Metal partitioning in a sulfidic canal sediment: metal solubility as a function of pH combined with EDTA extraction in anoxic conditions

The chemical forms of low concentrations of metals (Zn, Pb, Cu, Cd, Co and Mn) and main components (Fe and Ca) were determined under the original reducing conditions of a sulfide-rich sediment from the Gent-Terneuzen Canal (Belgium). Therefore, dissolution experiments as a function of pH were made in a salt solution mimicking the canal water. The centrifugates remaining after the dissolution vs. pH experiments were subsequently further extracted with 0.1 M EDTA/0.5 M NaAc (pH 4.65) to determine eventually readsorbed and/or reprecipitated metal ions. The experimental dissolution vs. pH edge of calcium, iron, manganese and cobalt had a lower slope than theoretically expected on the basis of the solubility of, respectively, calcium carbonate, iron sulfide/iron carbonate, manganese sulfide/manganese carbonate and cobalt sulfide and was explained by the combination of (a) the solubilities of the various minerals and (b) metal readsorption onto clay minerals and organic matter. Higher metal recoveries were measured in the 0.1 M EDTA/0.5 M NaAc mixture and proved that in addition coprecipitate formation with iron sulfide/iron carbonate minerals may occur. The solubility of zinc, lead, cadmium and copper was very low in the mimicking salt solution even at very low pH values (up to pH 1) in agreement with the theoretical solubility of their discrete metal sulfides. However, by using an additional 0.1 M EDTA/0.5 M NaAc extraction on the centrifugates remaining after the dissolution vs. pH experiments, it was qualitatively shown that zinc and lead were partly associated with iron sulfide/iron carbonate phases in the real sediment (in addition to their presence in discrete metal sulfides). Cadmium was present solely in discrete cadmium sulfide phases. It was not possible to verify whether copper was present in discrete copper sulfide phases and/or in mixed coprecipitates with iron sulfide/iron carbonate minerals.     

Effect of a magnetic water treatment on homogeneous and heterogeneous precipitation of calcium carbonate

In this paper are reported experimental results on the effect of a magnetic field on the precipitation process of calcium carbonate scale from a hard water. Carbonically pure water was circulated at a constant flow rate in a magnetic field. After this treatment, calcium carbonate precipitation was induced by degassing dissolved carbonic gas. The nucleation time was identified from the variations of the pH and the Ca(2+) concentration. The ratio between homogeneous and heterogeneous nucleation was determined from the measurement of the mass of precipitated calcium carbonate. It is shown that the magnetic treatment increases the total amount of precipitate. This effect depends on the solution pH, the flow rate and the duration of the treatment. In addition, the magnetic treatment modifies the ratio between homogeneous/heterogeneous nucleation. Homogeneous nucleation is promoted by an increasing the pH of water, the flow rate as well as the residence time. The magnetic treatment enhances these effects with a maximum for a 15 min treatment time. It is shown that the presence of calcium carbonate colloid particles is not necessary. It is advanced that the main magnetic effects concern the associations of ionic species which are present in the solution and which are involved in the nucleation process of calcium carbonate precipitation.     

Rate of precipitation of ferrous iron and formation of mixed iron-calcium carbonates by naturally occurring carbonate materials

The precipitation of iron from an iron sulphate containing wastewater by aragonite, calcite and a variety of limestones and limesands was examined at pH values near 6 in an atmosphere of carbon dioxide. Siderite (ferrous carbonate) and a calcium siderite containing 10 mol% calcium were the only iron-containing products detected by XRD, and accounted for substantially all the iron removed from solution. Calcium siderite was the major product and constituted between 50 and 100% of the iron containing product.The rate of precipitation of iron was proportional to the square of the supersaturation of the solution with respect to siderite, and increased with increase in pH and alkalinity. Precipitation occurred at different rates with different carbonate materials, tending to increase with increasing proportion of aragonite in the material. Precipitation did not occur in suspensions of calcite and was slow even in suspensions of aragonite where equilibrium was not reached within 40 days.These data suggest that the frequent occurrence of groundwaters apparently supersaturated with respect to siderite may be due either to slow precipitation of siderite, or to equilibrium of the groundwater with respect to a more soluble calcium siderite. The minerals present in an aquifer thus need to be identified before the saturation state of the associated groundwater with respect to siderite can be ascertained.     

Mechanisms of scale formation and carbon dioxide partial pressure influence. Part I. Elaboration of an experimental method and a scaling model

Scale formation in industrial or domestic installations is still an important economic problem. The existence of a metastable domain for calcium carbonate supersaturated solutions and its breakdown are observed under conditions rarely well defined. In most cases it is the pH rise caused by the carbon dioxide loss that involves calcium carbonate precipitation. Before studying this problem, we suggest in this first part, a new model for the evolution of the calcocarbonic system that takes into account the hydrated forms of CaCO3: CaCO3 amorphous, CaCO3 x 6H2O (ikaite) and CaCO3 x H2O (monohydrate). According to this model, the precipitation of any one of these hydrated forms could be responsible for the breakdown of the metastable state. After this first step, the solids evolve into dehydrated forms. At first, the metastable domain spread of the calcium carbonate supersaturated solutions was studied by the elaboration of computer programs in which the formation of CaCO3(0)(aq) ion pairs was taken into account. These ion pairs are supposed to evolve through dehydration to form the various calcium carbonate solid form precursors. This thermodynamic study was then compared to the experimental methods of the critical pH. Here the pH rise was caused by adding sodium hydroxide under different conditions for sodium hydroxide addition speed, agitation mode and ageing of solutions. For the highest speed of sodium hydroxide addition, the CaCO3 ionic product reached the value of the amorphous calcium carbonate solubility product, and the reaction of the amorphous calcium carbonate precipitation was of the homogenous type. Decreasing the reagent's addition speed caused an extension of the titration time. Then, the breakdown of the metastable state was obtained with the CaCO3 x H2O heterogeneous precipitation. This clearly illustrates the probable ageing of the precursors of the solid states that are considered in this model.     

Particle size distribution dynamics during precipitative softening: declining solution composition

Particle removal is a critical step in the treatment of surface water for potable use, and the majority of drinking water treatment plants employ precipitative coagulation processes such as alum and iron “sweep-floc” coagulation or lime softening for particle pre-treatment. Unfortunately, little is quantitatively known about how particle size distributions are shaped by simultaneous precipitation and flocculation. In an earlier paper, we demonstrated the effects of the saturation ratio, the mixing intensity and the seed concentration on the rates of homogeneous nucleation, precipitative growth and flocculation during precipitation of calcium carbonate at constant solution composition using electronic particle counting techniques. In this work, we extend those findings to systems more closely emulating the conditions in actual softening processes (i.e., declining solution composition). Key findings include the strong dependence of the rate of flocculation on the initial saturation ratio and demonstration of the benefits of seeding precipitative softening from the perspective of optimizing the effluent particle size distribution. The mixing intensity during precipitation was also shown to strongly influence the final particle size distribution. Implications of the findings with respect to softening practice are discussed.     

低温条件微生物MICP沉淀产率试验研究

低温导致微生物固化沉淀产率低,制约着该技术的应用。选取巨大芽孢杆菌,通过控制不同温度和pH值分析该菌种的生长繁殖特性和脲酶活性,并研究不同温度条件下的碳酸钙沉淀产率,通过采用营养液中添加尿素和低温驯化两种方法来提高低温条件下较低的沉淀产率,最后通过砂土固化试验,对比研究尿素添加方法和低温驯化对固化效果的影响。结果表明:温度越高,巨大芽孢杆菌的生长繁殖越快,脲酶活性越强,低温明显抑制其生长繁殖和脲酶活性;pH为8时,巨大芽孢杆菌生长繁殖最快,且脲酶活性最强;温度越高,沉淀产率越大;营养液中添加尿素和对巨大芽孢杆菌进行低温驯化都可以明显提高生长繁殖速度和沉淀产率,可以有效解决低温条件下碳酸钙沉淀不足问题,而将两者结合起来,沉淀产率提升更为明显;营养液中添加尿素和低温驯化都能提高砂土固化效果,而同时采用这两种方法固化效果提升更明显,该研究能有效解决低温条件沉淀少阻碍实际工程应用的问题,为后续低温条件微生物固化技术的应用打下基础。     

Electrochemical treatments for selective growth of different calcium carbonate allotropic forms on carbon steel

Different allogropic forms of calcium carbonate scales (calcite and aragonite) were electrochemically deposited on carbon steel surfaces, using different electrochemical techniques: cyclic voltammetry or potentiostatic pulses. To simulate conditions of Mexican refinery cooling systems, this study was performed in the presence of known concentrations of other salts at pH 7.8 and 40 degrees C with low and high calcium carbonate concentrations. Reduction reactions for dissolved oxygen and water occurring in such systems modified the pH at the substrate-solution interface to promote scaling of the calcium carbonate present. A systematic scanning electron microscopy and X-ray diffraction analysis of the carbon steel surface showed that the formation of calcite and aragonite depended on the initial state of substrate surface (clean or damaged) and on the concentration of calcium carbonate present in the system.     

Calcite saturation in the River Dee,NE Scotland

The spatial and temporal variations in calcite (calcium carbonate) solubility within the Dee basin of NE Scotland were assessed using water chemistry data gathered from a network of 59 sites monitored for water quality from June 1996 to May 1997. Calcite solubility, expressed in terms of a saturation index (SI_calcite), was determined from measured streamwater pH, Gran alkalinity and calcium concentrations and water temperature. In general, the waters of the Dee system are undersaturated with respect to calcite, though the saturation index is higher during the summer months indicating a dependency on flow conditions and biological activity. Under low-flow conditions, the streamwaters are dominated by water derived from the lower soil horizons and deeper groundwater stores and therefore, ions such as Gran alkalinity and calcium are at their highest concentrations as they are derived mainly from bedrock weathering. The influence of biological activity on the carbonate system is also evident as the observed pH and estimated EpCO₂ values indicate strong seasonal patterns, with the highest pH and lowest EpCO₂ values occurring during the summer low-flow periods. Only at three sites in the lowland region of the catchment, during the summer low-flow period, are the waters oversaturated. As such, the Dee system represents an extreme ‘end-member’ case when compared to many UK rivers that span both under- and oversaturated conditions during the year. Regression analysis highlights a systematic change in the SI_calcite–pH relationship in a broad east-west direction across the Dee system. At sites draining the relatively impermeable upland areas, the regression of SI_Calcite against pH gives a straight line with a gradient in the range 1.6–2.4. Correspondingly, under the most extreme alkaline conditions found at sites draining lowland agricultural areas, a straight-line relationship exists but with a gradient of unity. It is concluded that these changes in the SI_calcite–pH relationship are due to variations in the bicarbonate system induced by the flow conditions and biological activity. Given the waters are undersaturated, then calcite precipitation and hence phosphorus co-precipitation cannot occur within the water column.     

Effect of antiscalants on precipitation of an RO concentrate: Metals precipitated and particle characteristics for several water compositions

Inland brackish water reverse osmosis (RO) is economically and technically limited by the large volume of salty waste (concentrate) produced. The use of a controlled precipitation step, followed by solid/liquid separation (filtration), has emerged as a promising side-stream treatment process to treat reverse osmosis concentrate and increase overall system recovery. The addition of antiscalants to the RO feed prevents precipitation within the membrane system but might have a deleterious effect on a concentrate treatment process that uses precipitation to remove problematic precipitates. The effects of antiscalant type and concentration on salt precipitation and precipitate particle morphology were evaluated for several water compositions. The primary precipitate for the synthetic brackish waters tested was calcium carbonate; the presence of magnesium, sulfate, minor ions, and antiscalant compounds affected the amount of calcium precipitated, as well as the phases of calcium carbonate formed during precipitation. Addition of antiscalant decreased calcium precipitation but increased incorporation of magnesium and sulfate into precipitating calcium carbonate. Antiscalants prevented the growth of nucleated precipitates, resulting in the formation of small (100-200 nm diameter) particles, as well as larger (6-10 μm) particles. Elemental analysis revealed changes in composition and calcium carbonate polymorph with antiscalant addition and antiscalant type. Results indicate that the presence of antiscalants does reduce the extent of calcium precipitation and can worsen subsequent filtration performance.     

Calcium carbonate crystallization kinetics

The kinetics of crystallization of calcium carbonate were studied by following changes in pH only on batch solutions of CaCl₂, NaHCO₃ and NaOH in the presence of calcite seed crystals. The Ca⁺² and CO⁻²₃ concentrations were derived by taking account of the equilibrium relationships between the species of the carbonic—and calcium systems with changes in pH and ionic strength. Reactant concentrations and seed crystal masses studied were in the same range as that found in water softening practice. At these high concentrations the crystallization kinetics continued to conform to the crystallization model of Reddy and Nancollas. Removal of Ca⁺² from solution follows a rate equation first order with respect to Ca⁺² and CO⁻²₃ concentrations, the activation energy for crystal growth is 10·3 ± 0·9 kcal mol⁻¹ and the rate is independent of the stirring speed, all suggesting a surface controlled process. The rate constant is a function of the pH established immediately after mixing of the reactants. The rate of Ca⁺² removal from solution is directly proportional to the mass of crystals present in the reaction solution. Numerical integration of the rate equation gives a solution for the design of calcium carbonate crystallization reactors.     

Improving settlement and reinforcement uniformity of marine clay in electro-osmotic consolidation using microbially induced carbonate precipitation

This study explored the effect of microbially induced carbonate precipitation (MICP), a natural bio-geotechnical process, on mitigating the uneven settlement in electro-osmotic consolidation of soft soil. The real-time drainage and settlement were measured, and the control behavior of drainage to settlement was discussed. MICP solution components were also selected as the different additives to determine the control mechanism of MICP in improving settlement and reinforcement uniformity of clay. After the tests, the chemical properties and microstructure were analyzed according to pH, conductivity, and SEM. The addition of MICP solution in clay significantly even reduced the coefficient of settlement variation by 53.2%, and the upper surface profile tended to be uniform. Contrary to control, the coefficient of settlement variation of MICP-treated soil decreased gradually with drainage volume, mainly due to the filling of solid substances such as calcium carbonate, biofilm, and/or calcium hydroxide produced within soil pores. MICP significantly improved the uneven soil reinforcement generated during the electro-osmotic consolidation but resulted in the lower strength near the anode due to the less drainage. The contribution of MICP solution components to the improvement of settlement and strength uniformity obviously varies. Bacterial cells improved the settlement uniformity but had no effect on the strength improvement of soil. The co-existence of Ca²⁺ and bacterial cells maximized the modification effect, which determined the production of mineral precipitation. Microstructure observation proved the formation of calcium carbonate. The results demonstrated that MICP is an effective technique to improve the settlement and reinforcement uniformity of marine clay in electro-osmotic consolidation.

     

Hydrogeochemische Reaktionen im Sicker- und Grundwasserbereich von Braunkohlentagebaukippen

The changes in the chemical composition of water and solid phases in pyrite oxidation zones and in lignite mining dump aquifers are described and modelled. At pH-values below pH 4, sulfate and ferrous iron as pyrite oxidation products, are mobile. They are observed in a stoichiometric ratio of 1 to 2 in the aqueous phase. If pyrite oxidation takes place with no pH-buffering with the solid phases, pH-values below 1 are possible. This leads to more intensive silicate weathering which increases the pH-value of the solution to pH 2–3. However, equilibrium with silicates is not reached. At pH-values above pH 4, the iron concentration of water under oxidized conditions is limited by the precipitation of ferric hydroxide. This has an impact on sulfide oxidation itself, because ferric iron is involved in the oxidation reactions of pyrite. Products of sulfide oxidation stored in acidic material in overburden are leached by seepage water and ground water filling the dumps. Reaction with carbonate phases in non-acidic material mainly takes place during ground water recharge. The geogenic calcite minerals of the non-acidic sediments are dissolved. Gypsum and iron-rich carbonates precipitate as secondary minerals in the dump aquifer. The pH-value increases and the concentration of iron and trace metals (Co, Ni, Zn) in the ground water are controlled by equilibrium with iron-rich carbonates at pH-values above pH 5.     

The effect of antiscalant addition on calcium carbonate precipitation for a simplified synthetic brackish water reverse osmosis concentrate

The primary limitations to inland brackish water reverse osmosis (RO) desalination are the cost and technical feasibility of concentrate disposal. To decrease concentrate volume, a side-stream process can be used to precipitate problematic scaling salts and remove the precipitate with a solid/liquid separation step. The treated concentrate can then be purified through a secondary reverse osmosis stage to increase overall recovery and decrease the volume of waste requiring disposal. Antiscalants are used in an RO system to prevent salt precipitation but might affect side-stream concentrate treatment. Precipitation experiments were performed on a synthetic RO concentrate with and without antiscalant; of particular interest was the precipitation of calcium carbonate. Particle size distributions, calcium precipitation, microfiltration flux, and scanning electron microscopy were used to evaluate the effects of antiscalant type, antiscalant concentration, and precipitation pH on calcium carbonate precipitation and filtration. Results show that antiscalants can decrease precipitate particle size and change the shape of the particles; smaller particles can cause an increase in microfiltration flux decline during the solid/liquid separation step. The presence of antiscalant during precipitation can also decrease the mass of precipitated calcium carbonate.     

Influence of clay minerals, used as supports in anaerobic digesters, in the precipitation of struvite

This work studies the influence of clay minerals used as supports in anaerobic digesters on the precipitation of ammonium magnesium phosphate. The magnesium exchange cation present in the sample originally or from experimental saturation plays an important role in the precipitation of struvite crystal growth depending on the greater or lesser facility of its liberation to the medium. Vermiculite, and probably stevensite, influence the formation of struvite, transferring the magnesium cation only from their exchange positions, while sepiolite does so by contributing from its structure. In vermiculite, struvite crystal formation is on the edges or fractures of the laminas. The presence of other cations in the supports, such as exchange Ca²⁺—either structural or as a component of accessory minerals, as in the case of stevensite—causes precipitation of calcium ammonium phosphate. In minerals with a high iron content, such as nontronite, iron phosphates are produced. The type of digester influences the size of the struvite crystals, these being larger in the semi-continuous digesters.     

Variation on physico-mechanical properties of Kota stone under different watery environments

The physico-mechanical properties of rocks play an important role in planning and designing of civil constructional works. These properties are adversely affected by acidic and alkaline environments, where they are exposed for a longer time. The natural forces and agents of weathering have degrading effect on the appearance and structural soundness of Kota stone. These agents include rain, temperature, wind and atmospheric pollutants. Weathering agents almost never work individually or in isolation, they always act in combination with one another or with other agents of deterioration.

The durability of building stones is primarily judged by its reactivity with acidic and basic water of different pH values. The conditions are very obvious in any of the large scale construction and use of building stone like sandstones and Kota stones. During the mining of carbonate hosted mineral deposits, the interaction of ground water with ore minerals affect the surrounding environment and rock types. This may cause problem in exploitation of minerals and may cause failure of slope or pillars. To avoid such problems, the prior study of effect of ground water charged with ions released from ore minerals on carbonate rock is important.

In this paper, an attempt has been made to see the variation in the physico-mechanical properties of Kota stone under different watery environment. In the present study, the NX size cylindrical cores were prepared with the help of diamond core drilling machine as per ISRM standard. For each mechanical property, eight samples were prepared and tested. The prepared samples were put into the oven for 24 h at 104 °C to eliminate the moisture present and later submerged in to the water having different pH values ranging from nearly 0.89–12 for 24 h under stirring mode till it gets full saturation at room temperature. The study reveals that there is prominent change in strength properties under acidic and alkaline environments. Rock is considered as a neutral substance so at pH 7, Kota stone shows maximum strength due to non-reactive nature of the solution. Majority of rock mass is not neutral but some of the rock mass shows very minor changes in its strength. The chemical composition of the rock mass dominates the reaction process when it is submerged in varying pH solution.     

Is there a Technique for the Determination of Sedimentation Rates based on Calcium Carbonate Content? A Comparative Study on the Southeastern Brazilian Shelf

In this paper we evaluate the utilization of the sedimentation rate values obtained by the carbonate content technique (Fukue et al., 1996, 1999) as compared with those obtained by traditional radiometric techniques (²¹⁰Pb_xs, ¹³⁷Cs and radiocarbon), using sediment samples from the Southeastern Brazilian shelf. Our results indicate that the sedimentation rates calculated by the calcium carbonate content technique do not show any significant differences from those obtained from ¹³⁷Cs and ²¹⁰Pb_xs but do reveal a significant difference from radiocarbon estimates. Nevertheless, no significant correlation was obtained in the comparisons between ¹³⁷Cs, ²¹⁰Pb_xs and calcium carbonate. Thus it would seem that the calcium carbonate technique may be used for raw estimates of sedimentation rates. Taking a carbonate level which can be correlated with a tsunami that occurred in 1542 as well as the peak activity of ¹³⁷Cs in 1963-1965 as age references we obtained an average value for the calcium carbonate precipitation rate for the study area which is close to that given by Fukue et al. (1996, 1999). Nevertheless our results suggest that the assumption of a single carbonate precipitation value may be too generic to be taken as valid for the calculation of sedimentation rates.     

Calcium carbonate precipitation catalyzed by soybean urease as an improvement method for fine-grained soil

Although great advances have recently been made in biological soil-improvement methods, the treatments of fine-grained soil using these methods are still challenging. In this study, we tested a new method for the improvement of silty soil. This method adopts the calcium carbonate precipitation process catalyzed by soybean urease. Crude urease is derived simply by collecting the liquid formed by soaking soybean powder in water. The activity of crude urease is linearly related to the amount of soybean powder added to the water, and is high enough to be used for soil treatment. Under batch conditions, the reactions catalyzed by crude urease can be completed, although the reaction rates are slower than those using live bacteria with the same initial activity. In triaxial consolidated undrained tests, the treated silty soil shows more dilative responses in the stress-strain curves and larger peak deviatoric stresses as compared with the untreated soil, indicating significant improvements in the mechanical behaviour. In the results of CaCO₃ measurements, it is seen that there are continuous increases in the CaCO₃ content of the samples during the treatment process, which implies that the silty soil samples are not clogged when treated by crude urease. Such results indicate that calcium carbonate precipitation catalyzed by soybean urease can be used for the improvement of fine-grained soil.     

Equalization/neutralization modeling An application to fluoride removal

The chemistry of a fluoride removal system (gas scrubbing to capture hydrogen fluoride and subsequent precipitation of calcium fluoride with lime) was studied, with particular emphasis on the effects of and on pH. Carbonate, when present, was found to interfere with the precipitation in the pH range 7–11 but little carbonate was in the system studied because of the low pH and high gas/liquid exchange in the scrubber. A general approach to modeling equalization/neutralization problems, using mass balances on conservative substances and a charge balance to determine pH, was presented and illustrated via application to the fluoride removal system.     

造粒反应器处理高硬度水试验研究

为降低水的硬度,以一定粒径的细砂为填料,构建新型造粒反应器软化高硬度水。反应器在中温（20℃）条件下运行,通过改变 pH 值、填料粒径、水力条件、反应时间检测填料中碳酸钙的含量,考察反应器性能。试验结果表明,控制原水的 pH 值大于12、砂石填料粒径为0．2～0．5 mm、原水进水流量为10～35 mL／s,反应器的运行效果达到最佳。随着反应器运行时间的延长,细砂填料表面附着的碳酸钙晶体逐渐增多,运行15 d 左右填料表面所附着的碳酸钙晶体达到饱和,将沉下的填料取出,更换成新的填料。反应器对原水硬度的去除率为58％～67％,出水水质良好。     

基于数值模拟的砂柱微生物注浆影响因素分析

微生物注浆是一种较为新型的地基处理方式,其过程是一个复杂的物理化学生物过程,然而大量实验表明仅靠实验很难全面而系统地研究影响微生物注浆效果的主要因素。通过砂柱中微生物注浆过程的数值模拟,可以探究渗流传输与反应过程中各因素的影响作用,其影响主要体现在注浆结束后砂柱中碳酸钙的沉淀率和碳酸钙沿砂柱长度范围内的分布特征。本文首先通过已有实验数据与模拟结果的对比分析验证了一维数值模型的合理性,然后通过数值模型计算不同初始菌液酶活、营养盐浓度、营养盐传输速率和静置反应时间等因素作用下碳酸钙沉淀沿砂柱长度的分布变化和沉淀率,分析各影响因素对注浆过程的作用。结果表明初始酶活和营养盐传输速率是控制碳酸钙沉淀分布均匀性的重要因素,较小的尿素水解速率与较大的营养盐传输速率有利于碳酸钙的均匀沉淀,而适宜的营养盐浓度能够生成更多的碳酸钙沉淀,同时需要足够长的静置时间使砂柱中的营养盐完全反应。