetland approach is a passive remediation option that has proven to be a cost effective and long-lasting solution in abating toxic pollutant concentrations.

OBJECTIVES

The present study investigates the applicability of water hyacinth (Eichhornia crassipes), a tropical aquatic plant with reported heavy metal hyper-accumulation in microcosm floating wetland treatment systems designed to remediate AMD with copper (Cu) and cadmium (Cd) concentrations exceeding threshold limits.

METHODS

Twelve water hyacinth samples were prepared with varying concentrations of Cu (1 mg/L, 2 mg/L, 4 mg/L) and Cd (0.005 mg/L, 0.01 mg/L, 0.02 mg/L). Water samples of 5 ml each were collected from each sample at 24-hour intervals for analysis with an atomic absorption spectrometer.

RESULTS

Plant growth varied according to Cu and Cd concentrations and no plants survived for more than 14 days. There was a significant discrepancy in the rate at which the Cd concentrations abated. The rate of reduction was rapid for higher concentrations and after 24 hours a substantial reduction was achieved. There was a reduction in Cu concentration after the first 24-hour period, and after the next 24-hour period the concentrations were again elevated in the samples at initial concentrations of 2 mg/L and A4 mg/L. 4 mg/L Cu concentration was shown to be toxic to the plants, as they had low accumulations and rapid dying was evident.

CONCLUSIONS

Water hyacinth has the capability to reduce both Cu and Cd concentrations, except at an initial concentration of 4 mg/L of Cu, which was toxic to the plants.

COMPETING INTERESTS

The authors declare no competing financial interests.

Hydraulic and biotic impacts on neutralisation of high-pH waters

The management of alkaline (pH11-12.5) leachate is an important issue associated with the conditioning, afteruse or disposal of steel slags. Passive in-gassing of atmospheric CO₂ is a low cost option for reducing Ca(OH)₂ alkalinity, as Ca(OH)₂ is neutralised by carbonic acid to produce CaCO₃. The relative effectiveness of such treatment can be affected by both the system geometry (i.e. stepped cascades versus settlement ponds) and biological colonization. Sterilized mesocosm experiments run over periods of 20days showed that, due to more water mixing and enhanced CO₂ dissolution at the weirs, the cascade systems (pH11.2→9.6) are more effective than settlement ponds (pH11.2→11.0) for lowering leachate alkalinity in all the tested conditions. The presence of an active microbial biofilm resulted in significantly more pH reduction in ponds (pH11.2→9.5), but had a small impact on the cascade systems (pH11.2→9.4). The pH variation in biofilm colonized systems shows a diurnal cycle of 1 to 1.5pH units due to CO₂ uptake and release associated with respiration and photosynthesis. The results demonstrate that, where gradient permits, aeration via stepped cascades are the best option for neutralisation of steel slag leachates, and where feasible, the development of biofilm communities can also help reduce alkalinity.

Evaluation of thermally-modified calcium-rich attapulgite as a low-cost substrate for rapid phosphorus removal in constructed wetlands

The cost-effective and geographically available substrates are vital for the design of constructed wetlands (CWs), especially the saturated subsurface flow CWs, which are deemed as an efficient way to remove the inlet-lake phosphorus concentrations. In this study, phosphorus removal of thermally-treated calcium-rich attapulgite (TCAP) with varied particle sizes (0.2-0.5 mm, 0.5-1 mm and 1-2 mm) was assessed using batch and long-term column experiments to evaluate its feasibility as a CWs substrate. The phosphorus-bound mechanism in TCAP was identified in various initial phosphorus concentrations. Batch studies indicated that more than 95% of P can be rapidly (<1 h) removed by TCAP from solution with a concentration of 20 mg P/L, and P sorption can be well fitted by a pseudo-second-order equation. The maximum P sorption capacity of TCAP was in the range of 4.46-5.99 mg P/g, and the availability of Ca²⁺ concentration might limit the P removal capacity of TCAP at high phosphorus concentrations. Both the P removal rate and capacities decreased with the increase of TCAP particle sizes. Column P removal experiments indicated that hydraulic retention time (HRT) exerts great influence on P removal performance and longer HRTs favor the dissolution of CaO in TCAP, consequently increasing the P removal rate. In a 150-day P removal experiment, TCAP removed an average of 93.1%-95.4% of the influent P with a HRT of eight hours. Both the batch and chemical extraction of the P fraction of TCAP showed that the P removed by TCAP was mainly through formation of Ca phosphate precipitation. However, the species of Ca-P precipitation formed might be varied in different phosphorus concentrations. All results indicated that TCAP can be a suitable substrate when used in CWs, and field experiments should be carried out to test its real P removal performance in the future.

Effectiveness of a constructed wetland for treating alkaline bauxite residue leachate: a 1-year field study

Increasing volumes of bauxite residues and their associated leachates represent a significant environmental challenge to the alumina industry. Constructed wetlands have been proposed as a potential approach for leachate treatment, but there is limited data on field-scale applications. The research presented here provides preliminary evaluation of a purpose-built constructed wetland to buffer leachate from a bauxite residue disposal site in Ireland. Data collected over a 1-year period demonstrated that the pH of bauxite residue leachates could be effectively reduced from ca. pH 10.3 to 8.1 but was influenced by influent variability and temporal changes. The wetland was also effective in decreasing elemental loading, and sequential extractions suggested that the bulk of the sediment-bound metal inventory was in hard-to-leach phases. Elemental analysis of Phragmites australis showed that although vegetation displayed seasonal variation, no trace elements were at concentrations of concern.

Removal and recovery of vanadium from alkaline steel slag leachates with anion exchange resins

Leachable vanadium (V) from steel production residues poses a potential environmental hazard due to its mobility and toxicity under the highly alkaline pH conditions that characterise these leachates. This work aims to test the efficiency of anion exchange resins for vanadium removal and recovery from steel slag leachates at a representative average pH of 11.5. Kinetic studies were performed to understand the vanadium sorption process. The sorption kinetics were consistent with a pseudo-first order kinetic model. The isotherm data cannot differentiate between the Langmuir and Freundlich models. The maximum adsorption capacity (Langmuir value q_max) was 27 mg V g^-1 resin. In column anion exchange, breakthrough was only 14% of the influent concentration after passing 90 L of steel slag leachate with 2 mg L^-1 V through the column. When eluting the column 57-72% of vanadium was recovered from the resin with 2 M NaOH. Trials on the reuse of the anion exchange resin showed it could be reused 20 times without loss of efficacy, and on average 69% of V was recovered during regeneration. The results document for the first time the use of anion exchange resins to remove vanadium from steel slag leachate. As an environmental contaminant, removal of V from leachates may be an obligation for long-term management requirements of steel slag repositories. Vanadium removal coupled with the recovery can potentially be used to offset long-term legacy treatment costs.

The potential for constructed wetlands to treat alkaline bauxite-residue leachate: Phragmites australis growth

High alkalinity (pH > 12) of bauxite-residue leachates presents challenges for the long-term storage and managements of the residue. Recent evidence has highlighted the potential for constructed wetlands to effectively buffer the alkalinity, but there is limited evidence on the potential for wetland plants to establish and grow in soils inundated with residue leachate. A pot-based trial was conducted to investigate the potential for Phragmites australis to establish and grow in substrate treated with residue leachate over a pH range of 8.6-11.1. The trial ran for 3 months, after which plant growth and biomass were determined. Concentrations of soluble and exchangeable trace elements in the soil substrate and also in the aboveground and belowground biomass were determined. Residue leachate pH did not affect plant biomass or microbial biomass. With the exception of Na, there was no effect on exchangeable trace elements in the substrate; however, increases in soluble metals (As, Cd and Na) were observed with increasing leachate concentration. Furthermore, increases in Al, As and V were observed in belowground biomass and for Cd and Cr in aboveground biomass. Concentrations within the vegetation biomass were less than critical phytotoxic levels. Results demonstrate the ability for P. australis to grow in bauxite-residue leachate-inundated growth media without adverse effects.

Coprecipitation of (14)C and Sr with carbonate precipitates: The importance of reaction kinetics and recrystallization pathways

This study investigated the simultaneous removal of Sr(2+) and (14)CO3(2-) from pH>12 Ca(OH)2 solution by the precipitation of calcium carbonate. Initial Ca(2+):CO3(2-) ratios ranged from 10:1 to 10:100 (mM:mM). Maximum removal of (14)C and Sr(2+) both occurred in the system containing 10mM Ca(2+) and 1mM CO3(2-) (99.7% and 98.6% removal respectively). A kinetic model is provided that describes (14)C and Sr removal in terms of mineral dissolution and precipitation reactions. The removal of (14)C was achieved during the depletion of the initial TIC in solution, and was subsequently significantly affected by recrystallization of the calcite precipitate from an elongate to isotropic morphology. This liberated >46% of the (14)C back to solution. Sr(2+) removal occurred as Ca(2+) became depleted in solution and was not significantly affected by the recrystallization process. The proposed reaction could form the basis for low cost remediation scheme for (90)Sr and (14)C in radioactively contaminated waters (<$0.25 reagent cost per m(3) treated).

Effect of photosynthetically elevated pH on performance of surface flow-constructed wetland planted with Phragmites australis

Combination of emergent and submerged plants has been proved to be able to enhance pollutant removal efficiency of surface flow-constructed wetland (SFCW) during winter. However, intensive photosynthesis of submerged plants during summer would cause pH increase, which may have adverse effects on emergent plants. In this study, nitrogen transformation of lab-scale SFCW under pH gradient of 7.5, 8.5, 9.5 and 10.5 was systematically investigated. The results showed that total nitrogen (TN) removal efficiency decreased from 76.3 ± 0.04 to 51.8 ± 0.04 % when pH increased from 7.5 to 10.5, which was mainly attributed to plant assimilation decay and inhibition of microbe activities (i.e., nitrite-oxidizing bacteria and denitrifiers). Besides, the highest sediment adsorption in SFCW was observed at pH of 8.5. In general, the combination of submerged and emergent plants is feasible for most of the year, but precaution should be taken to mitigate the negative effect of high alkaline conditions when pH rises to above 8.5 in midsummer.

Performance of an open limestone channel for treating a stream affected by acid rock drainage (León, Spain)

The generation of acid rock drainage (ARD) was observed after the oxidation dissolution of pyrite-rich black shales, which were excavated during the construction of a highway in León (Spain). ARDs are characterized by the presence of high concentrations of sulfate and metals (Al, Fe, Mn, Zn, Cu, Co, Ni, Th, and U) that affect the La Silva stream. Dissolved element concentrations showed values between one and four orders of magnitude higher than those of natural waters of this area. A passive treatment system was constructed; the aim of which was to improve the quality of the water of the stream. This work provides a hydrochemical characterization of the La Silva stream after its transit through the different elements that constitute the passive treatment system (open limestone channel (OLC), small ponds, and a wetland), during its first year of operation. The passive treatment system has two sections separated by a tunnel 230 m long. The first section, which stretches between the highway and the tunnel entrance, is an OLC 350 m long with a slope of 16 %. The second section, which stretches from the tunnel exit to the end wetland, has a length of 700 m and a slope of 6 %; it is in this section where six small ponds are located. In the first section of this passive treatment system, the OLC was effectively increasing the pH from 3 to 4-4.5 and eliminating all of the dissolved Fe and the partially dissolved Al. These elements, after hydrolysis at a pH 3-3.5 and 4-4.5, respectively, had precipitated as schwertmannite and hydrobasaluminite, while other dissolved metals were removed totally or partially for adsorption by the precipitates and/or by coprecipitation. The second section receives different inputs of water such as ARDs and natural waters. After exiting the treatment system, the stream is buffered by Al at a pH of 4-4.3, showing high Al concentrations (19-101 mg/L) but with a complete removal of dissolved Fe. Unfortunately, the outflow shows similar or higher acidity than the inflow into the system due to the discharge of ARDs (mainly from the tunnel) that is received and to the existence of a natural stream, which is affected by a waste-rock pile. The predictions and calculations necessary for the design of any remediation/attenuation techniques are quite difficult. Despite the fact that the selected design is the most adequate one for this valley and type of passive treatment system (including adequate slopes), we must admit that the physicochemical characteristics of the ARD were not the most appropriate according to the literature. Moreover, during the design, engineers were unaware of the existence of the inflow from two highly polluting sources, which have rendered the passive treatment system ineffective and which therefore suggest that certain improvement measures could be considered.

The potential for constructed wetlands to treat alkaline bauxite residue leachate: laboratory investigations

High alkalinity (pH > 12) of bauxite residue leachates presents challenges for the long-term storage and managements of the residue. Whilst the use of constructed wetlands is gaining in interest for its use in the treatment of alkaline waters, thus far, there is limited evidence of its suitability for treating NaOH dominated bauxite residue leachate. A series of batch trials were conducted to investigate the potential for constructed wetland conferred mechanisms (dilution water quality, contact with CO2, and substrate type) for treating NaOH solutions to levels permissible for discharge (p < 9). Results demonstrate that significant reductions in solution pH can be achieved depending on the diluting water quality. Levels achieved may not always be suitable for direct discharge (i.e. pH ≤ 9), but further reductions occur with carbonation and soil contact. The extent of pH decrease and the timeframe required are influenced by soil quality, with greater efficiency observed in soils with higher organic matter content. Decrease in solution pH to discharge permit values are possible through a combination of the mechanisms occurring in a constructed wetland. Formation of a calcite precipitate was observed in some treatments and further characterisation by XRD and XPS suggested surface coating with Na2CO3. It is therefore suggested that, under suitable conditions, constructed wetland technology can reduce leachate pH to <9 through mechanisms supporting the precipitation of sodium carbonate from solution. Further trials should investigate the activity under biological conditions representative of an operating constructed wetland.

Basic Oxygen Furnace steel slag aggregates for phosphorus treatment. Evaluation of its potential use as a substrate in constructed wetlands

Basic Oxygen Furnace (BOF) steel slag aggregates from NW Spain were tested in batch and column experiments to evaluate its potential use as a substrate in constructed wetlands (CWs). The objectives of this study were to identify the main P removal mechanisms of BOF steel slag and determine its P removal capacity. Also, the results were used to discuss the suitability of this material as a substrate to be used in CWs. Batch experiments with BOF slag aggregates and increasing initial phosphate concentrations showed phosphate removal efficiencies between 84 and 99% and phosphate removal capacities from 0.12 to 8.78 mg P/g slag. A continuous flow column experiment filled with BOF slag aggregates receiving an influent synthetic solution of 15 mg P/L during 213 days showed a removal efficiency greater than 99% and a phosphate removal capacity of 3.1 mg P/g slag. In both experiments the main P removal mechanism was found to be calcium phosphate precipitation which depends on Ca(2+) and OH(-) release from the BOF steel slag after dissolution of Ca(OH)2 in water. P saturation of slag was reached within the upper sections of the column which showed phosphate removal capacities between 1.7 and 2.5 mg P/g slag. Once Ca(OH)2 was completely dissolved in these column sections, removal efficiencies declined gradually from 99% until reaching stable outlet concentrations with P removal efficiencies around 7% which depended on influent Ca(2+) for limited continuous calcium phosphate precipitation.

Temporal evolution of bacterial communities associated with the in situ wetland-based remediation of a marine shore porphyry copper tailings deposit

Mine tailings are a serious threat to the environment and public health. Remediation of these residues can be carried out effectively by the activation of specific microbial processes. This article presents detailed information about temporal changes in bacterial community composition during the remediation of a section of porphyry copper tailings deposited on the Bahía de Ite shoreline (Peru). An experimental remediation cell was flooded and transformed into a wetland in order to prevent oxidation processes, immobilizing metals. Initially, the top oxidation zone of the tailings deposit displayed a low pH (3.1) and high concentrations of metals, sulfate, and chloride, in a sandy grain size geological matrix. This habitat was dominated by sulfur- and iron-oxidizing bacteria, such as Leptospirillum spp., Acidithiobacillus spp., and Sulfobacillus spp., in a microbial community which structure resembled acid mine drainage environments. After wetland implementation, the cell was water-saturated, the acidity was consumed and metals dropped to a fraction of their initial respective concentrations. Bacterial communities analyzed by massive sequencing showed time-dependent changes both in composition and cell numbers. The final remediation stage was characterized by the highest bacterial diversity and evenness. Aside from classical sulfate reducers from the phyla δ-Proteobacteria and Firmicutes, community structure comprised taxa derived from very diverse habitats. The community was also characterized by an elevated proportion of rare phyla and unaffiliated sequences. Numerical ecology analysis confirmed that the temporal population evolution was driven by pH, redox, and K. Results of this study demonstrated the usefulness of a detailed follow-up of the remediation process, not only for the elucidation of the communities gradually switching from autotrophic, oxidizing to heterotrophic and reducing living conditions, but also for the long term management of the remediation wetlands.

Long-term evolution of highly alkaline steel slag drainage waters

The disposal of slag generated by the steel industry can have negative consequences upon the surrounding aquatic environment by the generation of high pH waters, leaching of potentially problematic trace metals, and rapid rates of calcite precipitation which smother benthic habitats. A 36-year dataset was collated from the long-term ambient monitoring of physicochemical parameters and elemental concentrations of samples from two steel slag leachate-affected watercourses in northern England. Waters were typified by elevated pH (>10), high alkalinity, and were rich in dissolved metals (e.g. calcium (Ca), aluminium (Al), and zinc (Zn)). Long-term trend analysis was performed upon pH, alkalinity, and Ca concentration which, in addition to Ca flux calculations, were used to highlight the longevity of pollution arising as a result of the dumping and subsequent leaching of steel slags. Declines in calcium and alkalinity have been modest over the monitoring period and not accompanied by significant declines in water pH. If the monotonic trends of decline in alkalinity and calcium continue in the largest of the receiving streams, it will be in the region of 50-80 years before calcite precipitation would be expected to be close to baseline levels, where ecological impacts would be negligible.

Treatment of industrial effluents in constructed wetlands: challenges, operational strategies and overall performance

The application of constructed wetlands (CWs) has significantly expanded to treatment of various industrial effluents, but knowledge in this field is still insufficiently summarized. This review is accordingly necessary to better understand this state-of-the-art technology for further design development and new ideas. Full-scale cases of CWs for treating various industrial effluents are summarized, and challenges including high organic loading, salinity, extreme pH, and low biodegradability and color are evaluated. Even horizontal flow CWs are widely used because of their passive operation, tolerance to high organic loading, and decolorization capacity, free water surface flow CWs are effective for treating oil field/refinery and milking parlor/cheese making wastewater for settlement of total suspended solids, oil, and grease. Proper pretreatment, inflow dilutions through re-circulated effluent, pH adjustment, plant selection and intensifications in the wetland bed, such as aeration and bioaugmentation, are recommended according to the specific characteristics of industrial effluents.

Potential for use of industrial waste materials as filter media for removal of Al, Mo, As, V and Ga from alkaline drainage in constructed wetlands--adsorption studies

The potential to remove Al, Mo, V, As and Ga from alkaline (pH 8.0-8.6) drainage originating from seawater neutralized bauxite processing residue storage areas using constructed wetland technology was studied in a laboratory study. Bauxite processing residue sand, bauxite, alum water treatment sludge and blast furnace slag were investigated as potential active filter materials. Al was shown to precipitate as Al(OH)3 in the pH range 7.0-8.0 in aqueous solution and 6.0-8.5 in the presence of silica sand particles that provided a surface for nucleation. For V As Mo and Ga, adsorption to the surfaces of the adsorbents decreased greatly at elevated pH values (>pH 6-9). Water treatment sludge and bauxite had a greater ability to adsorb V, As and Mo at high pH (As and V at pH 7-9 and Mo at pH 5-7) than processing sand and slag. Adsorption isotherm data for As and V onto all four adsorbent than processing sand and slag. Adsorption isotherm data for As and V onto all four adsorbent materials fitted equally well to the Langmuir and Freundlich equations but for Ga, and to a lesser extent Mo, the Freundlich equation gave higher R(2) values. For all four ions, the maximum adsorption capacity (Langmuir value qmax) was greatest for water treatment sludge. Bauxite adsorbed more Mo, Ga and V than residue sand or slag. The pseudo-second order equation gave a better fit to the experimental kinetic data than the pseudo-first order model suggesting that chemisorption rather than diffusion/exchange was the rate limiting step to adsorption. It was concluded that water treatment sludge and bauxite were the most effective adsorbents and that for effective removal of the target ions the pH of the drainage water needs to be decreased to 6.0-7.0.

Biosorption of toxic metals using freely suspended Microcystis aeruginosa biomass

This paper describes the potential application of a freely suspended cyanobacterial biomass of Microcystis aeruginosa as a sorption material for toxic metals, cadmium (Cd) and lead (Pb) from aqueous solutions. In order to identify the most suitable conditions for removal of these elements (concentration range: 1–20 mg L−1), the laboratory experiments were carried out during different incubation times (1–48 h) and under various temperatures (10–40°C), pH (5–9) and with or without light irradiance. Competitive biosorption of Cd and Pb was also investigated. We found that M. aeruginosa demonstrates high efficiency in removing both of the studied metals from aqueous solutions. Sorption of Pb occurred, however, more rapidly and effectively, and was less disturbed by changes in physico-chemical conditions. Under pH=7, 25°C and light, the removal rates after 3 h and 6 h of incubation, were 90–100% for Pb and 79.5–100% for Cd, respectively. The co-occurrence of the metals decreased the rate of metal biosorption. Pb was sequestered preferentially over Cd. From the results we conclude that freely suspended M. aeruginosa can constitute a promising low-cost, easy-producible biosorbent material for toxic metals in contaminated wastewater.

How to increase microbial degradation in constructed wetlands: influencing factors and improvement measures

Microorganisms play a vital role in degradation of multiple pollutants in constructed wetlands (CWs). Thus, the search for methods to improve microbial degradation in CWs is crucial. This study provides a review of critical parameters including availability of organic carbon, redox condition, temperature, pH, presence of plants, media characteristics and their influences on microbial processes. Current strategies focusing on regulation of carbon source, redox condition, and choice of substrates to enhance microbial activity in CWs are also described. A special emphasis is given to the application of bioaugmentation to enhance microbial activities in wetland in future research.