Alkaline Groundwater Research Articles

Identification of low fluoride areas using conceptual groundwater flow model and hydrogeochemical system analysis in the aquifer system on the flanks of an active volcano: Mount Meru, Northern Tanzania

This study investigates the localities of low and high F− groundwaters in the aquifer system on the flanks of Mount Meru to come up with guidelines to provide groundwater that can be used for drinking water supply without health impacts on the population. Our study focuses on parts of the flanks which were only partially or not at all covered by previous research. Results show that the groundwater chemistry of F−-rich NaHCO3 alkaline groundwater in the area is controlled by dissolution of weathering aluminosilicate minerals, dissolution of F−-bearing minerals, the precipitation of carbonate minerals as secondary products and the dissolution of magmatic gases. The low F− groundwaters which can be used for drinking water supply without health impacts under the WHO limit (1.5 mg/L) are the low-fluoride springs from the high altitude recharge areas on the eastern and north-western flanks of Mount Meru inside Arusha National Park, whereas on the western flank the groundwater meets the Tanzanian limit (4.0 mg/L). On the south-western flank, the shallow aquifer composed of alluvium deposits at lower elevations, shows F− values that meet the Tanzanian limit. One of the three investigated deep boreholes on this flank also meets the Tanzanian limit, suggesting a possibility of finding relatively low F− groundwaters in the deep aquifer. Yet, in general, the deposits at lower elevations are found to contain high to very high F− values, whereas the deposits at high elevations contain groundwater of low F− values. Thus, the internal texture and grain size of geological formations, the burial depth of these formations and the water residence times are the factors determining the groundwater mineralisation and F− concentrations in the area. The study identified that the deep hydrothermal system has influence on the high F− groundwaters on the eastern and north-eastern flanks of Mount Meru.

INTENSIFICATION OF IRON REMOVING OF LOW ALKALINE GROUNDWATER

The problem of providing humanity with quality drinking water is exacerbated in the modern world. According to international organizations, a half of the world's population by 2025 will live in areas with significant water shortages. The sufficient standard of living and environmental safety is one of the citizens’ rights defined by the main law of Ukraine, which provides for the provision of quality drinking water in the required amounts and in accordance with established standards for the quality of drinking water. The Cabinet of Ministers of Ukraine has approved the concept of the state target social program "Drinking Water of Ukraine" for 2022-2026 for this purpose. The unsatisfactory state of water quality in Ukraine is associated with moral and physical depreciation of main resources, underfunding of water supply and sewerage industry. The situation is complicated by the lack of qualified personnel in rural areas. A large part of the inhabitants’ rural settlements use groundwater aquifers for their drinking purposes, which often contain excessive concentrations of total iron. The water treatment facilities were built according to standard designs using non-reagent methods for water iron removal in small settlements. The practice of individual treatment plants operation has shown their low efficiency, due to the unreasonable use of such methods. The improvement of existing water treatment schemes should be carried out taking into account the peculiarities of the physicochemical composition of water and the use of existing water treatment equipment. The purpose of the work is to improve the technological scheme of water iron removal and improve the quality of the filtrate. It is established that the reason for the unsatisfactory operation of treatment plants is due to the low alkalinity of groundwater. The rational types of alkalizing reagents and their calculated doses are substantiated by the results of experimental studies, the dependences of changes in the hydrogen index on the type and dose of reagents, residual alkalinity of water, the efficiency of water iron removal.

Uranium (VI) Adsorbate Structures on Portlandite [Ca(OH)2] Type Surfaces Determined by Computational Modelling and X-ray Absorption Spectroscopy

Portlandite [Ca(OH)2] is a potentially dominant solid phase in the high pH fluids expected within the cementitious engineered barriers of Geological Disposal Facilities (GDF). This study combined X-ray Absorption Spectroscopy with computational modelling in order to provide atomic-scale data which improves our understanding of how a critically important radionuclide (U) will be adsorbed onto this phase under conditions relevant to a GDF environment. Such data are fundamental for predicting radionuclide mass transfer. Surface coordination chemistry and speciation of uranium with portlandite [Ca(OH)2] under alkaline groundwater conditions (ca. pH 12) were determined by both in situ and ex situ grazing incidence extended X-ray absorption fine structure analysis (EXAFS) and by computational modelling at the atomic level. Free energies of sorption of aqueous uranyl hydroxides, [UO2(OH)n]2–n (n = 0–5) with the (001), (100) and (203) or (101) surfaces of portlandite are predicted from the potential of mean force using classical molecular umbrella sampling simulation methods and the structural interactions are further explored using fully periodic density functional theory computations. Although uranyl is predicted to only weakly adsorb to the (001) and (100) clean surfaces, there should be significantly stronger interactions with the (203/101) surface or at hydroxyl vacancies, both prevalent under groundwater conditions. The uranyl surface complex is typically found to include four equatorially coordinated hydroxyl ligands, forming an inner-sphere sorbate by direct interaction of a uranyl oxygen with surface calcium ions in both the (001) and (203/101) cases. In contrast, on the (100) surface, uranyl is sorbed with its axis more parallel to the surface plane. The EXAFS data are largely consistent with a surface structural layer or film similar to calcium uranate, but also show distinct uranyl characteristics, with the uranyl ion exhibiting the classic dioxygenyl oxygens at 1.8 Å and between four and five equatorial oxygen atoms at distances between 2.28 and 2.35 Å from the central U absorber. These experimental data are wholly consistent with the adsorbate configuration predicted by the computational models. These findings suggest that, under the strongly alkaline conditions of a cementitious backfill engineered barrier, there would be significant uptake of uranyl by portlandite to inhibit the mobility of U(VI) from the near field of a geological disposal facility.

Fluoride removal by hybrid cation exchanger impregnated with hydrated Al(III) oxide nanoparticles (HCIX-Al) with novel closed-loop recyclable regeneration system

A high concentrations of fluoride (F-) in groundwater limits drinking water supplies and results in fluorosis for hundreds of millions of people globally. A novel F- selective adsorbent (HCIX-Al) with a closed-loop regeneration system was developed, characterized, and assessed for its effectiveness in F- removal using both synthetic and natural groundwater. The system used an inexpensive polymeric macroporous cation exchanger doped with hydrous aluminum oxide (HAlO) nanoparticles. The HAlO nanoparticles were loaded into the matrix of the polymer phase by ion-exchange reactions between Al3+ and Ca2+ ions that bind with sulfonic acid functional groups of the cation exchangers. Hydrogen ions were generated in-situ during the HAlO formation process, creating acidic conditions inside the polymeric phase, which significantly increased F- sorption capacity (maximum sorption capacity = 5.75 mg/g at pH 7.5). While most other aluminum oxide-based F- selective sorbents suffer from low F- removal capacity at neutral to alkaline pH of typical groundwater, the HCIX-Al exhibited high F- sorption capacity even using alkaline (pH 8.5) groundwater. The treated water had a pH greater than 6.5 and non-detectable levels of aluminum (detection limit 0.002 mg/L), which complies with drinking water regulations. The HCIX-Al material can be reused by conditioning the exhausted material with a closed-loop regeneration chemical containing a recyclable 12% AlCl3 solution. The F- was precipitated from the regeneration system as CaF2 using lime at pH 11. This material, including a novel regeneration scheme, offered high F- removal capacity, cost-effectiveness, and provided a sustainable defluoridation process.

Study of the Diagenetic Characteristics of Uraniferous Sandstone and Its Relationship with Uranium Mineralization of the Bayin Gobi Formation, Lower Cretaceous Tamusu Deposit

The Tamusu uranium deposit in the southern part of the Bayin Gobi Basin is a typical hard-sandstone uranium deposit， but with very different diagenetic characteristics from other sandstone-type uranium deposits in northern China. The characteristics of uraniferous sandstone （K1b2-2） and its relationship to uranium mineralization in the Tamusu deposit have been studied In recent years by petrology， petrography， geochemical， scanning electron microscope （SEM） and so on. The detritus is composed of quartz （average content 2.38%） and feldspars （average content 50.33%）. The cements comprise gypsum （average content 12.57%）， ankerite （average content 3.20%）， ferruginous dolomite （average content 9.33%）， dolomite （average content 9.76%） and a small amount of complex matrix material （average content 2.62%）， causing both basal cementation and porous cementation. The diagenetic process is characterized by early stage B diagenesis and later stage A diagenesis， with alkaline water （pH 7.52）. Early stage B diagenesis is characterized by chemical cementation， water-rock interaction， dissolution and metasomatism. The formation sequence of the cements is ankerite-ferruginous dolomite-dolomite-gypsum； the carbonate cements， from center to edge， are automorphic ankerite， automorphic-hypidiomorphic ferruginous dolomite， and hypidiomorphic-xenomorphic dolomite. Gypsum forms last， with metasomatism of quartz， feldspar and dolomite. In this process， plagioclase is altered to albite due to the presence of Na+ in the highly saline water replacing Ca2+， with dissolution microcavities formed on the surface of the plagioclase and microcracks along cleavage planes. During early stage B diagenesis， no auto-cementation takes place， and clay minerals are affected by alkaline groundwater. During late stage A diagenesis， acidic surface water containing S O 4 2 - dissolves the carbonate cement， forming cavities. The uraniferous sandstone does not favor interlayer oxidation due to its low porosity and permeability. It is inferred that the genesis of uranium mineralization in the Tamusu deposit occurs by sedimentary diagenesis and interlayer oxidation.

Hydrochemical Characterisation of High-Fluoride Groundwater and Development of a Conceptual Groundwater Flow Model Using a Combined Hydrogeological and Hydrochemical Approach on an Active Volcano: Mount Meru, Northern Tanzania

This study characterises high-fluoride groundwater in the aquifer system on the flanks of Mount Meru, focusing on parts of the flanks that were only partially or not at all covered by previous research. Additionally, we analyse the impact of rainwater recharge on groundwater chemistry by monitoring spring discharges during water sampling. The results show that the main groundwater type in the study area is NaHCO3 alkaline groundwater (average pH = 7.8). High F− values were recorded: in 175 groundwater samples, the concentrations range from 0.15 to 301 mg/L (mean: 21.89 mg/L, median: 9.67 mg/L), with 91% of the samples containing F− values above the WHO health-based guideline for drinking water (1.5 mg/L), whereas 39% of the samples have Na+ concentrations above the WHO taste-based guideline of 200 mg/L. The temporal variability in F− concentrations between different seasons is due to the impact of the local groundwater recharge. We recommend that a detailed ecohydrological study should be carried out for the low-fluoride springs from the high-altitude recharge areas on the eastern and northwestern flanks of Mount Meru inside Arusha National Park. These springs are extracted for drinking purposes. An ecohydrological study is required for the management of these springs and their potential enhanced exploitation to ensure the sustainability of this water extraction practice. Another strategy for obtaining safe drinking water could be to use a large-scale filtering system to remove F− from the groundwater.

Groundwater suitability estimation for sustainable drinking water supply and food production in a semi-urban area of south India: A special focus on risk evaluation for making healthy society

The main objective of the present investigation is to estimate groundwater suitability for sustainable drinking water supply and food production in a semi-urban area of south India with a focus on risk assessment for making healthy society. As urbanization and industrialization make the water unfit for water supply and crop rising in most parts of the world this study is very much significant for the fast growing Edappadi region in the southern part of India. A total of sixty-nine groundwater samples were obtained during May 2019 (summer season) and analyzed for Total Dissolved Solids (TDS), Electrical Conductivity (EC), pH, potassium (K+), magnesium (Mg2+), sodium (Na+), calcium (Ca2+), bicarbonate (HCO3−), nitrate (NO3−), sulphate (SO42-), chloride (Cl−), and fluoride (F−). Fluoride and nitrate ranged from 0.38 to 3.23 mg/L and 12 to 136 mg/L with an average of 1.7 mg/L and 62 mg/L, respectively. About 36 % of samples occupying 80.74 km2 area surpass the allowable limit of fluoride (1.5 mg/L). Similarly, 42 % of samples occupying 98.75 km2 area surpass the allowable limit of nitrate as per WHO and BIS standards (45 mg/L). Correlation studies point out that groundwater contamination is happening due to manmade activities. The Improved Water Quality Index (IWQI) suggested that about 57% of groundwater samples can be used for drinking utility. The alkalinity of groundwater is within the suitable level for crops but groundwater salinity is high to very high in most of the areas. The Total Hazard Index (THI) showed that 72%, 59%, 33% and 29% and 86%, 77%, 51% and 43% of groundwater samples are within the non-carcinogenic health risk category based on fluoride and nitrate contents, respectively, for infants, children, teens, and adults. This study will help the decision and policymakers such as municipal corporation, pollution control board, public works department, water supply and drainage boards, agricultural and public health departments, etc., to use appropriate groundwater resources for providing safe drinking water supply and food production from crops. The findings of this study can aid in the development of appropriate management strategies by the above government departments for ensuring safe water supply and health protection measures for inhabitants.

Metal geochemistry in suspended and bed sediments in the eutrophic lowland Salado River basin (Buenos Aires, Argentina)

The Salado River is a lowland river that drains a vast and productive region of central Argentina into the Río de la Plata estuary. To evaluate water quality and the geochemistry of major and trace metals (Fe, Mn, Zn, Cu, Pb, Cr and Ni), water and sediments samples were collected along 500 km of the Salado River basin (n = 21). Waters were highly alkaline (pH: 8.8 ± 0.4), saline (5.2 ± 1.8 mS/cm), turbid (467 ± 237 NTU) and eutrophic (chlorophyll a: 187 ± 119 µg/l) with a decreasing gradient towards the river mouth reflecting the contribution of alkaline ground waters and eutrophication at the river head. Suspended particles (70–595 mg/l) were enriched in total organic carbon, nitrogen (5–7 times) and metals (1–4 times) relative to bed sediments, but metal concentrations were 2–10 times below the world river’s average. Consistently, metal concentrations in bed sediments were lower than sediment quality guidelines with an increasing downriver trend following the enrichment of clays. The enrichment factors (EF) and geoaccumulation index (Igeo), calculated with local background metal levels from the deepest layers of a dated sediment core, indicate a general prevalence of natural metal sources (EF < 1.5) with unpolluted sediments (Igeo ≤ 0), excepted for the Chivilcoy stream, which showed a significant enrichment of Zn, Cu and Pb (EF = 2–12; 0 ≤ Igeo ≤ 1) constituting a clear pollution hot spot in the basin.

Groundwater fluoride contamination in Coimbatore district: a geochemical characterization, multivariate analysis, and human health risk perspective

In this paper, fluoride geochemistry and health risk of groundwater in Coimbatore district is studied. The order of dominance of ions were HCO3 > Cl > SO4 > CO3 and Na > Ca > Mg > K. Alkaline groundwater and the dominance of HCO3 and Na is favourable for the fluoride enrichment. Around 46% of the samples, N–NE regions, have F− higher than permissible limit of 1.5 mg/L. Pink granites, charnockite and gneisses in lithology is the possible origin of F−. However, NO3 vs F plot shows positive trend in some samples, indicating anthropogenic inputs of F−. Correlation plots of F− was trending positive with pH, HCO3 and Na and but negative for Ca, indicating the control of these ions in F− mobility. This result is supported by undersaturation of fluorite and supersaturation of carbonates. Four significant principal components were derived, which have explained 87% of the total variation. PC1 has high factor loadings for EC, Ca, Mg Na, Cl, SO4, NO3 indicating both natural and anthropogenic influences. PC2, PC3 and PC4 have higher loading for pH and HCO3, K and HCO3 and F−, respectively, indicating geogenic factors in the F− enrichment. Human health risk assessment (HHRA) by ingestion and dermal pathways were calculated using Hazard Quotient HQ and Hazard Index (HI). 27% of males, 36% of females and 39% of the children have HI > 1, posing noncarcinogenic risks.

Does the natural carcinogen ptaquiloside degrade readily in groundwater?

BackgroundPtaquiloside (PTA) is a natural carcinogen found in bracken ferns. PTA is released from the plants via soil to surface and groundwaters from where humans can be exposed via drinking water. Primary degradation of PTA is due to hydrolysis with formation of pterosin B (PTB). Temperature and pH determine the rate of hydrolysis under pure experimental conditions. To assess the applicability of the experimental model to natural groundwaters, PTA degradation kinetics were examined in a range of natural groundwaters at environmentally relevant conditions.ResultsPTA was quantified by UPLC-MS/MS. Over an 80-day study period, PTA half-lives ranged from 6.5 to 47 days (natural pH; 8.0 °C). The fastest degradation was observed for the most alkaline groundwaters with pH of around 8. Rates of degradation were well predicted using an existing mathematical model for hydrolysis. However, deviations from this model were found, especially at the extremes of the examined pH-range (4.7–8.2). The degree of conversion of PTA to PTB was close to unity around neutral pH. However, at slightly acidic conditions, formation of PTB could only count for 9% of the degraded PTA, indicating formation of other products.ConclusionsDegradation of PTA in groundwater is determined by pH and temperature, and PTA can prevail for months under slightly acid to neutral pH conditions. The existing laboratory-based model for PTA hydrolysis is generally applicable for groundwaters but needs further validation at high and low pH.

Geochemical sources, hydrogeochemical behaviour of fluoride release and its health risk assessment in some fluorosis endemic areas of the Brahmaputra valley of Assam, India

High fluoride (F−) concentration was detected in groundwater samples collected from three districts of the Brahmaputra valley of Assam, Nagaon, Kamrup Metropolitan (M) and West Karbi Anglong. The maximum F− value of 9 mgL−1 was found in southern Nagaon. About 38% of the total groundwater samples exceeded the WHO permissible limit of 1.5 mgL−1. Petrographic analyses revealed that micaceous minerals such as biotite and muscovite are the dominant contributors of F− in Nagaon and West Karbi Anglong; in Kamrup (M), it is biotite. The alkaline groundwater played a significant role in governing the dissolution and mobilization of F− from its sources. Silicate weathering was more prominent in Nagaon and West Karbi Anglong, while carbonate weathering was dominant in samples collected from Kamrup (M). Both cationic (Ca2+/Na+) and anionic (F−/OH−) exchange processes occurred extensively across the studied hydrogeochemical regime. PHREEQC simulations revealed that fluorite and calcite undersaturation govern the F− values in groundwater. This finding is well supported by the results of the dissolution experiment of rock samples, where powered samples (<600 μm) were mixed in ultrapure water (pH = 7) for 2000 h. A prominent variation, which ranged between 0.22 and 0.95 mgL−1, was observed in the F− concentration for Nagaon's rock sample solution. The experimental results gave a glimpse of real-time rock-water interaction scenarios in the study area, revealing Ca2+ removal processes to primarily govern F− release into aquifers. A non-carcinogenic risk assessment study of F− indicated a potential risk for the exposed population, especially, children in the study area.

Ammonium Removal from Alkaline Groundwater Using A Dropping Nitrification Unit with Sponge or Biofringe Material

Ammonium Removal from Alkaline Groundwater Using A Dropping Nitrification Unit with Sponge or Biofringe Material

Nano-Hydroxyapatite Encapsulated inside an Anion Exchanger for Efficient Defluoridation of Neutral and Weakly Alkaline Water

The efficient removal of fluoride from weakly alkaline groundwater still remains a great challenge, owing to the high hydration energy of fluoride. Herein, we conducted in situ precipitation of nan...

Algal softening followed by ozonation: The fate of persistent micropollutants and natural organic matter in groundwater

A proof-of-concept study evaluates the performance of a novel strategy using photosynthetic microorganisms to soften groundwater instead of using caustic chemicals. The microalga Scenedesmus quadricauda was used to increase the pH of the groundwater via natural photosynthesis. This work applied softening as a pretreatment to ozonation of hard groundwater and mainly focused on investigating the multiple effects of algal softening on the degradation of persistent micropollutants upon subsequent ozonation. The algae-induced alkaline conditions (pH > 10) were favorable to catalyze the formation of OH radicals directly from O3 molecules. Moreover, algal softening removed the strong radical-scavenging carbonate species (HCO3− and CO32−) to a much greater extent than that achieved by chemical softening, which was attributed to the combination of mineral carbonation and metabolic CO2 reduction. The fate of the natural organic matter (NOM) was characterized with spectroscopy, chromatography, and bioassay, which indicates that algal treatment decomposed the NOM to be less susceptible to attack by OH radicals. Consequently, the ozonation of alkaline groundwater achieved a better removal of the micropollutant residues in groundwater. Carbamazepine and diclofenac were used as model chemicals of persistent groundwater contaminants and were almost completely removed with an addition of 1.25 mg O3 L−1 (0.63 mg-O3 mg-C−1).

Defluoridation by fluorapatite crystallization in a fluidized bed reactor under alkaline groundwater condition

Fluoride removal using fluidized bed crystallization (FBC) technology with phosphate rock (PR) and calcite as seed particles under an alkaline groundwater condition was investigated. The results indicated that the optimized operating parameters were: upflow velocity = 7.21 m h−1, Ca:P:F molar ratio = 10:4:1, and initial bed height = 120 cm. The turbidity of FBC reactor effluent was 1.5–5.9 NTU at the first phase, and then reduced to less than 1.0 NTU at the second phase, while the effluent fluoride level remained at a constant level (0.4–0.9 mg L−1) during the entire procedure. The exfoliation and dissociation of newly-generated nuclei from the seed particles surfaces after aging and drying for three days could serve as an additional filtration layer (located on the top of the expanded bed with a thickness of 1.0 cm) to intercept the fine particles in the second phase. Consequently, a rational strategy for fluidized bed operation was provided, which focused on turbidity control and fluoride removal. FBC performance can be divided into ripening and running periods according to the evolution of turbidity in effluent. Fluoride removal was achieved by the formation of amorphous calcium phosphate and then recrystallization into fluorapatite and/or FCO3-apatite and induced crystallization of fluorapatite and/or FCO3-apatite. The FBC process with additional filtration function of dissociated newly-generated nuclei could be regarded as a cleaner technology for fluoride removal; additionally, the spent seed particles could be utilized as resources in the phosphatic fertilizer industry. However, the fertilizer industry should continue to recover fluoride for further economic benefits.

Hydrochemical Characteristic of Groundwater and Its Impact on Crop Yields in the Baojixia Irrigation Area, China

While irrigated crops produce much higher yields than rain-fed crops, the ionic components of irrigation water have important effects on crop yield. Groundwater is widely used for irrigation in the Baojixia irrigation area in China. The chemical characteristics and water quality of groundwater in the Baojixia irrigation area were analyzed and evaluated to study the impact of groundwater quality on crop yield. Results showed cations in the groundwater to mainly be Na+, Ca2+, and Mg2+, whereas the anions are mainly HCO3−, SO42−, and Cl−. Water-rock interaction and cation exchange were identified as the main factors affecting hydrogeochemical properties from west to east. The study found salinity and alkalinity of groundwater in the western region of the study area to be low, and therefore suitable for irrigation. Groundwater in the eastern part of the study area was found to have a medium to high salinity and alkalinity, and is therefore not recommended for long-term irrigation. The groundwater irrigated cultivation of wheat and corn in the research area over 2019, for example, would have resulted in a drop in the annual crop output and an economic loss of 0.489 tons and 0.741 × 104 yuan, respectively. Irrigation using groundwater was calculated to result in the cumulative loss of crop yields and an economic loss of 49.17 tons and 80.781 × 104 yuan, respectively, by 2119. Deterioration of groundwater quality will reduce crop yields. It is recommended that crop yields in the study area be increased by strengthening irrigation water management and improving groundwater quality.

Nitrogen fertilization of soils fuels carbonate weathering and translocation in calcareous watersheds

In calcareous watersheds, groundwater alkalinity results largely from dissolution of carbonate minerals in soils. The alkalinity increases initially approximately in proportion to nitrate (NO3−) concentration and eventually approaches an apparent maximum of approximately 8 mmol L−1 at high NO3− concentrations. This close positive relationship between alkalinity and NO3− concentration appears to be predominantly a result of three processes: (i) mineralization of organic nitrogen fertilizer, (ii) exchange of OH− and H+ during the uptake of NO3− or ammonium by crop plants, and (iii) CO2 released by roots as a result of fertilizer-stimulated plant growth. We suggest that the asymptotic approach to a maximum groundwater alkalinity at NO3− concentrations exceeding 0.25 mmol L−1 may be caused by (i) a maximum possible areal crop production at excessive N fertilization and (ii) an increasing CO2 loss to the atmosphere due to the increasing CO2 production in the soil. Our analysis provides a general understanding and quantitative prediction of steady-state groundwater NO3− concentration, alkalinity, pH, the degree of CO2 supersaturation in the soil, and soil CO2 emissions to the atmosphere. The positive correlation between alkalinity and NO3− concentration observed in groundwaters persists in rivers and lakes. We conclude that an economically efficient agricultural practice that avoids over-fertilization might accelerate the in-soil carbonate weathering rate up to approximately threefold compared to unfertilized soils, but it will not jeopardize the use of aquifers for drinking water.

Comprehensive environmental monitoring and assessment of protected wetland and lake water quality in Croatia and Serbia.

The irresponsible and uncontrolled use of water bodies for long period of time leads to pollution and water quality degradation, particularly in sensitive protected terrestrial and aquatic areas as lakes and wetlands situated in the vicinity of human habitations. Vulnerable ecosystems, the Lake Zobnatica, Serbia and the Wetlands of Tompojevci, Croatia, are characterized as protected natural areas and are surrounded by agricultural land and developed anthropogenic activities. In the samples of surface, ground water and agricultural runoff the key physicochemical parameters were determined. The results of analysed water characterized the Lake water quality class as IV-V. The measured parameters for the Lake show the high values of nutrients, particularly in ground water and agricultural runoff, pH value higher than 8.5 and lower values of dissolved oxygen (DO) than 5mg/L in the most of the examined water samples. According to the results and maximal allowable values Wetlands belong to the class V water quality, with high trophic state and poor eco-status according to Croatian and Serbian national Laws and By-Laws. The pH value higher than 8.5 of ground water in Wetlands of Tompojevci signifies alkaline groundwater. The maximum concentrations of orthophosphates in ground water and agricultural runoff (10.875 and 10.886mg/L, respectively) imply the high level of agricultural pollution. Research study of water samples has been performed from 2017 to 2019 within the Interreg IPA CBC Croatia-Serbia Project AF_HR-RS135_SeNs_Wetlands. This type of parallel and comparative research in Serbia and Croatia was conducted for the first time.

The critical role of oxidative debris in the adsorption and desorption of Pb(II) to graphene oxides under alkaline groundwater conditions

The application of graphene oxide (GO) and its derivatives as adsorbents to remove heavy metal contaminants from industrial wastewaters has attracted increasing attention worldwide. Here, we investigated the role of oxidative debris (OD) on GO surfaces in the adsorption of Pb(II) under natural conditions. OD attached to GO surfaces can be removed under alkaline conditions, and plays a critical role in the adsorption of Pb(II) by GO. Specifically, the maximum adsorption capacity of Pb(II) on GO decreased from 926.50 to 357.64 mg·g−1 after OD removal. Under simulated natural alkaline groundwater conditions, some adsorbed Pb(II) was released with OD that was stripped from spent GO adsorbents (i.e., GO-Pb(II)). Further, batch experiments indicated that 1.30−1.43% of adsorbed Pb(II) could stably disperse in water as dissolved PbCO3 and OD-Pb(II) complexes, at a pH of ~8.3. The stripping and dissociation of OD under alkaline conditions promoted the mobility of Pb(II) on GO adsorbents. This study enhances our understanding of heavy metal transport in natural alkaline groundwater environments that contain GO particles.

Silica fouling during groundwater RO treatment: The effect of colloids’ radius of curvature on dissolution and polymerisation

Silica fouling during groundwater reverse osmosis (RO) treatment can have a significant impact on filtration performance. To better understand this phenomenon, the equilibrium kinetics of amorphous colloidal silica were studied at conditions relevant to RO of silica-rich alkaline groundwater. The impact of particle size was investigated using synthetic monodisperse silica nanoparticles. Bench scale experiments were conducted by monitoring dissolved silica concentration of aqueous suspensions of colloids of 100 and 300 nm diameter and pH 8.5 to 9.5. The equilibrium data was determined from existing established rate law equations. This study concluded that surface energy has a major impact on silica dissolution rate constant, particularly for colloidal silica. Observations of Ostwald ripening in bidisperse silica dispersions further confirmed these results, which indicate that dissolution and redeposition is responsible for the problematic silica fouling behaviour during RO treatment. 2D modelling based on inferred equilibrium data allows visualization of scale layer growth in agreement with cross-sectional scanning electron micrographs of autopsied membranes.