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The Effect of the Order of Reagent Addition on the Settling Rate of Aluminium Hydroxide in the Al(III)-Na2CO3 System
Abstract
The effects of several factors on the settling rate of aluminiumhydroxide were investigated during chemical coagulation using aluminium salts. Experimental variables were pH, aluminium (III) concentration and the order of addition of reagents. Experiments were carried out at pH 5–8 and rapid settling was achieved when aluminium (III) solutions were added to Na2CO3 solutions near neutral pH, close to the minimumsolubility pH of Al(OH)3. For a narrow range of total Al concentration where Al(III) species were supersaturated with respect to the solid phase, Al(III)-added-to-carbonate type mixtures yielded a higher settling rate than mixtures obtainedby the reverse order of reagent addition. The results were interpreted by comparing the rates of formation of polymer andsolid (amorphous Al(OH)3) phases. It was concluded that Al(III) coagulants should be added to water containing natural or artificially incorporated carbonate alkalinity for rapid settling of Al(OH)3 flocs.
... Chemical purification is based on the ability of the chemicals to precipitate SS and dissolved substances present in the water (Gregory and Duan, 2001;Demirata et al., 2002;Duan and Gregory, 2003;Sinha et al., 2004;Pernitsky and Edzwald, 2006). It consists of three individual but well-interconnected processes: coagulation, flocculation and sedimentation. ...
... However, dissolved substances and very fine particles or colloids cannot be removed via sedimentation alone and their removal is the main objective of chemical treatment (Koohestanian et al., 2008). Factors known to influence the efficiency of chemical purification include water quality characteristics such as temperature, alkalinity, pH and the concentration and type of pollutant substances (Volk et al., 2000;Franceschi et al., 2002;Sinha et al., 2004;Sansalone and Kim, 2008;Yan et al., 2008) as well as process parameters such as coagulant type (Lee et al., 1998;Liu and Chin, 2009;Zhao et al., 2009Zhao et al., , 2012, dosage and mixing during and after coagulant addition (Dentel, 1988;Rossini et al., 1998;Demirata et al., 2002;Exall and vanLoon, 2003;Zhan et al., 2011). ...
... As expected, the removal of SS was higher (20%) in the purification of water samples 1 and 2 with increased pH. At pH conditions which are closer to neutral the amorphous iron and aluminium hydroxide precipitates have minimum solubility, favouring the sweep coagulation mechanism and enhancing the removal of SS (Demirata et al., 2002;Sansalone and Kim, 2008;Yan et al., 2008). ...
The drainage of peatland areas for peat extraction, agriculture or bioenergy requires affordable, simple and reliable treatment methods that can purify waters rich in particulates and dissolved organic carbon. This work focused on the optimisation of chemical purification process for the direct dosage of solid metal salt coagulants. It investigated process requirements of solid coagulants and the influence of water quality, temperature and process parameters on their performance. This is the first attempt to provide information on specific process requirements of solid coagulants. Three solid inorganic coagulants were evaluated: aluminium sulphate, ferric sulphate and ferric aluminium sulphate. Pre-dissolved aluminium and ferric sulphate were also tested with the objective of identifying the effects of in-line coagulant dissolution on purification performance. It was determined that the pre-dissolution of the coagulants had a significant effect on coagulant performance and process requirements. Highest purification levels achieved by solid coagulants, even at 30% higher dosages, were generally lower (5%-30%) than those achieved by pre-dissolved coagulants. Furthermore, the mixing requirements of coagulants pre-dissolved prior to addition differed substantially from those of solid coagulants. The pH of the water samples being purified had a major influence on coagulant dosage and purification efficiency. Ferric sulphate (70 mg/L) was found to be the best performing solid coagulant achieving the following load removals: suspended solids (59%-88%), total organic carbon (56%-62%), total phosphorus (87%-90%), phosphate phosphorus (85%-92%) and total nitrogen (33%-44%). The results show that the use of solid coagulants is a viable option for the treatment of peatland-derived runoff water if solid coagulant-specific process requirements, such as mixing and settling time, are considered.
... As a consequence, the degradation rate of FA by the Fe(II) PMS addition order was higher than that by the PMS-Fe(II) addition order. Demirata et al. (2002) reported that the order of reagent addition had an effect on the settling rate of aluminium hydroxide in the Al(III)-Na 2 CO 3 system. Wang and Chu (2011) also indicated that the depletion efficiency of rhodamine B was lower at an order of PMS-Fe(II). ...
It is well known that chloride ions could affect the oxidation kinetics and mechanism of contaminant based on SO4•− in the wastewater. Here, the degradation of an organic acid, fumaric acid (FA), was investigated in the presence of chloride (0–300 mM) by the Fe(II)/peroxymonosulfate (Fe(II)/PMS) system. A negative impact of chloride was observed on the rates of FA degradation. The degree of inhibitory effect was higher in Fe(II)/PMS addition order. Some chlorinated byproducts were identified during the FA oxidation process in the presence of Cl− by the ultraperformance liquid chromatography and quadrupole-time of flight mass spectrometer (UPLC-QTOF-MS). With the increasing content of Cl−, an accumulation of adsorbable organic halogen (AOX), an increase in acute toxicity, and an inhibition of mineralization were observed. According to the results of kinetic modeling, the production and transformation of oxidative species were dependent on Cl− dosage and reaction time. SO4•− was supposed to be the main radical for FA degradation with Cl− concentration below 5 mM, whereas Cl2•− was primarily responsible for the depletion of FA at [Cl−] > 5 mM. A possible degradation pathway of FA was discussed. This study reveals the potential environmental risk of organic acid and is necessary to explore useful strategies for ameliorating the treatment of chloride-rich wastewater.
Nano TiO2 particles with different phase composition were synthesised using sol-gel method. Two different solvent were used for gelation. Effect of gelation temperature and to time on anatase/rutile phase formation had been studied. Particles crystallinity, crystal size, surface area, pore size had been calculated by x-ray diffraction, Raman spectroscopy, Brunauer-Emmett-Teller (BET) surface area) theory, and transmission electron microscopy analysis. How change in crystal size, surface area, pore size with respect to phase change also has been studied. Arsenate and arsenite removal by TiO2 particles of different phase compositions had been investigated by atomic absorption spectroscopy. Some parameters such as the contact time and the pH of the solution, which could affect the magnitude of adsorption, were examined. Sorption data have been interpreted in terms of the Freundlich and Langmuir equations. [Supplemental materials are available for this article. Go to the publisher's online edition of the Journal of Dispersion Science and Technology to view the free supplemental file.]
This study examined the removal of nC60 from wastewater by alum-enhanced primary treatment at different pH levels and alum dosages and identified wastewater characteristics affecting the removal. The efficiency of the removal depended on the alum dosage, pH, alkalinity, and the concentrations of suspended solids, sewage organic matter and salinity. Alum dosage and pH influenced the removal efficiency by changing the hydrolyzed aluminum species distribution at acidic and near neutral pH levels. At pH 9–10, nC60 removal was associated with the formation of magnesium (or calcium) carbonate or hydroxide precipitates. Alkalinity dramatically enhanced the nC60 removal efficiency (by up to 71%) through the formation of sodium aluminum hydroxycarbonate precipitates. Increasing the concentration of suspended solids enhanced the nC60 removal efficiency by up to 46% at 25 mg/L of alum as the solids act as adsorbents and the nuclei around which flocs can grow. Increasing the concentration of sewage organic matter decreased the nC60 removal efficiency (by 17% or less) while salinity improved it, albeit only slightly so. The results of this study demonstrate that nC60 can be effectively removed from wastewater by the alum-based chemically enhanced primary treatment process under appropriate conditions and better removal can be achieved by increasing alum dosage.
An aluminum hydroxide coprecipitation method for the determination of cadmium, copper and lead by flame atomic absorption spectrometry in aqueous solutions, seawater and mineral water samples has been investigated. The coprecipitation conditions, such as the effect of the pH, the amount of carrier element, the effect of possible matrix ions and the time were examined in detail for the studied elements. It was found that cadmium, copper and lead are co-precipitated quantitatively (≥95%) with aluminum hydroxide at pH 7 with low R.S.D. values of around 2 to 3%. Detection limits (38) were 6 ng ml−1 for Cd, 3 ng ml−1 for Cu and 16 ng ml−1 for Pb. The method proposed was validated by the analysis of HPS 312205 seawater standard reference material and spiked mineral water samples.
In this research, a waste water treatment plant is systematically optimized. The waste water treatment plant is used to remove aluminium from waste water using precipitation, flocculation and flotation. In total 40 variables influence the combined unit. After systematic selection, the number of variables was reduced to six: the waste water flow, pH, agitation velocity, amount of poly-electrolyte, amount of dissolved air and aluminium concentration. For these variables an experimental design was set up and executed and the results were analyzed by means of ANOVA. With the results of the ANOVA, an empirical model was constructed. The model was used for maximization of the aluminium removal. Subsequently, validation experiments were performed to confirm the findings. The study showed that the amount of poly-electrolyte is a key factor for combined unit operation.
Bromo-DBPs (disinfection by-products) are generated by bromide and disinfectant in drinking water disinfection, which have adverse effects on human health. In this study, effects of coexisting anions on removal of bromide by aluminium coagulation were investigated. It was observed that bromide was removed of 62.1-87.0% in raw water, while the removal efficiency of bromide was achieved 82.8-99.2% in deionized water through the combination of Br- with Al(III) in various pathways. The coexisting anions in raw water significantly affected the removal of bromide. Removal efficiency decreased by 11.5, 21.2, 14.6, 8.0 and 40.8% with the addition of HCO3-, SO4(2-), Cl-, NO3- and H2PO4-, respectively, for their affinities with Al(III) or accelerating the formation of Al(OH)3(am). These results demonstrated that bromo-DBPs in drinking water could be controlled though removing bromide by enhanced coagulation.
Particle formation and growth over the 1–40 μm size range in dilute aluminum solutions (approx. 2 × 10−4 M) have been studied using an electronic particle counter. Sulfate, fulvate and hydroxide ion accelerate the rate of particle formation and changes of the particle size distribution over time. Increasing ionic strength (inert electrolyte) produces similar but less dramatic effects. Combinations of sulfate and fulvic acid or sulfate and inert electrolyte further accelerate the rate of particle formation. Aluminum chloride solutions at moderate ionic strength are devoid of supramicron particles after several days. A conceptual pathway model is developed which suggests that two different solids are formed when aluminum is added to fulvic acid solutions: an aluminum-fulvate precipitate and Al(OH)2(s). The first solid dominates in fulvic acid solutions at pH ∼5.5.
Aluminum hydrolysis in mixed liquid reactors is a problem with multiple kinetic and mixing time scales Since some of these times scales overlap, and since aluminum hydrolysis has multiple pathways (some of which are slowly reversed), we have modeled aluminum precipitation as a coupled mixing and chemistry problem. For the base neutralization of dilute aluminum chloride in a standard stirred reactor with Rushton impeller, mixing submodels are combined with simplified aluminum chemistry models to predict the effect of mixing on steady-state pH at various formation or neutralization ratios. Our simulations and experiments clearly show that mixing speed influences the shape of the pH titration curve, which suggests that the conversion of aluminum into amorphous and polymeric species is mixing-sensitive. The experimental and simulated results show less sensitivity to position of base injection in the reactor.
PACl more effectively coagulates fine particles than alum.
The coagulation kinetics of polyaluminum chloride (PACl) and alum applied to kaolin clay suspensions were compared in batch experiments. Increasing aluminum (Al) dosages reduced the time required for particles destabilization and increased the rate at which the number of primary particles decreased. PACl worked faster than alum. Photometric dispersion analysis showed that increasing Al dosages decreased the time required for floc to form. Again, this rate was faster with PACl than with alum. Increasing the Al dosage accelerated the flocculation rate because it increased collision–attachment efficiency and increased particulate volume. The rates of particle destabilization, reduction of destabilized primary particles, and agglomeration varied with mixing intensity and water temperature. Although lower temperatures slowed the particle destabilization rate, similar effects on the flocculation rate could be avoided by maintaining a constant G value. PACl was less sensitive to changes in pH, implying that PACl performed better than alum in cold water.
The kinetics of agglomeration of silica dispersions coagulated with hydrolyzed AI (III) have been studied with respect both to physical parameters, such as diameter and concentration of dispersed colloids and extent of agitation of the medium, and to chemical parameters of the solution, such as pH and AI(III) concentration relative to surface concentration of the dispersed phase. The rate of colloid destabilization, i.e., the rate of surface coverage of the colloid with adsorbed aluminum hydroxo complexes, was evaluated on the basis of results from adsorption studies. The rate of collision between colloids was determined for Ludox colloids (15 mμ) from light adsorbance measurements and for Min-U-Sil colloids (1.1μ) from microscopic counts. A comparison of the rates of destabilization and particle collision shows that the latter reaction is rate determining in the coagulation of colloidal silica with hydrolyzed AI(III). The factor of collision efficience reflecting the relative colloid stability is found as the ratio of actual coagulation rates and theoretical collision rates at various concentrations of coagulant. The observed relationship between colloid stability and coagulant concentration confirms the assumed model of particle destabilization and restabilization: specific adsorption of isopolycations causes a decrease of the surface potential and, at sufficiently high coagulant concentrations, reversal of sign of the surface potential. The over-all coagulation rate is determined by physical parameters characterizing the collision frequency and by chemical solution parameters describing the factor of collision efficiency.
In order to understand the relationship of the hydrolysis reaction to the reactions of other nucleophilic or basic groups with the metal ion, a series of studies were initiated with the overall objective of determining the composition of these complexes under the conditions of concern in water. These conditions included very dilute solutions, and reaction times that may not permit equilibrium to be reached. The system initially studied was the simplest possible, containing only aluminum, hydrogen, hydroxide, and perchlorate ions. The procedures used were essentially the same as those reported by Singley and Black in the studies on the hydrolysis of iron (III), and therefore involved the potentiometric titration of solutions of aluminum perchlorate with sodium hydroxide under N2. The sample was stirred continuously, using a magnetic stirrer, and the pH was monitored, using a recording pH meter. The sodium hydroxide solution was added continuously, using a syringe pump.
It is possible to produce stable preparations of partially neutralized aluminum chloride solutions in which cationic aluminum polyelectrolytes predominate, but attempts to produce similar stable preparations containing cationic iron(III) polyelectrolytes were unsuccessful.
One method to improve the effectiveness of inorganic metal coagulants in water treatment is to partially neutralize concentrated metal salt solutions prior to their addition to the raw water and thus preform polymeric metal species, the actual coagulants. In this way coagulant chemistry can be controlled and proper solution conditions for the formation of desired coagulant species can be maintained. This article compares polymeric preparations of aluminum and ferric chloride salts with respect to preparation parameters and the extent and stability of polymerization.
The rate of dissolution of AI2O3 when an aqueous colloidal dispersion of AI2O3, is diluted is examined over a range from 30 sec to 50 hr after dilution as a function of pH. The aluminum aqueous solutions derived from dissolution of AI2O3 coagulate Ti02 dispersions in the same manner as Al(NO2)3 aqueous solutions of the same concentration and pH. Within the limits of the best available colorimetric technique, TiO2 shows no detectable solubility on dilution of TiO2 colloidal dispersions. The solubility of AI2O3 in the presence of colloidal TiO2 is also examined and is found to be similar to the solubility of aluminum hydroxides.
The detailed electrokinetic and coagulation behavior of TiO2 and Al203 hydrous oxide colloids, is reported as a function of pH and indifferent electrolyte concentration. The isoelectric point values for Ti02 (pH 5.9) and Al203 (pH 8.9) were obtained by both techniques.Comparison of the observed coagulation rates with those calculated on the basis of observed ζ-potentials using the Derjaguin—Landau—Verwey—Overbeek theory supports the general validity of the theory for oxide colloids. Specifically rapid coagulation, whether induced by pH or KNO3 concentration changes, occurs at |ζ| ⩽ 14 ± 4 mV in all systems.
Jar testing of 31 natural waters suggests many utilities may need the proposed alternative performance criterion to comply with enhanced coagulation specified in the proposed D/DBP Rule.
Jar tests were performed on 31 natural waters from a variety of sources across the United States. These tests indicate that the majority of utilities supplying samples will have difficulty meeting the Step 1 total organic carbon (TOC) removal requirements in the Disinfectants/Disinfection By‐products Rule. Utilities may need the proposed point of diminishing returns alternative performance criterion to achieve compliance with the enhanced coagulation requirement. Water sources with high TOC and low alkalinity were most likely to meet the proposed Step 1 TOC requirements. Those with low TOC were most likely to need the alternative performance criterion.
Naturally occurring compounds are defined as all nonsynthetic materials of constant composition. The emphasis in this review is placed on the removal of humic materials, other organic compounds, trace metals, and certain inorganic ligands. Removal of the alkaline earths is not reviewed. Associations between different compounds, or between compounds of interest and naturally occurring particulates, are important and complex. In this context, the formation and removal of such aggregates is reviewed. As a result, the removal of particles by coagulation and settling is briefly discussed. The removal of certain synthetic compounds is also reviewed, to the extent that removal of such compounds is dependent on the removal of naturally occurring compounds. Both inorganic and organic coagulants are considered.
Langmuir‐based semiempirical models are used to predict DOC removal during alum and ferric coagulation.
The concentration of dissolved organic carbon (DOC) remaining after enhanced coagulation can be predicted with a standard error of about 10 percent or 0.4 mg/L using a new model with inputs of coagulant dosage, coagulation pH, raw water UV 254 , and raw water DOC. Total organic carbon remaining after coagulation can be predicted with similar accuracy. The model may also be calibrated to a specific site, improving the standard predictive error to 4 percent or 0.27 mg/L (or ±10 percent for 90 percent confidence). Performance differences between equimolar dosages of alum and ferric coagulants in mediating DOC removal may be attributed to (1) equal or better removal of DOC using ferric at very high coagulant dosages, (2) equal or better removal of DOC using alum at lower coagulant dosages, or (3) differing acidity of coagulants, producing a performance advantage for the more acidic coagulant.
Turbid waters, containing suspended and colloidal particles, are normally treated by coagulation–flocculation followed by clarification; the process usually consists of the rapid dispersal of a coagulant into the raw water followed by an intense agitation commonly defined as rapid mixing. Rapid mixing is influenced by chemical and physical parameters and the final result of the whole treatment depends on this step. The study focused on the most important parameters of rapid mix design: velocity gradient and rapid mix time. Wastewaters from a tannery processing stored ovine hides were treated with aluminium(III) or iron(III) salts. Experimental results indicate that rapid mix time has a strong influence on final results; an optimal combination of rapid mix parameters was determined. The same approach was applied to highly turbid synthetic water, obtained through dissolving kaolin in distilled water and the results compared with those obtained for the tannery wastewater. It was confirmed that high turbidity removal can be achieved both with a short mixing time (about 10s) and a long mixing time (60–90s).
The optimal conditions for the preparation of polymeric aluminium(III) hydroxide species for water treatment has been described. The partially hydrolysed aluminium(III) solutions were characterized by ultrafiltration method, ferron assay, 27Al n.m.r. measurements, high pressure anion exchange chromatography and precipitation of polymeric aluminium hydroxo species with sodium sulphate. The results of these tend to suggest that the polymeric aluminium(III) hydroxide species have a size between 10–20 Å; these have an average residual positive charge of 0.54, and constitute about 80% of the total aluminium in the hydrolysed solution. They are retained by UM05 membrane. 27Al measurements indicate that Al13O4(OH)247− ion are the predominant polymeric species in these solutions. In addition to these the hydrolysed aluminium(III) solutions contain low proportions of monomeric aluminium hydroxo species and inert micro-crystalline aluminium hydroxide. The effect of ageing, dilution and pH on the stability and characteristics of the aluminium hydroxide polymer were also studied in detail.
Aluminium hydroxide particles, created by hydrolysis of aluminium nitrate solutions with sodium carbonate, aggregate rapidly to form mass fractal structures for which the fractal dimension varies from 1.7 to 2.1 depending on the OH/Al-ion ratio of the solution. This system has been studied by small-angle X-ray scattering (SAXS). The OH/Al ratios for the solutions were 1.5, 2.0 and 2.15. The maximum radius of gyration of the aggregates of about 50 Å is found in the solutions having the highest OH/Al ratio. SAXS measurements made at various stages of the aging process show that, in addition to fractal aggregation, there is a second process active in the solutions that leads to the break up of the fractal structure and formation of nonfractal particles, which are probably Al13 ions, [AlO4Al12(OH)24(OH2)12]7+. This process, which is strongly temperature dependent, is governed by an activation energy of approximately 162 kJ mol−1.
An analysis of the type of rapid mixing necessary in specific regions of the stability diagram suggests that it is possible to tailor the design of the rapid-mix operation to the predominant mode of alum coagulation.
Speciation resulting from fast hydrolysis of dilute ferric chloride salt (0.1 M) by NaOH at 25-degrees-C was studied versus the hydrolysis ratio n = NaOH/Fe from 0 to 3.0 at different aging time from t = 400 s up to 1 day by using quasi-elastic light scattering and small-angle X-ray scattering. Before the flocculation threshold (n approximately 2.7), large colloids are formed during the hydrolysis. Photon correlation spectroscopy showed that at t = 400 s the size range varied from approximately 10 nm (n = 1.0) up to 700 nm (n = 2.7). At larger times the colloid size rapidly decreased during the first 30 min. The mean sizes were centered around approximately 10 nm. Small-angle X-ray scattering experiments carried out with n = 1.0 to n = '3.0 permitted description of the semilocal structure of the colloids. The scattering curves are characteristic of colloids formed by aggregates of subunits. Whatever the value of n and the time (t less-than-or-equal-to 1 h), the submit size is equal to 16 angstrom in diameter. It is not modified by pH variation for t less-than-or-equal-to 1 h). The semilocal structure of the colloids depends on the time and n. For n = 1.0 and t = 400 s, locally the colloids are formed of linear aggregates. This local shape is originated from long-range magnetic dipolar interactions, which are known to linearize the fractal aggregates. The same sols aged for 1 h exhibited slightly more branched aggregates. From n = 2.0 up to n = 3.0 fractal structures occurred in the suspensions with fractal dimensions D(f), which was varied from approximately 1.7 (n = 2.0 and 2.5) up to approximately 2 (n approximately 3.0).
Critical coagulation concentrations of aluminum salt solutions for negative silver iodide sols (both aged and in statu nascendi) and for silver bromide sols in statu nascendi were obtained over the pH range from 2 to 10. At pH < 4, the critical coagulation concentration corresponds to that of trivalent counterions indicating the presence of simple hydrated Al+3. In the pH range 4-7 a tetravalent hydrolysis product is indicated, and at pH >7 a lower, possibly divalent species appears. The neutralization curve of Al(NO3)3 titrated with NaOH is consistent with a first hydrolysis product with a Al:OH ratio of 1:2.5. The only complex ion in agreement with these results is Al8(OH)204+. At higher pH's the existence of Al8(OH)222+ and Al8(OH)24 is assumed. Coagulation and electrophoretic measurements show that the hydrolyzed aluminum species reverse the charge of the originally negative silver halide sols, whereas the simple hydrated Al+++ does not.
Recent studies using Al-27-NMR spectroscopy indicate that the polynuclear hydroxo-Al fraction that exhibits a moderate reaction rate with the ferron reagent can be equated to the Al13 tridecamer, and we utilized this correspondence to examine the formation of Al13 over a broad range of synthesis conditions, including some pertinent to natural waters. Ferron and NMR analyses revealed that solutions ([Al]T = 2 x 10(-5)-5 x 10(-3) mol L-1) partially neutralized with aqueous NaHCO3 contained from 26 to 87% of Al(T) as the tridecamer. Solutions neutralized with solid CaCO3 or MgO, or with NH3 vapor, also contained substantial quantities (14-46% of Al(T)) of Al13. Bi-carbonate-neutralized solutions were also prepared by injecting base into an acidic Al solution (forward method), by injecting the Al solution into the base (reverse method), and by simultaneous injection of the two solutions. The latter two methods resulted in less Al13 formation than the forward method, and our results thus suggest that Al13 can, but does not always, form in a variety of settings characterized by inhomogeneous conditions at the interface between solutions of differing pH and alkalinity/acidity. Scrutiny of these and previously published results revealed that the "yield" of the tridecamer diminishes with decreasing Al(T), but that it can still be a significant component of solutions with Al(T) concentrations as low as 10(-5) mol L-1.
The kinetics of agglomeration of silica dispersions coagulated with hydrolyzed AI (III) have been studied with respect both to physical parameters, such as diameter and concentration of dispersed colloids and extent of agitation of the medium, and to chemical parameters of the solution, such as pH and AI(III) concentration relative to surface concentration of the dispersed phase. The rate of colloid destabilization, i.e., the rate of surface coverage of the colloid with adsorbed aluminum hydroxo complexes, was evaluated on the basis of results from adsorption studies. The rate of collision between colloids was determined for Ludox colloids (15 mμ) from light adsorbance measurements and for Min-U-Sil colloids (1.1μ) from microscopic counts. A comparison of the rates of destabilization and particle collision shows that the latter reaction is rate determining in the coagulation of colloidal silica with hydrolyzed AI(III). The factor of collision efficience reflecting the relative colloid stability is found as the ratio of actual coagulation rates and theoretical collision rates at various concentrations of coagulant. The observed relationship between colloid stability and coagulant concentration confirms the assumed model of particle destabilization and restabilization: specific adsorption of isopolycations causes a decrease of the surface potential and, at sufficiently high coagulant concentrations, reversal of sign of the surface potential. The over-all coagulation rate is determined by physical parameters characterizing the collision frequency and by chemical solution parameters describing the factor of collision efficiency.
Acidified aluminum nitrate solutions were titrated with alkali (NaOH or KOH) over a temperature range of 24°C to 90°C. A homogeneous distribution of added base was achieved by: (i) in situ decomposition of urea (90°C); and (ii) a novel method involving injection through a capillary submerged in the agitated salt solution.The experimental pH curves are characterized by two plateaus (or platforms) separated by a steep jump in pH. These characteristic features were not previously observed in continuous titration experiments. This is especially true of the second platform, which begins at an ratio larger than 2.5. Stable colloidal solutions or gels are formed on traversing this plateau. The hydrolysis-precipitation behavior was also followed by light scattering and x-ray diffraction measurements.A qualitative picture of the neutralization process is developed.
This paper describes the effects of several important water quality and water treatment variables on particle formation and growth during chemical coagulation. Experimental variables included initial pH, initial turbidity, aluminum sulfate dose, preozonation dose and flocculation time. An orthogonal experimental design was employed with statistical analysis of data by the Duncan's Multiple Range Test. The variables investigated proved to be capable of imparting, individually or collectively, statistically significant effects on particle formation and growth. Particle growth during coagulation is a necessary prerequisite for the effective use of subsequent solid-liquid separation processes.
Formation and Growth in Dilute Aluminum(III) Solution'
- W J Snodgrass
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Basic Analytical Chemistry The Hydrolysis of Cations
- R Apak
Apak, R.: 1997, Basic Analytical Chemistry, 2nd ed., (a text book of 640 p, in Turkish), Istanbul University Publ., Istanbul. Baes, C. F. and Mesmer, R. E.: 1976, The Hydrolysis of Cations, Wiley-Interscience, New York, pp. 112–120.
Coagulation in Wastewater Treatment The Scientific Basis of Flocculation’ NATO Adv The Netherlands
- O 'melia
Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing
- T Wood
Comparison of Aluminium Preparations as Coagulation in Water Treatment
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