Iron Hydroxide Flocs Research Articles

In-situ generation of both hydroxyl radical and adsorptive flocs in electro-coagulation process with air breathing cathode

In order to achieve considerable generation of free radicals during electro-coagulation (EC) process, EC with air breathing cathode (EC-ABC) was designed to allow for the coexistence of electro-Fenton (EF) and EC functions. This EC-EF-ABC synergetic system allows for both in-situ free radicals and adsorptive flocs generation in an undivided electrolytic cell. The yield of hydroxyl radicals and adsorptive flocs were quantified for accurate mechanism analysis. The control mechanism of the transition from EF to EC function during EC-ABC system was investigated. The air cathode material was studied and optimized to obtain high hydroxyl radical generation. The influences of both dissolved oxygen (DO) and pH condition on hydroxyl radical and flocs generation were studied. Whiles the latter was found to dominate the successive EF and EC functions, and further controls the hydroxyl radicals and flocs yield, the former only has a dominant effect on the hydroxyl radical yield. The adsorptive iron (oxy)hydroxide flocs’ structure was also found to be fully depend on the DO condition. The control mechanism of initial pH and DO condition on radical and flocs generation was simplified as a ‘Sudoku-like’ relationship. The optimized initial pH and DO condition was studied to maximize the generation of hydroxyl radicals and adsorptive flocs. The design of EC-ABC process could have both advanced oxidation and adsorption abilities for wastewater pollutant removal.

Removal of levofloxacin and ciprofloxacin from aqueous solutions and an economic evaluation using the electrocoagulation process‏

ABSTRACT For this research, the utilisation of electrocoagulation (EC) toremove theciprofloxacin (CIP) and levofloxacin (LVX) from aqueous solutions was examined. The effective removal efficiencies are 93.47% for CIP and 88.00% for LVX, under optimum conditions. The adsorption isotherm models with suitable mechanisms were applied to determine the elimination of CIP and LVX utilizingtheEC method. Thefindingsshowed the adsorption of CIP and LVX on iron hydroxide flocs followed the Sips isotherm, with correlation coefficient values (R2) of 0.939 and 0.937. Threekinetic models were reviewed to determine the accurate CIP and LVX elimination methods using the EC method. The results showed that itfittedfor the second-order model, which indicated that the chemical adsorption mechanism controlled the removal of CIP and LVX. The R2 with CIP is 0.944, and LVX is 0.941. For binary system removal, efficiencies were 93.00, 90.10, and 96.30% for CIP, and 91.80, 96.10, and 92.97% for LVX, at the CIP ratio: LVX of 1:1, 1:4, and 4:1. The electrode consumption (ELC) and electrical energy consumption (EEC) were found at 0.208 g and 3.21 kWh−3 for a single operation. The operating cost was estimated at 0.613 US$ m−3.

OPTIMIZATION OF LEVOFLOXACIN REMOVAL FROM AQUEOUS SOLUTION USING ELECTROCOAGULATION PROCESS BY RESPONSE SURFACE METHODOLOGY

This study was aimed to investigate the response surface methodology (RSM) to evaluate the effects of various experimental conditions on the removal of levofloxacin (LVX) from the aqueous solution by means of electrocoagulation (EC) technique with stainless steel electrodes. The EC process was achieved successfully with the efficiency of LVX removal of 90%. The results obtained from the regression analysis, showed that the data of experiential are better fitted to the polynomial model of second-order with the predicted correlation coefficient (pred. R2) of 0.723, adjusted correlation coefficient (Adj. R2) of 0.907 and correlation coefficient values (R2) of 0.952. This shows that the predicted models and experimental values are in good agreement. The results of the kinetic study showed that the second-order kinetic model was in good agreement with the experimental results and suggested that the mechanism of chemisorption controlled the LVX adsorption. The experimental results indicated that the adsorption of LVX on iron hydroxide flocs follows Sips isotherm with the value of the correlation coefficient (R2) of 0.937. Sips isotherm shows that both homogenous and heterogeneous adsorption can occur.

Removal of polyethylene glycols from wastewater: A comparison of different approaches

Physicochemical methods such as adsorption on activated carbon, oxidation with either ozone or Fenton reagent, and chemical precipitation (coagulation), were assessed for the removal of polyethylene glycol (PEG) from wastewater. This contaminant is rarely investigated due to its low toxicity, although its presence limits the use of large water resources. The experimental tests showed that adsorption on activated carbon is well approximated by a Langmuir isotherm, and influenced by contact time, PEG molecular weight, pH, temperature, and initial PEG concentration. Ozonation allowed fragmenting the polymeric chains but was unable to remove completely the PEG, while about 85% of the total organic carbon (TOC) was removed by Fenton oxidation reaction by using a ratio between H2O2 and FeII close to 4. Coagulation did not produce results worthy of note, most likely because the uncharged PEG molecule does not interact with the iron hydroxide flocs. However, when performed after the Fenton oxidation (i.e., by simply raising the pH to values > 8), it allowed a further reduction of the residual TOC, up to 96% of the total, in the best case. Based on the resources used by each process studied and in consideration of the effectiveness of each of them, a semi-quantitative comparison on the sustainability of the different approaches is proposed.

Improved electrolysis of colloid-polluted wastes using ultrasounds and electrocoagulation

In this work, a new strategy for the treatment of wastes polluted with micelles is evaluated. It consists on the concentration of the pollutant by electrocoagulation, the pre-treatment of the concentrated sludge with US and the later application of an electrolytic oxidation. As it is well-known that the electrolytic treatment of diluted solutions generally fits well a first-order kinetic, this concentration is expected to lead to a more efficient process. This approach has been studied using oxyfluorfen polluted wastes. Electrocoagulation has been carried out for the concentration of a wastewater containing oxyfluorfen reducing its volume from 50.00 to 1.39 L. Once obtained the small volume of concentrated liquid waste, several treatments have been applied such as Ultrasonic irradiation or heat followed by electrooxidation. Results show that application of US has a very positive influence on the performance, in particular LF-US. Application time is also an important parameter and heating has a negative effect on results because iron hydroxide flocs compress and seal the pollutant inside making more difficult their destruction by the direct and indirect oxidation reactions.

Modeling adsorption kinetic of crystal violet removal by electrocoagulation technique using bipolar iron electrodes

This paper studies the removal of crystal violet (CV) dye by electrocoagulation (EC) process using bipolar iron electrodes. Numerous operating parameters such as initial CV solution concentration, speed of agitation, number of electrodes, type and quantity of supporting electrolyte, temperature- and initial pH were investigated. A complete removal of 10 mg/L CV was achieved within 10 min at pH 7, 0.5 g NaCl, 1 LCV, 750 rpm, 9 sheets, 17.36 mAcm2, and 25 °C. The performed energy-dispersive X-ray spectroscopy (EDAX), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analyses confirmed the adsorption of CV onto the insoluble iron hydroxide flocs. Amongst the studied adsorption isotherms models, Langmuir and Dubinin-Radushkevich were the most applicable. The kinetic of adsorption of CV onto flocs during the EC operation was studied using the pseudo-first-order, pseudo-second-order, and intraparticulate diffusion models, with results affirming that the adsorption process proceeded according to the pseudo-second-order model. The study of thermodynamic parameters (ΔG0, ΔH0, and ΔS0,) of the CV removal at different temperatures reflected the feasibility of the spontaneous randomness of endothermic adsorption, especially at lower temperatures. The multiple regression equation of the removal of CV by EC technique under the different studied conditions was predicted.

Simultaneous removal of cadmium, zinc and manganese using electrocoagulation: Influence of operating parameters and electrolyte nature

In the present study, the influence of operating parameters and electrolyte nature on the simultaneous removal of toxic metals (cadmium, zinc and manganese) from synthetic smelting wastewater by batch electrocoagulation was investigated. This wastewater contained high concentrations of anion–cation electrolytes. Results indicated that the efficiency of heavy metals removal can be enhanced by increasing the solution pH and current density. The Fe–Fe electrode combination is more effective than the other combinations (Al–Al, Al–Fe and Fe–Al). The interaction of heavy metal ions showed that the increase of initial Zn2+ concentration adversely affects on Cd2+ removal. In addition, the single chloride system exhibits the optimum removal efficiency on Mn2+. Single sulfate and binary anion systems exert a more positive effect on Cd2+ and Zn2+ removal because of the stronger charge neutralization and destabilization of iron hydroxide flocs. Increases of Ca2+ and Mg2+ ions exert a significant negative effect on metal removal. However, the addition of a small amount of sodium chloride into a high sulfate and hardness solution can accelerate the removal of heavy metals. Finally, the sludge samples generated from electrocoagulation were characterized by XRD and SEM–EDS analyses.

Characteristics of fluoride adsorption onto aluminium(III) and iron(III) hydroxide flocs

ABSTRACTThe fluoride adsorption onto the hydroxide flocs of Al(III), Fe(III), or a mixture of the two was studied. The optimum pH was influenced by the flocs’ solubility and surface charge. Although the Al(III) hydroxide flocs had a maximum adsorption capacity at the equilibrium concentration of 8 mg-F/L, the two-stage adsorption process revealed that the mixture of the Al(III) and Fe(III) hydroxide flocs required the smallest adsorbent dose as the concentration changed from 40 to 8 mg-F/L. Thus, the simultaneous dosing of Al(III) and Fe(III) combined with a two-stage adsorption process appears to be an effective option for fluoride removal.

Efficient removal of perfluoroalkyl acids (PFAAs) from aqueous solution by electrocoagulation using iron electrode

Electrocoagulation (EC) technique was used to investigate the removal performance of aqueous perfluoroalkyl acids (PFAAs) with relatively high concentration as simulating the wastewater from organic fluorine industry. A comparison between iron and aluminum electrode was made to remove PFAAs with the similar electrolysis condition. Effective removal of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) was obtained by EC process, especially for Fe anode. Several key factors were studied to optimize the EC process using Fe electrode. At the optimal operating parameters including 25.0mA/cm2 of current density, 180rpm of stirring speed, and 2g/L NaCl as supporting electrolyte, more than 99% of PFOS could be removed with 0.25mM of initial concentration after 50-min electrolysis. Additionally, the adsorptive removal of aqueous PFOS on iron hydroxide flocs during EC was also verified by Fourier transform infrared spectra. The investigation of removal efficiency for PFAAs with different carbon chain length after EC and the characteristic of zeta potential on Fe flocs indicated that the electrostatic adsorption was primarily responsible for the PFAAs sorption on the iron hydroxide flocs.

Comparison of the Capacity of Chitosan and Iron Hydroxide Flocs for Phosphate Adsorption in Seawater

Adsorption is proposed as an effective method for phosphate removal in real seawater with low phosphate concentration. This paper aimed to investigate the capacity of chitosan and iron hydroxide flocs for phosphate adsorption in real seawater, and the results shown iron hydroxide flocs had better performance of phosphate adsorption than chitosan.

Effect of dye molecules and electrode material on the settling behavior of flocs in an electrocoagulation induced settling tank reactor (EISTR)

A simple electrocoagulation induced settling tank reactor (EISTR) was fabricated to study the dye removal and sedimentation kinetics of Amido Black 10B (AB), Methyl Violet (MV), Eosin Yellow (EY), Malachite Green (MG), Methylene Blue (MB), Rhodamine 6G (R6G), their binary, tertiary mixtures and actual textile effluent. Optimum conditions such as current density, electrolysis time, initial pH and NaCl concentration to achieve high color removal efficiency (CRE) and sludge settling velocity were evaluated for the removal of 100mg/L AB dye. The CRE and chemical oxygen demand (COD) reduction, initial sludge settling velocity, final sludge height and turbidity reduction were measured. These experiments indicate that 81–99% CRE, 54–68% COD reduction and 73–88% turbidity reduction can be achieved for the synthetic as well as textile dye wastewater systems. Around 3.8Ah of charge loading per gram of dye was found to be sufficient for high CRE and satisfactory sedimentation kinetics for most of the dyes. The energy requirement was found to be 24±3Whg−1 of dye molecule. Fe electrodes perform better when compared to Al electrode in similar operating conditions. XRD and SEM studies indicate that relatively large porous low density aluminum hydroxide flocs formed under the induced sedimentation conditions do not settle effectively. Relatively smaller and denser iron hydroxide flocs generated using Fe electrode leads to relatively better performance.

Mechanisms of virus control during iron electrocoagulation – Microfiltration of surface water

Results from a laboratory-scale study evaluating virus control by a hybrid iron electrocoagulation – microfiltration process revealed only 1.0–1.5 log MS2 bacteriophage reduction even at relatively high iron dosages (∼13 mg/L as Fe) for natural surface water containing moderate natural organic matter (NOM) concentrations (4.5 mg/L dissolved organic carbon, DOC). In contrast, much greater reductions were measured (6.5-log at pH 6.4 and 4-log at pH 7.5) at similar iron dosages for synthetic water that was devoid of NOM. Quantitative agreement with Faraday’s law with 2-electron transfer and speciation with phenanthroline demonstrated electrochemical generation of soluble ferrous iron. Near quantitative extraction of viruses by dissolving flocs formed in synthetic water provided direct evidence of their removal by sorption and enmeshment onto iron hydroxide flocs. In contrast, only approximately 1% of the viruses were associated with the flocs formed in natural water consistent with the measured poor removals. 1–2 logs of virus inactivation were also observed in the electrochemical cell for synthetic water (no NOM) but not for surface water (4.5 mg/L DOC). Sweep flocculation was the dominant destabilization mechanism since the ζ potential did not reach zero even when 6-log virus reductions were achieved. Charge neutralization only played a secondary role since ζ potential → 0 with increasing iron electrocoagulant dosage. Importantly, virus removal from synthetic water decreased when Suwanee River Humic Acid was added. Therefore, NOM present in natural waters appears to reduce the effectiveness of iron electrocoagulation pretreatment to microfiltration for virus control by complexing ferrous ions. This inhibits (i) Fe2+ oxidation, precipitation, and virus destabilization and (ii) virus inactivation through reactive oxygen species intermediates or by direct interactions with Fe2+ ions.

Floc Volume Effects in Suspensions and Its Relevance for Wastewater Engineering

The aim of this paper is to better understand oxygen transfer reduction caused by floc suspensions. We demonstrate that the overall floc volume significantly influences oxygen transfer depletion. Submerged fine bubble and coarse bubble diffusers are affected in the same way by this phenomenon. The mixed liquor suspended solids concentration (MLSS concentration) is not an appropriate parameter for describing or relating phenomena that are caused by the overall floc volume in activated sludge (e.g., oxygen transfer depression and sludge sedimentation characteristics). A better correlation is achieved by using the mixed liquor volatile suspended solids concentration (MLVSS concentration). To characterize the effects of the overall floc volume in suspensions whose MLVSS concentration cannot be determined (e.g., inorganic iron hydroxide flocs), a new method-the hydrostatic floc volume (HFV)-that approximates the overall floc volume in floc suspensions is introduced. Application of this method demonstrates that oxygen transfer depression caused by iron hydroxide flocs and activated sludge flocs is similar.

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.

Free Water Content and Sludge Retention Time: Impact on Oxygen Transfer in Activated Sludge

To investigate the effect of sludge retention time (SRT) and the concentration of mixed liquid volatile suspended solids (MLVSS) on oxygen transfer in activated sludge, we studied mass transfer coefficients (k(L)a) in a pilot-scale membrane bioreactor, which treated synthetic greywater. Additionally, experiments with iron hydroxide flocs were performed to examine the role of free water content in oxygen transfer. The results demonstrate that the alpha-factor is reduced when free water content decreases and floc volume increases. Because the MLVSS concentration in activated sludge mainly regulates floc volume, a reduction of oxygen transfer with increasing MLVSS concentration was observed. If the floc volume remains unchanged, oxygen transfer increases with increasing SRT.

Immobilisation of Fe floc: Part 2, encapsulation of floc in composite cement

Radioactive iron (Fe 3+) hydroxide flocs are produced during reprocessing of nuclear fuel at Sellafield, UK. The flocs must be pre-treated with slaked lime before encapsulation in a pulverised fuel ash/ordinary Portland cement composite cement to produce a crack-free wasteform. This paper reports results obtained after investigating the fate of the iron in the floc during cementation. Results indicate that the iron in the floc reacts by substituting into and adsorbing onto the C–S–H phase formed during hydration. Additionally, a small quantity is substituted into a crystalline katoite phase, Ca 3AlFe(SiO 4)(OH) 8. These substitutions are significant because the iron in the floc is rendered chemically immobilised within the cement rather than simply being physically encapsulated. No new phases are formed after twelve years cement hydration and examination of the twelve year old sample indicates that the durability of the wasteform appears to be high.

Immobilisation of Fe floc: Part 1, pre-treatment of floc with slaked lime

A series of radioactive iron (Fe 3+) hydroxide flocs are produced during reprocessing of nuclear fuel at Sellafield, UK. Before the flocs can be successfully encapsulated in a pulverised fuel ash/ordinary Portland cement composite cement to form a durable cemented wasteform, they need to be pre-treated with slaked lime. This paper reports results after investigating the reaction between a simulant floc and the slaked lime and shows that the floc reacts readily to form an X-ray amorphous hydrated calcium ferrite. Calcite (CaCO 3), formed from carbonation of Ca(OH) 2, appears to contribute to the formation of a crystalline iron hydroxy carbonate phase (Fe 6(OH) 12(CO 3)). This work concludes that pre-treatment allows a pozzolanic reaction to occur between the pulverised fuel ash and the Ca(OH) 2 produced during cement hydration. This increases the quantity of C S H formed and increases the compressive strength of the hardened wasteform enough to withstand forces generated during curing and prevent cracking.

A study of coagulation mechanisms of polyferric sulfate reacting with humic acid using a fluorescence-quenching method

A fluorescence-quenching method is developed to assess the effect of pH on the coagulation mechanism of humic acids (HA) reacting with metal ions. A polyferric sulfate (PFS) synthesized in our laboratory is adopted as the coagulant to simplify the hydrolysis process and increase the experimental precision. The following results are discovered. When the concentration of PFS increases from 2 to 10 mg/L, the effective pH range of HA removal changes from 4.0–5.0 to 4.0–7.5. At increased coagulation pH, the ferric ions may still react with HA but unable to neutralize the surface charge completely. The residual concentrations of HA measured by fluorescence spectrophotometer are lower than those by TOC, as a consequence of the fluorescence-quenching effect. This demonstrates that the coagulation of HA by PFS at low pH is mainly due to charge-neutralization. The adsorption of the HA on the pre-formed iron hydroxide flocs is accompanied by the dissociation of Fe ions from the floc structure until the equilibrium has been reached, which is evidenced by the presence of the Fe-HA complexes in the solution during adsorption experiment. This is quite different from the characteristics of flocs formed by PFS associated with HA in the coagulation. Within the pH range investigated, the complex-formation and the hydrolysis are the two competitive reactions happened between the hydroxide ions in solution and the functional groups of HA. Therefore, the removal of HA is not caused by adsorbing onto the iron hydroxide resulted from PFS hydrolysis, but through the complex-formation between the PFS and the HA.

Monitoring effects of shearing on floc structure using small-angle light scattering

In this work, small-angle light scattering was used to investigate the formation of iron hydroxide flocs from the hydrolysation of FeCl 3 solution. The freshly formed iron hydroxide flocs had a fractal dimension of 2.25 ± 0.05, and as the hydrolysation proceeded the flocs frew and restructured to a more compact form with a fractal dimension of 2.52 ± 0.05. The effect of shearing on the structure of the flocs was also studied. At low shearing rates, increasing the shear induced only the restructuring of the flocs, but at a high shearing rate the shear induced both the breakup and restructuring of the flocs.

On the Types of Bond Developing between the Aluminium and Iron (III) Hydroxides and Organic Substances

During the coagulation-flocculation processes involved in the treatment of surface waters, dissolved organics are also incorporated into the aluminium and iron (III) hydroxide flocs. A part of the dissolved organics can thus be removed by simple solid-liquid phase separation techniques, like sedimentation, flotation, sand filtration. According to laboratory evidence, durable bonds between the coagulant, or the metal hydroxides thereof, develop only over the extremely short (a few seconds) period which elapses from the formation of the metal hydroxides to the intense aggregation of the metal hydroxide sols. The bonds between these latter and the organic substances are mainly of the hydrogen bridge type, but other secondary chemical bonds and adsorption are also involved. Owing to the chemical structure and relatively low concentration levels of the organic substances, direct valence chemical bonds between the coagulants or the metal hydroxides thereof and the organic substances develop to a slight extent only.