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AbstractMicrobially induced calcium carbonate precipitation (MICP) is a geotechnical solution with a low ecological footprint. Conducting MICP inside a porous medium can be used for biocementation. Despite many models and tools being published for simulating MICP, a computationally light, easy-to-use, and freely-available simulation tool is lacking. In this work, we present a reactive transport model using the native capabilities of the Phreeqc software (a free and open-source geochemical solver). We model MICP inside a column packed with sands and inoculated with bacteria. Our model is a one-dimensional representation of the column that accounts for the kinetics of precipitation and ureolysis as well as porosity change. We verify our model by simulating previously published MICP experiments to show our model gives reasonably well predictions. Sensitivity analysis of our model’s input parameters and discussions around their range are presented. Additionally, we calculate the supersaturation of calcite as a function of space and time—a feature that could be important for precipitation, but it is commonly ignored in MCIP modeling. Our scripts developed for running simulations are accessible publicly. Due to the low computational effort required for the simulations, our Phreeqc scripts can be used to design and test different treatment recipes for conducting MICP. [ABSTRACT FROM AUTHOR]

The employment of granular ferric iron-(oxy)hydroxides, a well-known economic and effective method, lowers arsenic concentrations in different water types. However, for direct application in polluted groundwaters, there is a need to develop new injectable adsorbents for aquifers that could also neutralize acidic media. In this context, a granular ferric hydroxide-calcite (GFH-C) adsorbent was size-reduced to 0.4–50 µm by sonication with the aim of improving (i) the adsorption of As(III) and As(V) at different pHs and (ii) the pH control through the dissolution of calcite. Batch experiments were conducted to determine As(III) and As(V) adsorption isotherms and kinetics, as well as calcite dissolution kinetics, using GFH-C of two sizes (granular and sonicated). Results showed that the arsenic binding capacity of sonicated adsorbents did not improve significantly. On the contrary, the As(III) and As(V) adsorption kinetics improved with the sonication, as in the case of calcite dissolution kinetics. The dissolution of calcite from the adsorbent made the water pH increase to around 9.2–9.4. The sonicated adsorbent offers an advantage in depolluting As-containing groundwater due to its smaller size, which is linked with faster arsenic adsorption and effective acidic water neutralization. [ABSTRACT FROM AUTHOR]

The expanding geothermal energy sector still faces performance issues due to scalings in pipes and surface level installations, which require elevated operation pressure levels and costly maintenance. For facilities in the North Alpine Foreland Basin, the precipitation of CaCO 3 is the main problem which is a consequence of the disruption of the lime-carbonic acid equilibrium during production. The formation of gas bubbles plays a key role in the scaling process. This work presents experiments in a bubble column to quantify the effects of gas stripping on carbonate precipitation and an extension of PhreeqC to include kinetic exchange between a gas phase and water for the simulation of the experimental results. With the same hybrid model not only precipitation of CaCO 3 but also the dissolution of scalings by the injection of CO 2 could be quantified. The bubble column was filled with tap water and brine. By varying the ionic strength of the solution, a wider range of geothermal waters was covered. Air and CO 2 were introduced at the bottom. The precipitates built on the column wall were analyzed with Raman spectroscopy: injecting air into tap water at low ionic strength led to the formation of aragonite with 59.8% of the precipitates remaining at the column wall and the rest as particles in dispersion. At moderate ionic strength the dominant polymorph was calcite and 81.5% of the crystals were attached to the wall. At high ionic strength precipitation was inhibited. The presence of crystallization nuclei reduced the time for precipitation, but not the amount of scalings formed. Injecting CO 2 into the solution completely removed the scalings from the column wall. The model and its experimental backup lay the foundation for a process-based prediction of the scales (not only) in geothermal systems. [ABSTRACT FROM AUTHOR]

In hydrating Portland cements, more than one of the AFm family of calcium aluminates may exist. Depending on the amount of carbonate and sulfate present in the cement, the most common phase to precipitate is monosulfate, monocarbonate and/or hemicarbonate. It has been reported in the literature that hemicarbonate often appears in measurements such as XRD but not predicted to form/equilibrate in thermodynamic models. With the ongoing use of commercial cements such as CEM I and CEM II containing more and more limestone, it is important to understand which hydrate solids physically precipitate and numerically predict over time. Using 27 cement samples with three w/c ratios analysed at 1, 3 and 28 days, this paper shows that although hemicarbonate was observed in a hydrating commercial Portland cement, as well as being predicted based on its carbonate (CO2/Al2O3) and sulfate (SO3/Al2O3) ratios, thermodynamic analysis did not predict it to equilibrate and form as a solid hydrate. Regardless of the w/c ratio, thermodynamic analysis did predict hemicarbonate to form for calcite contents < 2 wt.%. It appears that the dominant stability of monocarbonate in thermodynamic models leads to it precipitating and remaining as a persistent phase. [ABSTRACT FROM AUTHOR]

In this innovative study, biogas has been associated with calcium carbonate [CaCO3] to promote the precipitation of fluorite, aiming at the treatment of wastewater with high content of fluoride. The work associates distinct sources of calcium and CO2 for the precipitation of fluorite according to previous simulation with the free software PHREEQC. Considering the reaction at equilibrium, the minimal predicted Ca dosage was 215 mg/L, lower than the 430 mg/L that was experimentally determined, independent of Ca source. The simultaneous use of CaCO3 and CO2 from distinct gas sources (pure CO2, 1:1 CO2:N2, and biogas) exhibited high performance permitting the reduction of fluoride content from 134 to 10 mg/L, with low gas consumption. The biogas consumption of 66.0 mmol/L, equivalent to 33.4 mmol/L of CO2 (1.47 kgCO2/m3treated wastewater) was predicted, indicating that the biogas storage bag of 700 L would be able to treat 469 L of wastewater. Furthermore, the inert fraction of biogas (CH4) did not impact the reaction and it may be used after the reaction as an alternative source of power, equivalent to 8.25 kWh/m3treated wastewater. Final solids were composed by fluorite and non-dissolved calcite, confirming the predictions obtained by PHREEQC. [ABSTRACT FROM AUTHOR]

The majority of scales observed at geothermal facilities exploring the Malm Aquifer in the Bavarian Molasse Basin are carbonates. They form due to a disruption of the lime–carbonic acid equilibrium during production caused by a reduction of the partial pressure of carbon dioxide due to pressure change and degassing. These scales are found at the pumps, production pipes, filters, heat exchangers, and occasionally in the injection pipes. In this study, scales of all sections of geothermal facilities were taken. The database consists of scale samples from 13 geothermal pumps, 6,000 m production pipe (sample interval 10 - 12 m), 11 heat exchanger revisions, 2 injection pipes, and numerous filter elements. The samples were analyzed by SEM-EDX, XRD, Raman spectroscopy, and acid digestion to assess their chemical and mineralogical composition. From direct gauge measurements at six facilities during pump changes, scale rates were determined along the production pipes. From indirect measurements (multifinger caliper measurements) scale rates are derived for the region below the pump. Hydrochemical analyses from the wellhead were taken from 13 sites to feed the hydrogeochemical models. The calcite scale rates in the production pipes increase from the pump to the wellhead, where they reach 1.5 - 4.1 μ mol/( m 2 · s). Scale rates below the pump reach up to 1.5 μ mol/( m 2 · s). Given the slight change of hydrochemistry on the rise through the production pipe, where < 4 % of dissolved calcium ions precipitate as scale, scale rates cannot be derived from water samples at the wellhead, but require direct gauge measurements. The small amount of precipitation, together with fully turbulent conditions suggests that all measured rates are controlled by the surface-reaction of calcite crystallization following the nomenclature of Appelo and Postma (2004). Two approaches are used for the modeling of the scale rates. The first approach is based on hydrogeochemical modeling with PHREEQC. Scale rates calculated by this method are one order of magnitude higher than the measured ones. The second approach is based on correlations between the measured scale rates at the wellhead at six facilities and identified thermodynamic scale drivers (Δ log (p CO 2 ), Δ total pressure, Δ pH, and SI calcite ). The correlations allow linear regressions which are used for the prediction of the scale rate at the wellhead, along the whole production pipe, and below. The modeling results show that scale prediction based on the new regressions that rely on thermodynamic scale drivers works better than existing hydrogeochemical models, already without implementation of kinetic parameters ( CO 2 -stripping and magnesium inhibition). [ABSTRACT FROM AUTHOR]

Sorption of cobalt (CoII) ions from aqueous solution by waste calcite, kaolinite and zeolite was investigated in a series of batch experiments. The sorption capacity of the sorbents was a function of the initial solution pH, contact time and sorbent/sorbate ratio. For these three sorbents, the kinetic and isotherm experimental data were well fitted to pseudo-second-order and Langmuir equations, respectively. The maximum sorption capacity (mg g−1) of Co(II) was 4.67, 3.76 and 2.23 for waste calcite, zeolite and kaolinite, respectively. Desorption experiments showed that the desorption capacities were in the order of zeolite > kaolinite > waste calcite. The equilibrium and kinetic results indicated that waste calcite had the best performance for the removal of Co(II) compared to zeolite and kaolinite. To simulate and predict Co(II) sorption mechanisms, the surface complexation and cation exchange models in PHREEQC program were used. The model results suggested that the main mechanisms of Co(II) sorption on waste calcite and zeolite were surface complexation and cation exchange, respectively. In the case of kaolinite, the model predicted that both mechanisms were involved in the sorption of Co(II), but the surface complexation was the predominant mechanism. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the negative impact of a newly-formed manganese (Mn)-layer on calcite dissolution in the long-term operation of a calcite contactor. Simultaneous removal of Mn and remineralization of soft water in an up-flow calcite contactor was conducted and led to a progressive loading of Mn into the calcite matrix. The calcite contactor demonstrated high Mn removal; however, the hardness release decreased from 32 to 20 mg CaCO 3 L−1 after 600 h of operation on a high Mn concentration (5 mg L−1) feed. For an elevated Mn concentration (i.e. 5 mg Mn L−1) in the feed water, the coated layer was mainly composed of Mn which inhibits the mass transfer from the calcite core to the liquid phase. The superficial layer was identified as 5.2% Mn oxides (MnOx) by X-ray photoelectron spectroscopy (XPS). Therefore, it is postulated that Mn removal starts with an ion exchange sorption reaction between soluble Mn2+ from aqueous phase and Ca2+ from the CaCO 3 matrix which is followed by a slow recrystallization of MnCO 3 into MnO 2. On the other hand, when the Mn content in the feed water was lower (i.e. 0.5 mg Mn L−1), a considerably lower amount of MnOx was detected on the coated media. For all the examined conditions, the formation of this coating improved Mn removal due to the autocatalytic nature of the adsorption/oxidation of dissolved manganese by MnOx. A mechanistic model based on calcite dissolution and the progressive formation of a MnO 2 layer was implemented in PHREEQC software to predict the reduction in hardness release expected in long-term operation. The model was calibrated with experimental data and resulted in realistic breakthrough curves. In order to accurately predict the pH of the effluent stream, a slow-rate recrystallization of MnCO 3 into MnO 2 was implemented (compared to the fast precipitation of MnO 2 or the absence of MnO 2 formation). Image 1 • Calcite removed >95% of Mn2+, effluent concentration reached ≤45 μg Mn2+L−1. • At 5.0 mg Mn2+ L−1, Mn integrated into calcite and then recrystalized to MnOx. • Newly-formed MnOx layer deteriorates calcite dissolution. • At 0.5 mg Mn2+L−1, sorbed Mn incorporates into calcite matrix. [ABSTRACT FROM AUTHOR]

The first version of the P-Hydroslag model for numerical simulations of steel slag filters is presented. This model main original feature is the implementation of slag exhaustion behavior, crystal growth and crystal size effect on crystal solubility, and crystal accumulation effect on slag dissolution. The model includes four mineral phases: calcite, monetite, homogeneous hydroxyapatite (constant size and solubility) and heterogeneous hydroxyapatite (increasing size and decreasing solubility). In the proposed model, slag behavior is represented by CaO dissolution kinetic rate and exhaustion equations; while slag dissolution is limited by a diffusion rate through a crystal layer. An experimental test for measurement of exhaustion equations is provided. The model was calibrated with an experimental program made of three phases. Firstly, batch tests with 300 g slag sample in synthetic solutions were conducted for the determination of exhaustion equation. Secondly, a slag filter column test fed with synthetic solution was run for 623 days, divided into 9 cells and sampled at the end of the experiment. Finally, the column was dismantled, sampled and analyzed with XRD, TEM and SEM. Experimental column curves for pH, oPO 4 , Ca and inorganic carbon were well predicted by the model. Crystal sizes measured by XRD and TEM validated the hypothesis for homogeneous precipitation while SEM observations validated the thin crystal layer hypothesis. A preliminary validation of the model resulted in successful predictions of a steel slag filter longevity fed with real wastewater. [ABSTRACT FROM AUTHOR]

This study focuses on the investigation regarding impact of Ba2+ and Cd2+ on defluoridation by calcite because Spent Pot Lining (SPL, a byproduct generated during the aluminum production) leachate is a complex chemical cocktail. To better understand defluoridation processes and replicate the situation in the field, column tests were conducted. Results indicate that presence of neither Ba2+ nor Cd2+ has significant impact on fluoride retention when compared with the blank (no metals) because the relative concentration values ( F/Fo) as well as the transport parameters like retardation factor ( Rc) and distribution coefficient ( Kd) are almost the same. PHREEQC, a geochemical software, was then applied to simulate the column test processes ( i.e., 1D reactive/transport processes) based on the mechanism of CaF2 precipitation followed after calcium dissolution. As indicated in the batch reactor test, addition of Cd2+ may inhibit calcite dissolution; therefore, a new phase 'CalciteSupp' (a phase representing the suppressed solubility of calcite) was defined. Modeling results exhibit good predictions of observed data, and the log_k solubility of the hypothetical 'CalciteSupp' equilibrium phase implies the suppression of calcite dissolution in the metal samples. Moreover, saturation indices obtained by modeling indicate the precipitation of CdCO3 and BaCO3, revealing that addition of metals may influence fluoride removal at some point during the process, but has no big impact on overall defluoridation. The initial development of the PHREEQC geochemical model presented shows potential in being able to be used as a predictive tool for the design of fluoride remediation strategies, such as permeable reactive barriers, and transport parameters obtained on the basis of the column tests, and CXTFIT (a computer program for evaluating and estimating transport parameters) modeling can be applied in practical design. [ABSTRACT FROM AUTHOR]

Lacustrine carbonate deposits with spherulitic facies are poorly understood, but are key to understanding the economically important “Pre-Salt” Mesozoic strata of the South Atlantic. A major barrier to research into these unique and spectacular facies is the lack of good lacustrine spherulite-dominated deposits which are known in outcrop. Stratigraphy and petrography suggest one of the best analogue systems is found in the Carboniferous of Scotland: the East Kirkton Limestone. Here we propose a hydrogeochemical model that explains why the CaCO 3 , SiO 2 , Mg-Si-Al mineral suite associated with spherular radial calcite facies forms in alkaline lakes above basaltic bedrock. Demonstrating links between igneous bedrock chemistry, lake and spring water chemistry and mineral precipitation, this model has implications for studies of lacustrine sediments in rift basins of all ages. Using empirical and theoretical approaches, we analyze the relationship between metal mobilization from sub-surface volcaniclastic rocks and the potential for precipitation of carbonate minerals, various Mg-bearing minerals and chalcedony in a lacustrine spherulitic carbonate setting. This suite of minerals is most likely formed by in-gassing of CO 2 to a carbon-limited alkaline spring water, consistent with the reaction of alkali igneous rocks in the subsurface with meteoric groundwater. We suggest that an analogous system to that at East Kirkton caused development of the ‘Pre-Salt’ spherulitic carbonate deposits. [ABSTRACT FROM AUTHOR]

A study of surface water chemistry evolution was conducted by multivariate statistical analysis and inverse geochemical modelling using the PHREEQC computer program. Using hierarchical cluster analysis the 14 sampling sites were classified into three groups (recharge, transition and discharge areas). Water chemistry changed along a flow path so that waters with Ca–HCO3 and Mg–Cl composition changed to Mg–Cl–HCO3 waters. The order of abundance of the major cations was Mg > Ca > Na > K. Their average concentrations were 21, 19, 3.6 and 2.5 mg L-1, respectively. Inverse geochemical modelling along flow paths indicated that the dissolution of sylvite and kaolinite, and precipitation of feldspars and andalusite, happened with Na entering the solution and Ca, Mg and K leaving the solution. Editor D. Koutsoyiannis; Associate editor not assigned [ABSTRACT FROM AUTHOR]

Abstract: Ettringite–gypsum sludge, formed by neutralization of acid mine drainage with lime, has been stored temporarily in the open pit of a uranium mine that floods periodically. The present study characterized samples of this sludge, named according to the time of placement as Fresh, Intermediate, and Old. Standard leaching and sequential extraction procedures assessed the associations and stabilities of U, Zn, Fe, Mn, and other contaminants in the solid phases. Corresponding mineralogical transformations associated with sludge weathering were modeled using PHREEQC. The main crystalline phases were ettringite, gypsum and calcite; the minor constituents were fluorite and gibbsite. This mineral assemblage could be attributed to the incongruent dissolution of ettringite to form gibbsite, calcite, and gypsum. Sequential extractions indicated high contents of U, Ca, SO4, and Zn in the water-soluble (exchangeable) and carbonate fractions. Thus, the analytical and modeling results indicated that despite being classified as non-toxic by standard leaching protocols, the minerals composing the sludge could be sources of dissolved F, SO4, Fe, Zn, Mn, U, and Al under various environmental conditions. Decommissioning strategies intended to prevent contaminant migration will need to consider the chemical stability of the sludge in various environments. [Copyright &y& Elsevier]

Abstract: The mineral fixation of CO2 has a significant but highly varying relevance in carbon capture and storage (CCS) models. One reason is the impact of the minerals’ concentrations, but the choice of the thermodynamic database on which the geochemical model calculations are based, has probably an equivalent or even more important impact on the model results. In this example, natural groundwater and a mineral assemblage consisting of calcite, dolomite, albite, and K-feldspar in two different concentrations were used to perform scenario simulations with the geochemical model PHREEQC. The four different thermodynamic datasets phreeqc.dat, wateq4f.dat, llnl.dat, and minteq.dat were used to calculate the amount of CO2 which could be fixed through the reactions with the described minerals. In the simulations using low reactive mineral concentrations (1–2.5 wt%), the range of the amount of fixed CO2 in the mineral and dissolved phases lies between 0.29 and 0.31 mol/kgw for the four chosen thermodynamic databases, which reflects the complete transformation of albite and K-feldspar to secondary minerals in all the simulations and only minor differences in the amounts of dissolved calcite and dolomite. On the other hand, the amount of fixed CO2 shows a variance of between 1.51 and 2.17 mol/kgw when using high reactive mineral concentrations (10–25 wt%). Especially the transformation of the K-feldspar depended significantly on the chosen thermodynamic database which affected the calculated product minerals of the reaction. Using the phreeqc.dat, the wateq4f.dat, and the minteq.dat databases, the amount of CO2 fixation due to its reaction with K-feldspar is up to six times larger than the amount calculated with the llnl.dat database. It is currently not clearly possible to decide which thermodynamic database should be used to receive the most realistic modeling results. Consequently, the uncertainty in the modeling results due to varying thermodynamic databases should be regarded in addition to the uncertainty resulting from factors like the heterogeneous distribution of mineral phases in the aquifers. It still has to be discussed whether a risk-based approach offers a secure way of modeling CCS related scenarios or whether worst-case scenarios have to be applied. [Copyright &y& Elsevier]

Abstract: A borehole in the Callovo–Oxfordian clay rock in ANDRA''s underground research facility was sampled during 1 year and chemically analyzed. Diffusion between porewater and the borehole solution resulted in concentration changes which were modeled with PHREEQC''s multicomponent diffusion module. In the model, the clay rock''s pore space is divided in free porewater (electrically neutral) and diffuse double layer water (devoid of anions). Diffusion is calculated separately for the two domains, and individually for all the solute species while a zero-charge flux is maintained. We explain how the finite difference formulas for radial diffusion can be translated into mixing factors for solutions. Operator splitting is used to calculate advective flow and chemical reactions such as ion exchange and calcite dissolution and precipitation. The ion exchange reaction is formulated in the form of surface complexation, which allows distributing charge over the fixed sites and the diffuse double layer. The charge distribution affects pH when calcite dissolves, and modeling of the experimental data shows that about 7% of the cation exchange capacity resides in the diffuse double layer. The model calculates the observed concentration changes very well and provides an estimate of the pristine porewater composition in the clay rock. [Copyright &y& Elsevier]

Abstract: Inverse geochemical modeling from PHREEQC, and multivariate statistical methods were jointly used to define the genetic origin of chemical parameters of groundwater from the Voltaian aquifers in the Afram Plains area. The study finds, from hierarchical cluster analysis that there are two main hydrochemical facies namely the calcium–sodium–chloride–bicarbonate waters and the magnesium–potassium–sulfate–nitrate waters in the northern and southern sections, respectively, of the Afram Plains area. This facies differentiation is confirmed by the distribution of the SO4 2−/Cl− ratio, which associates groundwater from the northern and southern sections to areas influenced by contact with evaporites and seawater, respectively. Principal component analysis (PCA) with varimax rotation using the Kaiser criterion identifies four principal sources of variation in the hydrochemistry. Mineral saturation indices calculated from both major ions and trace elements, indicate saturation—supersaturation with respect to calcite, aragonite, k-mica, chlorite, rhodochrosite, kaolinite, sepiolite, and talc, and undersaturation with respect to albite, anorthite, and gypsum in the area. Inverse geochemical modeling along groundwater flowpaths indicates the dissolution of albite, anorthite and gypsum and the precipitation of kaolinite, k-mica, talc, and quartz. Both the PCA and inverse geochemical modeling identify the incongruent weathering of feldspars as the principal factors controlling the hydrochemistry in the Afram Plains area. General phase transfer equations have been developed to characterize the geochemical evolution of groundwater in the area. A very good relationship has been established between calcite and aragonite saturation indices in the Afram Plains area, with R 2=1.00. [Copyright &y& Elsevier]

Thermal water of Thermopylae and from other geothermal fields located in the southern part of the Sperchios basin (central Greece) are characterized by high salinity (total dissolved salts, or TDS, range from 1.2 to 30.3 g L−1) associated with a degassing of CO2. To determine the mineralization processes, geochemical and isotopic investigations (major elements, 18O, 2H and 13C) have been carried out upon 17 thermal waters from springs and boreholes. This study emphasizes that all the thermal waters result from the mixing of a seawater end-member, several fresh water components depending on the field location, and a mantle-derived CO2 rising upward through an E–W fault system. The seawater identified in the thermal mixture is likely to be evolved Aegean seawater (ASW). Once intruded into the basin sediments, the trapped seawater has its chemical content modified by both water–rock interactions and massive dissolution of the deep CO2 (pCO2 of 100.5 atm). The modelling performed with PHREEQC indicates that the anomalous major ion ratios measured in the so-called evolved ASW are explained by the dissolution of calcite and dolomitization process associated to precipitation of gypsum within the thermal aquifer. [ABSTRACT FROM AUTHOR]

The article presents abstracts of several studies on environmental geochemistry. Some of the abstracts presented include "Biogeochemical Cycling of Nutrients in Karstic Catchments, Southwestern China: Linkages to Changes of Eco-Environments," by Congqiang Liu and "Emerging Organic Contamination in China," by Guibin Jiang and colleagues.

The calcium carbonate (CaCO3) precipitation potential (CCPP) can predict the potential for corrosion and lime scaling in drinking water systems. CCPP can be calculated by different standards, but none of these consider all of the conditions in drinking water systems where temperatures can reach 100 °C and the water exchanges CO2 with the atmosphere. We provided and demonstrated a procedure for CCPP calculations using the open-source software PHREEQC with the phreeqc.dat database at temperatures relevant for drinking water systems (10–90 °C) and for open systems in equilibrium with atmospheric CO2. CCPP increased by 0.17–1.51 mmol/kg when the temperature was increased from 10 °C to 90 °C and increased by 0.22–2.82 mmol/kg when going from closed to open systems at 10 °C. Thus, CaCO3 precipitation may be underestimated if CCPP is only considered for the lower sample temperature and for closed systems. On the other hand, CCPP10 decreased by 0.006–0.173 mmol/kg when including the ionic species from the German DIN 38404-10 standard in addition to calcium, alkalinity and pH, indicating that all relevant ionic species should be included in CCPP calculations. CCPP values should always be reported with the calculation procedure and temperature to avoid inconsistency in literature. [ABSTRACT FROM AUTHOR]

The objective of this study was to propose a conceptual model of clogging in alkaline granular filters. Two slag columns were operated for 600 days and monitored using piezometers and tracer tested at regular intervals. The type of influent (organic or inorganic) affected the loss of effective porosity in the filters. Well organized and loose crystal structures were observed by scanning electron microscopy in columns with inorganic and organic influents, respectively. It was postulated that the formation of crystals in unorganized structures results in confined voids that are not accessible for water flow, thus accelerating porosity loss. The effect of the combination of chemical clogging and biofilm on the porosity loss is higher than the effect of these two factors separately. The Kozeny-Carman equation for hydraulic conductivity could not efficiently predict the evolution of head losses in the column fed with an inorganic influent. The crystal structure and connectivity in the presence of homogeneous or heterogeneous precipitation are concepts that could improve predictions of hydraulic conductivity. The results of this study highlighted the importance of the inlet zone on the development of pressure head in alkaline granular filters. Future research on clogging should focus on precipitation mechanisms in the inlet zone and on the design of the feeding system. [ABSTRACT FROM AUTHOR]

The objective of this work was to demonstrate the removal of the phosphorus and carbon dioxide capture potential of a conventional septic system upgraded with a sidestream steel slag filter used in recirculation mode. A pilot scale sidestream experiment was conducted with two septic tank and drainfield systems, one with and one without a sidestream slag filter. The experimental system was fed with real domestic wastewater. Recirculation ratios of 25%, 50% and 75% were tested. Limestone soils and non-calcareous soils were used as drainfield media. The tested system achieved a satisfactory compromise between phosphorus removal and pH at the effluent of the septic tank, thus eliminating the need for a neutralization step. The phosphorus removal efficiency observed in the second compartment of the septic tank was 30% in the slag filter upgraded system, compared to −3% in the control system. The slag filter reached a phosphorus retention of 105 mg/kg. The drainfield of non-calcareous soils achieved very high phosphorus removal in both control and upgraded systems. In the drainfield of limestone soil, the slag filtration reduced the groundwater phosphorus contamination load by up to 75%. The removal of chemical oxygen demand of the drainfields was not affected by the pH rise induced by the slag filter. Phosphorus removal in the septic tank with a slag filter was attributed to either sorption on newly precipitated calcium carbonate, or the precipitation of phosphate minerals, or both. Recirculation ratio design criteria were proposed based on simulations. Simulations showed that the steel slag filter partly inhibited the biological production of carbon dioxide in the septic tank. The influent alkalinity strongly influenced the recirculation ratio needed to raise the pH in the septic tank. The recirculation mode allowed clogging mitigation compared to a mainstream configuration, because an important part of chemical precipitation occurred in the septic tank. The control septic tank produced carbon dioxide, whereas the slag filter-upgraded septic tank was a carbon dioxide sink. [ABSTRACT FROM AUTHOR]

Understand the origin of the exotic strontium-rich calcite/magnesium-rich clay/silica deposits formed during the Early Cretaceous in the South Atlantic Pre-salt lakes is challenging our ability to comprehend the chemical evolution of alkaline volcanic lakes. Here we present a new hydrochemical model based on an open basin concept, thermodynamic equilibrium, and chemical data from Lake Baringo (East African Rift) to shed light on the mechanisms facilitating carbonate-clay-silica precipitation. This model explores the effects that the leakage to adjacent waterbodies, the combination of lake evaporation and lake recharge (with river, hydrothermal and marine waters), and the bathymetric effects of the partial CO 2 pressure have in the stationary precipitation of Pre-salt lacustrine mineral assemblages. Sediment thickness calculations suggest that the facies cyclicity in the Pre-salt and elsewhere are likely reflecting temporal fluctuations in the type of waters sourcing the lake, and in the degree of leakage to aquifers over multiple evaporative-freshening cycles. A plausible link between enhanced strontium uptake into Pre-salt calcite allochems and [Ca2+]/[CO 3 2−] lake water stoichiometries allows to infer different scenarios for strontium-rich calcite formation with or without magnesium clays. Our conceptual model allows to quantify the role that hydrology (groundwater charge, discharge to aquifers; evaporation) exert on carbonate-clay-silica precipitation providing with a refined framework to understand the basin-scale chemical sedimentation in alkaline lakes. Furthermore, this model represents a robust template upon which critically test the superimposed kinetic factors likely attached to the genesis of specific mineral factories such as those encountered in the South Atlantic Pre-salt alkaline lakes. [ABSTRACT FROM AUTHOR]