Ferric Hydroxide Flocs Research Articles

Determination of main influence mechanism of fulvic acid on arsenic removal by ferric chloride

In this study, synthetic wastewater containing 110 µg/L arsenate (As(V)), 0-20 mg/L fulvic acid (FA), and 0-12.3 mg/L phosphate was treated with 3 mg/L Fe3+. The mechanisms of FA and phosphate effects on As(V) removal by ferric chloride were determined using 0.22-10 µm pore-size filtration, Zetasizer analysis, and in situ flow through cell ATR-FTIR. The results showed that up to 20 mg/L FA had almost no effect on the solubility of ferric hydroxide precipitates and adsorption of As(V) by the precipitates. When FA concentration increased from 0 to 20 mg/L, the adsorption of FA led to higher negative zeta potential of the precipitates and the strong electrostatic repulsion between the precipitates decreased the particle size of ferric hydroxide flocs from larger than 10 µm to smaller than 1 µm. In the presence of 5-20 mg/L FA, 46%-63% As(V) was adsorbed onto the flocs with particle size in the range of 0.45-1 µm. On the other hand, phosphate did not affect the size of ferric hydroxide flocs and significantly increased the dissolved As(V) concentration because it competed with As(V) for adsorption sites on ferric hydroxide precipitates. The addition of 5 mg/L cationic organic flocculant significantly reduced the effect of FA on As(V) removal, but did not reduce the effect of phosphate on As(V) removal. The findings of this study will help develop effective arsenic treatment techniques and predict the mobility of arsenic in the environment.

Development of arsenic removal unit with electrocoagulation and activated alumina sorption: Field trial at rural West Bengal, India

In the present study, electrocoagulation (EC) with iron electrodes along with adsorption by activated alumina has been evaluated as a treatment process for arsenic removal from groundwater. Batch scale experiments were performed with iron electrodes to assess the removal efficiency of electrocoagulation. The effects of inter-electrode distance and electro-charge loading (ECL) were evaluated on arsenic removal. EC was able to decrease the arsenic concentration in the aqueous phase below 10 ppb at ECL of 60 coulomb/L with 1 cm inter-electrode distance. The optimal condition was obtained by Response Surface Methodology (RSM) with the help of central composite design (CCD). The regression equation and analysis of variance were originated using Design-Expert software and F value (131.56), p-value (<0.05), and desirability value of 1 were obtained for the optimized model. Activated alumina adsorption has also been shown to be a promising arsenic removal technology (As5+) with 15 min of equilibrium time. An arsenic removal system was designed with EC unit, sand filter, and activated alumina column in series for field application. The field unit was able to deliver safe water to the local community. During the operation, the electrodes were passivated, and calcium-based deposition was found on the cathodes. Periodic electrode reversal was able to minimise the passivation. Due to the accumulation of ferric hydroxide flocs, the pores of the filter bed become clogged and they must be cleaned by backwashing. The Backwash system should be continued until the water runs clear. An arrangement of a suitable soak pit is made to contain the backwash water avoiding any environmental contamination. EC could be able to deliver safe water locally in rural areas at affordable prices due to its high efficiency and low operation and maintenance costs.

Electrocoagulation for Arsenic Removal: Field Trials in Rural West Bengal.

Arsenic contamination in drinking water is a great concern in different regions of the world as well as in India. Several technologies have been investigated to remove arsenic from water, such as coagulation and co-precipitation, ion exchange, adsorption, and reverse osmosis. In the present research, electrocoagulation with iron electrodes has been assessed as a treatment technology for arsenic removal from groundwater to reach concentrations below 0.01mg/L (WHO limit) and which is technically effective, affordable for the local area, and easy to operate and maintain. Electrochemically generated iron is converted to hydrated ferric oxide within the contaminated water, which takes up the arsenic from water. A downstream filtration unit (sand or activated alumina) is applied to remove ferric hydroxide flocs produced during the process. The laboratory experiments were conducted in a batch reactor using iron plates as electrodes with monopolar configuration to study the effects of initial pH and electro-charge loading (ECL) on arsenic removal. The optimum operating condition was observed for an electro-charge loading of 25-30 Coulombs/L at pH 7.0 and an initial arsenic concentration of 0.2mg/L. Two field trials were implemented in West Bengal after suitably designing the electrocoagulation system. Arsenic removal was significant (75-80%) delivering safe water with arsenic below 0.01mg/L (acceptable limit). Passivation of the electrodes occurred during the operation and calcium-based (including iron) deposition was observed on the cathodes. Passivation is avoidable after running regular polarity reversal of the electrodes.

Removal of linear alkylbenzene sulfonate (LAS) by a cetyltrimethylammonium bromide (CTAB)-aided coagulation-filtration process

ABSTRACT This paper reports on the removal of linear alkylbenzene sulfonate (LAS) by conventional and cetyltrimethylammonium bromide (CTAB)-aided coagulation-filtration processes. The addition of CTAB led to the formation of CTAB-LAS aggregates, whose surface charge was negative at a CTAB-LAS molar ratio < 1 and positive at a molar ratio > 1. The efficacy of the coagulation-filtration process strongly depended on the electrostatic interactions between hydroxide floc and both free LAS and CTAB-LAS aggregate. Since free LAS is anionic, its adsorption onto the hydroxide floc was enhanced when the floc was positively charged. Ferric hydroxide floc (pH < 6.0) and aluminum hydroxide floc (pH < 7.4) showed positive surface charges and were thereby effective in removing free LAS. The optimal coagulation pH for removing the CTAB-LAS aggregate depended on the CTAB/LAS molar ratio. A higher coagulation pH was effective in removing LAS under the CTAB/LAS molar ratio > 1. However, in this case, excessive free CTAB remained after coagulation-filtration. Therefore, the lower coagulation pH with a CTAB/LAS molar ratio < 1, with the hydroxide floc and CTAB-LAS aggregate being positively and negatively charged, respectively, was recommended for LAS removal. In chemical industry wastewater, the optimal CTAB dose was higher than that in synthetic wastewater due to coexistent chemicals scavenging CTAB. Consequently, coagulation-filtration using a ferric chloride coagulant under the CTAB/LAS molar ratio of 0.7–2.0 and pH 4.5 was recommended for LAS removal from chemical industry wastewater, where the observed LAS removal efficiency reached 91 ± 3% without any deterioration of COD removal.

Influence of humic acid on the removal of arsenate and arsenic by ferric chloride: effects of pH, As/Fe ratio, initial As concentration, and co-existing solutes.

The influence of humic acid (HA) on the removal of arsenic by FeCl3 was systematically studied in this paper. Jar tests were performed to investigate the influence on arsenic during FeCl3 coagulation of the pH adjusting method, the initial As/Fe ratio, the equilibrium As concentration, and co-occurring anions and cations. Compared with results in HA-free systems, the removal trends of arsenic in HA solutions were quite different. It was found that As(V) removal was higher at low equilibrium concentration, yet the opposite was true for As(III) removal. The presence of HA influenced the effective number of active sites for arsenic removal by FeCl3 flocculation. In addition, in the presence of HA, the impacts of co-existing solutions on arsenic removal were also different from that of an HA-free system. This study examined the influence of co-occurring anions, such as phosphate, sulfate, and silicate on arsenic removal, depending on their ability to compete for sorption sites and to hinder or facilitate the aggregation of ferric hydroxide flocs. The presence of Ca2+ or Cd2+ significantly increased arsenic removal at higher pH. Low concentrations of dissolved HA and high concentrations of colloid affected the adsorption of arsenic onto iron oxide. The influence of HA on the adsorption of arsenic onto iron oxide primarily depended on the relative content of the dissolved and mineral combination states of HA and the interface combination forms.

Applying chemical sedimentation process in drinking water treatment plant to address the emergent arsenic spills in water sources

Arsenic (As) spills occurred more frequently and sometimes polluted water sources in recent years in China. It is as urgent need to develop emergency treatment technologies to address the arsenic threat for large-scale water treatment plants. In response, we developed a chemical sedimentation technology to remove arsenic contaminants for water treatment plants. Bench-scale experiments were conducted to investigate the efficiency of arsenic removal and the influencing factors of the chemical sedimentation treatment process. The influencing factors included the choice and dosage of coagulants, the valence of arsenic and pH value of solution. The As(V) contaminants can be almost completely removed by ferric or alum coagulants. The As(III) contaminants are more recalcitrant to chemical sedimentation, 75% for ferric coagulant and 40% for alum coagulant. The quantitative results of arsenic removal load by different ferric or alum coagulants were presented to help determine the para- meters for arsenic treatment technology. The dominant mechanism for arsenic removal is static combination, or adsorption of negative arsenic species onto positive ferric hydroxide or alum hydroxide flocs. The efficiency of this treatment technology has also been demonstrated by a real production test in one water treatment plant with arsenic- rich source water and one emergency response. This technology was verified to be quick to set-up, easy to operate and highly efficient even for high concentration of arsenic.

Enhanced removal of Methylene Blue by electrocoagulation using iron electrodes

The removal of pollutants from effluents by electrocoagulation has become an attractive method in recent years. The study deals with the enhancement of removal of Methylene Blue dye by using an electromagnetic field during the electrocoagulation process. Effects of electrolyte concentration, dye concentration, intensity and the direction of the electromagnet on the decolorization efficiency have been investigated. The formed ferric hydroxide flocs trap colloidal particles and make solid–liquid separation easier during the next stage. The electrocoagulation stages must be optimized in order to design an economically feasible process. The results showed that the optimum electrolysis was 10–20min at a current density of 8mA/cm2, while the optimum concentration of the electrolyte (NaOH) was found to be 2wt.% when the dye concentration was 50mg/L. The utilization of an electromagnetic field enhanced the dye removal due to the induced motion of paramagnetic ions inside the solution. The power consumption required to remove the dye was reduced by 45% in the case of applying an electromagnetic field.

Impact of NOM character on copper adsorption by trace ferric hydroxide from iron corrosion in water supply system

The fate of trace concentrations of cupric sulfate (<0.4mg/L) dosed into chloraminated distribution systems to inhibit nitrification has been shown in this study to be controlled by at least two crucial factors: the character of natural organic matter (NOM) and iron hydroxide corrosion products present at low concentrations (<2mg/L). This research quantified the removal of Cu(II) ions added into waters containing trace Fe(OH)3 flocs and the effect of NOM of different character on this removal. The dominant dissolved copper species in NOM-containing waters were found to be Cu(II)–NOM complexes. Both intramolecular chelation and intermolecular complexation can occur, with the latter occurring preferentially and resulting in the aggregation of smaller organic molecules to form larger molecules. The presence of ferric hydroxide flocs when Cu(II) ions were added into NOM-containing waters was shown to result in removal of Cu(II) ions, presumably as Cu(II)–NOM complexes. This removal was through adsorption processes obeying Freundlich isotherms, although the presence of larger NOM molecules and heterogeneous copper species (e.g. Cu(OH)2(s) and CuO(s)) appeared to shield smaller Cu(II)–NOM complexes from adsorption to some extent. For the strategy of inhibition of nitrification in distribution systems by the addition of Cu(II) ions, complexation of Cu(II) ions by NOM and adsorption of Cu(II)–NOM complexes by ferric hydroxide flocs released from pipe walls pose significant operational challenges to maintaining the concentration of Cu(II) ions through the distribution system.

Arsenate and arsenite removal by FeCl3: Effects of pH, As/Fe ratio, initial As concentration and co-existing solutes

Coagulation is one of the most commonly used technologies for arsenic removal from water and wastewater. Jar tests were carried out to determine the influence of pH, initial As/Fe molar ratio, equilibrium As concentration and co-occurring solutes on the crossover pH, where As(V) removal was equivalent to As(III) by FeCl3 coagulation. As(V) was more effectively removed than As(III) at lower pH values in FeCl3 coagulation process but the opposite was true at higher pH level. Increasing As/Fe ratio from 0.12 to 0.50 progressively lowered the crossover pH from 8.5 to 7.4. The arsenic isotherms revealed that As(V) removal was favored at low pH and low equilibrium concentration yet the opposite was true for As(III). The co-existing solutes had different influences on the crossover pH, depending on their ability to compete for sorption sites and to hinder or facilitate the aggregation of ferric hydroxide flocs. The presence of sulfate broadened the pH range where As(III) removal was better than As(V) removal. However, As(V) removal was always superior to As(III) removal in the presence of Ca2+ and no crossover pH was observed. The results from this study revealed that arsenic speciation, arsenic loading, and pH should be considered when predicting and managing arsenic removal by coagulation.

Comparative study of the effects of experimental variables on growth rates of aluminum and iron hydroxide flocs during coagulation and their structural characteristics

In this study, the dimensions of over six thousand flocs were analyzed to quantitatively and comparatively investigate the effects of several experimental variables on the growth rate of aluminum (Al) and ferric (Fe) hydroxide flocs. Results show that Fe hydroxide flocs have faster growth rate than Al hydroxide flocs; and the average size of the former is larger than that of the latter. Increasing the concentration of the bivalent sulfate ion (SO 4 2−), initial turbidity, or slow mixing rate, was able to increase the growth rate of both kinds of flocs. On the other hand, steady floc sizes were found to decrease with the increase in SO 4 2− concentration, initial turbidity, or shear rate. Fe hydroxide flocs are more prone to be influenced by the changes in the variables than Al hydroxide flocs. While the steady floc sizes became smaller when initial turbidity or slow mixing speed increased, the roundness and smoothness of flocs were found to increase, indicating that higher initial turbidity or larger slow mixing rate produces flocs with more regular and round shape. Furthermore, at a fixed shear rate, Fe hydroxide flocs are stronger than Al hydroxide flocs. However, Fe hydroxide floc sizes are much easier to decrease with the increase in slow mixing intensity.

Effect of organic matter on arsenic removal during coagulation/flocculation treatment

The aim of this study is to evaluate the influence of organic matter on arsenic removal by coagulation/flocculation on both a model water with low mineral content and a natural water sample. Ferric chloride was used as coagulant at concentrations avoiding the preoxidation step usually required to oxidize As(III) and increase its removal. Arsenic removal was accomplished by combining evaluation of arsenic residual concentrations and speciation analysis with zeta potential measurements. A preliminary study evaluated the influence of coagulant dose, coagulation pH, and organic matter on As(III) and As(V) removal. The main conclusions were: (i) As(III) removal depended on coagulant dose and on the number of sites available on hydroxide surfaces rather than on coagulation pH; (ii) As(V) removal depended on the zeta potential of colloidal suspension and was more influenced than As(III) by coagulation pH and the presence of organic matter; (iii) organic matter removal followed As(V) removal. This allowed determination of adsorption as the main mechanism occurring during As(V) and organic matter removal and supposing precipitation/coprecipitation as an important As(III) removal mechanism. Adsorption on preformed ferric hydroxide flocs experiments allowed then confirmation of these hypotheses.

Indecisiveness of electrophoretic mobility determination in evaluating Fe(III) coagulation performance

The magnitude of electrophoretic mobility (EM) is usually used to represent the repulsion force between colloid particles in an aqueous system. A generally accepted point is that particles will coagulate well when the suspension EM is close to zero, which is often realized by adding trivalent cations such as Fe(III). With regard to Fe(III) coagulation, however, we found that particles were primarily enmeshed by voluminous ferric hydroxide precipitate or flocs, and the enmeshment of particles by flocs was independent of the suspension EM during Fe(III) coagulation. A suspension with either highly negative or positive EM could be well coagulated and clarified if the solution pH and Fe(III) concentration favored the formation and fast growth of ferric hydroxide precipitate. For an effective Fe(III) coagulation, the EM was not necessarily zero.

Evaluation of waste activated sludge as a coagulant aid for the treatment of industrial wastewater containing mixed surfactants

Wastewater generated by the industry manufacturing detergents and various kinds of consumer products normally contains very high contents of mixed surfactants, organic matters expressed as chemical oxygen demand (COD), and phosphates that must be treated prior to discharge to the aquatic environment. In this study, jar-test experiments were conducted to evaluate the waste activated sludge (WAS) as a coagulation aid in the coagulation-flocculation process with ferric chloride or aluminum sulfate as a coagulant for the treatment of wastewater collected from the aforementioned industry. The WAS was selected because of its adsorption capability of anionic surfactants and its availability from the wastage stream of biological wastewater treatment process. The effective dosages of both coagulants with and without the WAS additions were determined in this study. Without the WAS addition, the concentrations of 800 mg/L aluminum sulfate at the optimum pH value of 8 and 2208 mg/L ferric chloride at the optimum pH value of 12 were the optimum chemical dosages. It appears that aluminum sulfate was more effective than ferric chloride based on the chemical dosage and removal efficiency. The turbidity, suspended particles, anionic surfactants, COD, and phosphates removal efficiencies of aluminum sulfate and ferric chloride under the optimum dosage were 95.6, 88.2, 78.4, 73.5, 47.3% and 98.8, 92.0, 72.7, 67.5, 53.1%, respectively. The addition of 200 mg/L WAS was sufficient to reduce the optimum dosages of both chemicals by 200 mg/L. The cationic surfactant existing in the wastewater worked as a flocculating agent leading to the flocculation of waste activated sludge resulting in the enmeshment of the suspended particles and colloids on which the COD, anionic surfactants, and phosphates were adsorbed. However, the substances removal efficiencies of ferric chloride and aluminum sulfate were slightly enhanced and reduced, respectively. It is possibly explained that the settling time is insufficient to settle the aluminum hydroxide floc when it is compared to the ferric hydroxide floc because the iron floc is normally heavier than the alum floc.

Enhanced pre-coat engineering (EPCE) for micro- and ultrafiltration: the solution for fouling?

The feasibility of ultra- and microfiltration depends strongly on the achieved net flux. Direct treatment of surface water frequently results in low net fluxes and high cleaning frequencies. Experiments have been conducted at constant flux on surface water with an ultrafiltration pilot plant with direct filtration, in-line coagulation and pre-coating (EPCE) with ferric hydroxide flocs. The aim was to control and reduce the rate of fouling. With the use of a pre-coat at the beginning of the filtration cycle a stable ultrafiltration process was obtained. This is contrary to the use of in-line coagulation and direct filtration which resulted, due to a very high fouling potential of the feed water, in high fouling rates at low fluxes. The result of this exploratory research is an important step towards a higher feasibility of micro- and ultrafiltration.

Flocs in Shear and Strain Flows

Preflocculated ferric hydroxide flocs were subjected to either a simple shear flow or a two-dimensional straining flow, and their motion was optically observed. Digital image analysis was applied to extract information on orientation and deformation from the digitized frames. It was found that the simple shear flow led to a rotation of the flocs whose motion can be understood from the behavior of a solid ellipsoid. In the extensional flow, no continuous rotation occurred and flocs were broken apart along the axis of straining. The rupture forces estimated from an ellipsoid model were found to be in the range of 0.1 N/m2.

Break-up of flocs in contraction and swirling flows

The break-up of ferric hydroxide flocs was observed by optical methods in a contraction and a swirling flow. The corresponding flow fields were produced by pumping water through two pipes of different diameter, which were either kept stationary or rotated around the axis of symmetry. Digital image analysis was applied to obtain data on the orientation and deformation of the flocs. It was found that flocs were broken apart by both flow fields before they passed the orifice. Based on a simplified model the rupture forces were estimated to be on the order of 0.1 N m −2.

Municipal wastewater treatment by lime/ferrous sulfate and dissolved air flotation

In this work a chemical treatment system for municipal wastewater using copper cementation effluent and lime as coagulants in combination with dissolved air flotation, was investigated. Copper cementation effluent (CCE) containing about 45% dissolved solids (density=1.31) is a liquid waste from a leaching process, which is disposed in large ponds by several copper mines in the North of Chile. CCE contains a combination of ferrous iron, sulfate, chlorine, magnesium, sodium, residual copper and other ions. For its use as a coagulant, CCE was treated using a low cost process. Ferric hydroxide flocs precipitated after CCE addition were efficiently removed by dissolved air flotation producing a low density froth. High suspended solid removal efficiency, process flexibility, short overall residence time in addition to low cost of chemicals used and precipitation of varied proportions of contaminants, including bacteria from wastewater, are the main features of this chemical treatment system.

Removal of Arsenic from Aqueous Solution by Adsorbing Colloid Flotation

Adsorbing colloid flotation (ACF) with ferric hydroxide as the coprecipitant, anionic surfactant sodium dodecyl sulfate (SDS) as the collector, and nitrogen microbubbles has been shown to be effective in removing arsenic from low concentration of arsenic aqueous solution ([approximately]10 mg/dm[sup 3]). Experiments were conducted to assess the effects of pH, dosages of SDS and Fe(III), gas flow rate, foreign anions including NO[sub 3][sup [minus]], SO[sub 4][sup 2[minus]], and PO[sub 4][sup 3[minus]], and Al(III) addition on the efficiency of the arsenic removal by ACF. When pH is at the range of 4--5, 99.5% arsenic removal efficiency can be achieved. The optimal operating conditions are 35 ppm SDS, 80 ppm Fe(III), 40 cm[sup 3]/min gas flow rate, 500 rpm rotary speed of stirrer, and 5 min ferric hydroxide floc formation time. The analysis results indicate that foreign anions SO[sub 4][sup 2[minus]] and PO[sub 4][sup 3[minus]] have an inhibiting effect on the removal of arsenic, while AI(III) additions can compensate for the effect. An actual mine wastewater sample containing 10.4 ppm arsenic(V), 0.76 ppm chromium(VI), and 0.55 ppm lead was tested by adsorbing colloid flotation at the optimal operation conditions. The residual arsenic, chromium, and lead are 0.07, 0.06, and 0.04 ppm, respectively.more » The interaction mechanisms between Fe(OH)[sub 3]-bearing arsenic floc and SDS were analyzed and interpreted by means of [zeta]-potential, infrared spectra analysis, and molecular orbital theory. The adsorption of SDS can occur physically and chemically on Fe(OH)[sub 3] at the pH range of 4--5.« less

Preformed Ferric Hydroxide Flocs as Adsorbents of Humic Substances

The purpose of this study is to show that previously built flocs obtained by the coagulation-flocculation of kaolinite suspension with ferric chloride are a potential adsorbent of humic substances. Flocs are prepared from kaolinite (&amp;lt; 50 µm) and FeCl3 according to the jar-test technique:after 3 minutes of rapid mixing at pH = 5.4 ± 0.1, 30 minutes of slow mixing, humic acids are added for a contact time of 6 hours; the effects of calcium, sodium salts (chloride, sulphate) on the adsorptive capacity of preformed flocs are also studied. The results indicate that the preformed flocs are good adsorbent materials. Langmuir equation is used to determine adsorption capacities.

Kinetics of aggregate formation in rapid mix

The growth of porous aggregates during initial mixing is investigated in a series of numerical simulations. Hydrodynamic radii of aggregates composed of i primary particles are used to calculate collision rate constants as a function of fractal dimension, D, and aggregation number, i. D is assumed to be independent of aggregate size. The rate of floc volume formation increases by several orders of magnitude as the fractal dimension decreases from 3 to 2. Neglecting aggregate porosity is estimated to introduce an error of < 10% in calculating the collision efficiency factor from particle number measurements and a coalescing-sphere model for particle aggregation. For conditions approximating those in which ferric sulfate is added to a rapid mix basin, simulations with D = 2.5 produce a volume fraction of flocs larger than 1 μm that approximates the volume fraction greater than 1 μm measured using an electronic particle counter. A fractal dimension of 2.35 produces an aggregate volume distribution that most nearly approximates the volume distribution observed in jar tests with ferric sulfate. These values of D are within the range of values reported in the literature for larger ferric hydroxide flocs. Mixing conditions in most full scale installations are likely to favor the formation of aggregates of precipitated coagulant particles that are sufficiently large for breakup and aggregate restructuring to control the size distribution of materials leaving the rapid mix basin.