Hydroxide Flocs Research Articles

Removal of perfluoroalkanesulfonic acids (PFSAs) from synthetic and natural groundwater by electrocoagulation

Severe contaminations of perfluoroalkanesulfonic acids (PFSAs) existed in the natural groundwater beneath a fluorochemical industrial park (FIP) in Fuxin of China. In the present study, systematic researches were performed to determine the best conditions of efficient treatment for 1 mg L−1 of PFSAs in the synthetic groundwater samples with the periodically reverse electrocoagulation (PREC) using the Al–Zn electrodes. Based upon the orthogonal experiments, the removal efficiencies of perfluorobutane sulfonate (PFBS), perfluorohexane sulfonate (PFHxS), and perfluorooctane sulfonate (PFOS) could reach 87.4%, 95.6%, and 100%, respectively, within the initial 10 min, under the optimal conditions of voltage at 12.0 V, pH at 7.0, and stirring speed at 400 rpm. In addition, the optimized PREC technique was further applied to remove the PFSA contaminations from the natural groundwater samples of the Fuxin FIP, subsequently generating the removal efficiencies of three target PFSA analytes in the range between 59.0% and 100% at 60 min. Moreover, the SEM-EDS analyses showed the hydroxide flocs formed during the process of PREC treatment had clear characteristics of floc aggregates, with the major constituents of O, Al, C, N, Zn, and F elements. As a result, long-chain PFHxS and PFOS tended to be eliminated completely from the natural groundwater by their absorptions on the Al–Zn hydroxide flocs, potentially because of their higher hydrophobicity compared with short-chain PFBS.

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.

Highly efficient recovery of ruthenium from integrated circuit (IC) manufacturing wastewater by Al reduction and cementation.

Ruthenium (Ru) is a rare-earth metal, which is employed widely in metal-processing industries. This study recovered Ru from the wastewater of an IC foundry by cementation using metallic aluminum (Al) powder as the sacrificial agent. Ru ions were efficiently reduced to the metal and coagulated with the derived aluminum hydroxide flocs. Experimental parameters included the particle size of Al, molar ratio of Al to Ru, initial Ru concentration and operation temperature. The recovery rate reached 99% under these conditions: particle size Al powder = 88–128 μm, Al/Ru molar ratio = 2.0, initial Ru = 200 mg L−1, temperature = 338.16 K, reaction time = 120 min, stirring speed = 150 rpm. The cemented Ru over Al powder was spherical with a rough surface. Kinetic modelling suggested that the diffusion of Ru through the ash layer of Al powder controlled the reaction rate with an activation energy of 40.75 kJ mol−1. A brief cost analysis demonstrated that the cementation of Ru yielded a profit of $0.180 per 0.1 m3-wastewater.

Defluoridization of drinking water by electrocoagulation (EC): process optimization and kinetic study

In this work, removal of fluoride ions from drinking water using electrocoagulation (EC) was carried out using combination of different electrode materials in both monopolar and bipolar connections. The most effective pair of electrode for fluoride removal was found to be Al/Al in bipolar connection. Experiments were conducted to optimize the various operating parameters of EC such as pH, salt dose, stirring speed, inter electrode distance (IED) and current density for efficient removal of fluoride from water. Initial pH of 7.0, current density of 129.31 A/m2, IED of 1 cm, salt dose of 0.33 g/L, stirring speed of 500 and EC time of 40 minutes were found to be optimum conditions to reduce 10 mg/L fluoride ions from water below 1.5 mg/L, which is maximum contaminant level in drinking water as declared by WHO and Indian standard of drinking water. Efficacy of EC process on TDS and turbidity removal of the solution at different operating conditions had also been studied. First order kinetic model was found to fit for fluoride removal. Rate constant (k) increased from 0.02566 to 0.117 min−1 with increase in CD from 17.29 to 258.62 A/m2 due to availability of more aluminum hydroxide flocs in the solution.

Removal of perfluorooctanoic acid in simulated and natural waters with different electrode materials by electrocoagulation

Increasing attentions have been paid on widespread perfluorooctanoic acid (PFOA) contaminations in the environment globally, especially in the industrial wastewaters and natural water bodies. In order to perform the treatment of PFOA simulated solution effectively with periodically reverse electrocoagulation (PREC), intensive studies were implemented on the conditions of treatment and adsorption mechanism. The optimal conversion rate of 1 mg L−1 PFOA treatment could be achieved 99.6% with Al-Zn electrode, when the voltage and stirring speed were set to 9 V and 500 rmp. The energy consumption of treating PFOA simulated aqueous solution at 20 min using Al-Zn electrodes was 43.93 kWh kg−1. In addition, the PREC technique was further applied to eliminate the PFOA contaminations in the natural groundwater, generating the conversion rate up to 79.4% at 60 min. Moreover, it was verified that PFOA contamination was adsorbed on the flocs of Zn0.61Al0.39(OH)2(CO3)0.195·xH2O by the detection of X-ray diffraction spectra (XRD) and Fourier transform infrared (FTIR) spectrum, the mainly mechanism was adsorption bringing and enmeshment, primarily resulting from the hydrophobic interaction of PFOA and Al-Zn hydroxide flocs.

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.

Hybrid electrocoagulation/electroflotation/electrodisinfection process as a pretreatment for seawater desalination

The main objective of this study was to examine the feasibility of a hybrid electrocoagulation/electroflotation/electrodisinfection (EC/EF/ED) process in treating seawater prior to desalination treatment using reverse osmosis process. Residual dissolved organic carbon (DOC) and heterotrophic bacteria concentrations were measured. A continuous treatment using aluminium electrodes was investigated, which lead to a complex gas-liquid-solid mixture in which aluminium flocs and hydrogen gas bubbles enhanced pollution removal. Experimental results showed that DOC removal efficiency increased when the current density increased and when the inlet flow rate decreased. The best performance of DOC removal (69.0%) was recorded while imposing 20mA/cm2 and 320s residence time when inflow pH was 4. Heterotrophic bacteria were completely removed for the highest current densities (20mA/cm2). In the EC process, a low concentration of total chlorine species (up to 0.45mg/L) resulting from the oxidation of chloride anions was formed, which promoted the removal of microorganisms, even though these were mainly removed by the coupled effects of the electric field and the bacteria entrapment in hydroxide flocs. Finally, EC/EF/ED as a one-step pretreatment method was shown to be efficient to replace the conventional methods prior to desalination by reverse osmosis as it was proved to remove efficiently organic matter and microorganisms from seawater: with 0.8L/h and 14mA/cm2, 50% of DOC and nearly all the microorganisms could be removed at less than 0.5kWh/m3.

Modeling of the electrocoagulation process: A study on the mass transfer of electrolysis and hydrolysis products

Electrocoagulation (EC) which is characterized by in-situ generation of coagulant and hydroxide flocs with high absorption ability is an environmental-friendly process for treating wastewater with heavy metal ions and toxic organics. In order to get a systematic understanding of EC process, a steady state model considering electrochemical, hydrolysis reaction, mass and momentum transfer was established. The coagulant (Al3+), H+ and OH− are generated at the direction of streamline. However, the concentration of these species increases and reaches its maximum near inlet and after that they decrease gradually to a much lower level. We found that there are three areas in EC channel: acid front, base front and buffering area, which has also been found in electro-kinetic remediation. At the direction of streamline, the electro-generated Al3+ is gradually hydrolyzed to hydroxides. The anionic and cationic hydroxides accumulate in acid front and base front respectively. The insoluble hydroxides will accumulate in buffering area, which could be considered as a trap for hydroxide flocs and pollutants.

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.

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.

Removal and transformation of polycyclic aromatic hydrocarbons during electrocoagulation treatment of an industrial wastewater

Polycyclic aromatic hydrocarbons (PAHs) are an important class of water pollutants because of their known ecological and human toxicity. Electrocoagulation (EC) is a promising technology for mitigating industrial wastewater pollution, but the removal and transformation of PAHs during EC treatment has not yet been understood. Therefore, a paper-making wastewater effluent (PMWW) was employed in this study to investigate the relationship between PAHs’ removal and transformation during EC treatment. The results show that 86% of PAHs were effectively removed not only by the electro-oxidation reactions, but also by adsorption onto Fe hydroxide flocs. The removal and transformation of PAHs were related to the number of rings in their structures. Some PAHs composed of two aromatic rings (e.g., naphthaline and dimethylnaphthalene) were produced from humic acid-like and fulvic acid-like organics in PMWW, while PAHs with three to four rings were degraded, thus being removed efficiently. Therefore, PAH transformation during EC treatment exerted double-sided effects on the removal of PAHs; the net effect appeared to be positive. Overall, this study revealed the existence and importance of PAH transformation during EC treatment and provided useful guidance for pulp and paper mills to improve the design and operation of wastewater treatment facilities.

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.

Electrocoagulation mechanism of perfluorooctanoate (PFOA) on a zinc anode: Influence of cathodes and anions

Batch experiments were conducted to investigate the effects of cathode materials and anions (Cl−, SO42−, NO3−, and CO32−/HCO3−) on perfluorooctanoate (PFOA) removal in electrocoagulation process using zinc anode. The results indicated that the hydroxide flocs generated in-situ in the electrocoagulation process using the stainless steel rod as cathode were more effective than those using aluminum rod as cathode for the removal of PFOA after 20min of electrocoagulation at a current density of 0.5mAcm−2. Hydroxide flocs generated in-situ in the electrocoagulation in the presence of Cl−/NO3− could effectively remove PFOA from aqueous solution with the removal ratios of 99.7%/98.1% and 98.9%/97.3% using stainless steel rod and aluminum rod as cathode, respectively. However, the PFOA removal ratios were 96.2%/4.1% and 7.4%/4.6% using stainless steel rod and aluminum rod as cathode, respectively, in the presence of SO42− and CO32−/HCO3−. The different removal ratios of PFOA during the electrocoagulation process were primarily due to the fact that the hydroxide flocs generated in-situ were different in the presence of diverse cathodes and anions. We firstly demonstrated that Zn0.70Al0.30(OH)2(CO3)0.15·xH2O and ZnO generated in-situ in the electrocoagulation process (except for CO32−/HCO3−) using zinc anode and aluminum/stainless steel rod cathode governed the sorption of PFOA. The adsorbent hydroxide flocs in-situ generated in the presence of Cl− could effectively remove PFOA from aqueous solution containing CO32−/HCO3− anion at the initial hydroxide flocs concentration of 2000mgL−1. These results provided an effective and alternative method to remove PFOA from aqueous solution containing CO32−/HCO3− anion.

Regrowth of Broken Hydroxide Flocs: Effect of Added Fluoride.

Hydrous oxides of Al(III) and Fe(III) play a large part in environmental processes and in the action of coagulants used in water and wastewater treatment. Aggregates (flocs) of hydroxide precipitates can be rather weak and are easily broken by applied shear. It is usually found that broken flocs do not fully regrow under low-shear conditions, and this could be a serious disadvantage in practical applications. The irreversible nature of floc breakage suggests that some form of specific, chemical interaction between precipitate particles must be at least partly responsible. On the basis of experiments reported here and elsewhere, we propose that hydroxyl bridges between particles play a part. When these are broken, there is a reduction in the number of "active" surface groups that are able to form new bridges. When small amounts of fluoride are added during breakage of Al flocs, there can be significant improvement in floc regrowth, although this depends on a number of factors, especially pH. With Fe flocs, fluoride has no noticeable effect. These results can be explained by the formation of soluble Al-F complexes and some dissolution of the Al(OH)3 precipitate. This creates a new surface with more "active" groups that can form new hydroxyl bridges.

Electrocoagulation: an electrochemical process for water clarification

Electrocoagulation is a group of various procedures used for removing contaminants from water by electrochemically dissolving aluminum or iron. The contaminants of the solution may be incorporated in the in situ forming metal hydroxide flocs, which can be filtered as a precipitate or skimmed as a float. The main features of the procedure are highlighted on the example of cleaning of an oily waste-water. Design parameters of a 1 m 3 /h waste-water cleaning system are calculated from the results of small-scale experiments.

Effect of aluminum hydrolyte species on human enterovirus removal from water during the coagulation process

We prepared different types of aluminum-based coagulants, consisting of mainly monomeric aluminum species, polymeric aluminum species, or colloidal aluminum species, to investigate the effect of aluminum hydrolyte species on the removal of two types of human enteroviruses, poliovirus (PV) type 1 and the free-chlorine-resistant virus coxsackievirus (CV) B5, from lake and river water samples during the coagulation process. We found that differences in the distribution of the aluminum hydrolyte species in the coagulant affected the removal of these enteroviruses during coagulation: the removal ratios of PV and CV observed with polyaluminum chloride (PACl) with a high colloidal aluminum content and a basicity of 2.1 (i.e., PACl-2.1c) were larger than those observed with high monomeric aluminum content coagulant (i.e., AlCl3 solution) and with high polymeric aluminum content coagulant PACl (PACl-2.1b). Unlike AlCl3 or PACl-2.1b, PACl-2.1c contains Al30 species, indicating that Al30 species probably play a major role in the removal of enteroviruses. The PV and CV removal ratios were almost identical, regardless of the coagulant type or viral quantification method used (plaque-forming unit method or real-time polymerase chain reaction method), suggesting that PV and CV behaved similarly during the coagulation process. We also experimentally confirmed that the main mechanism for virus removal was coprecipitation into growing aluminum hydroxide during charge neutralization; virus adsorption onto formed aluminum hydroxide flocs also contributed to virus removal, but played a limited role.

Efficient Sorption and Removal of Perfluoroalkyl Acids (PFAAs) from Aqueous Solution by Metal Hydroxides Generated in Situ by Electrocoagulation.

Removal of environmentally persistent perfluoroalkyl acids (PFAAs), that is, perfluorooctanesulfonate (PFOS) and perfluorocarboxylic acids (PFCAs, C4 ∼ C10) were investigated through sorption on four metal hydroxide flocs generated in situ by electrocoagulation in deionized water with 10 mM NaCl as supporting electrolyte. The results indicated that the zinc hydroxide flocs yielded the highest removal efficiency with a wide range concentration of PFOA/PFOS (1.5 μM ∼ 0.5 mM) at the zinc dosage <150 mg L(-1) with the energy consumption <0.18 Wh L(-1). The sorption kinetics indicated that the zinc hydroxide flocs had an equilibrium adsorbed amount (qe) up to 5.74/7.69 mmol g(-1) (Zn) for PFOA/PFOS at the initial concentration of 0.5 mM with an initial sorption rate (v0) of 1.01 × 10(3)/1.81 × 10(3) mmol g(-1) h(-1). The sorption of PFOA/PFOS reached equilibrium within <10 min. The sorption mechanisms of PFAAs on the zinc hydroxide flocs were proposed based on the investigation of various driving forces. The results indicated that the hydrophobic interaction was primarily responsible for the PFAAs sorption. The electrocoagulation process with zinc anode may have a great potential for removing PFAAs from industrial wastewater as well as contaminated environmental waterbody.

Coagulation of methylated arsenic from drinking water: Influence of methyl substitution

Methylated arsenic can be found in virtually all earth surface environments. So far, however, little information has been collected regarding their removal by coagulation. In this study, the removal of monomethylarsenate (MMA) and dimethylarsenate (DMA) from drinking water by coagulation was investigated from the viewpoint of methyl substitution. Results indicated that FeCl3 was more efficient than AlCl3 and polyaluminum chloride (PACl) in methylated As removal. For the initial arsenic concentration of 200μg/L, an FeCl3 dosage of 0.2mmol Fe/L was sufficient to attain about 95% removal of MMA, while a dosage of 0.6mmol Fe/L achieved about 57% removal of DMA. Arsenic removal efficiency was negatively correlated with the degree of methyl substitution. With the increase in methyl group number, the quantity of negatively charged arsenic species decreased and molecular size increased, leading to the decrease of methylated As removal by coagulation. Adsorption on preformed hydroxide flocs was the major mechanism during coagulation. Both FTIR and XPS results indicated that the AsO group of As might substitute the OH group of Fe/Al hydroxide to form a Fe/AlOAs complex. Furthermore, the use of traditional oxidants and coagulation aids exhibited limited help for improving coagulation removal of DMA.

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.