Understanding the coagulation mechanism and floc properties induced by Fe(VI) and FeCl₃: population balance modeling

As a result of self-decay and/or reactions with reductants (Zhou et al. 2019), ferrate(VI) (i.e. ⁠) eventually becomes stable ferric ions (Fe(III)) via more active intermediate high valence iron species (i.e. Fe(V) and Fe(IV)) through electron flow. It is thus considered as a multifunctional water treatment agent due to its ability to oxidize, disinfect and coagulate simultaneously the undesirable substance in the water (Sharma et al. 2015). While most of the publication available focused on the oxidation functionality of Fe(VI) (Graham et al. 2010), only few studies have been conducted to evaluate its coagulation performance of Fe(VI) in water because it is hard to separate the coagulation effects from its multifunctionalities (Yu et al. 2016). Though the aforementioned difficulties, few efforts were still made to investigate the mechanisms of Fe(VI) coagulation along with its performance in the removal of colloidal particles from water. However, the results are inconsistent. Fe(VI) was reported as a better coagulant than Fe(III) when it was used to treat upland colored water (Jiang et al. 2001). Better algae and turbidity removal was observed when Fe(VI) was used as a pre-oxidation agent before alum coagulation (Ma & Liu 2002a, 2002b). It is reported Ferrate(VI) achieved comparable, or better, floc formation to ferric chloride, but less dissolved organic carbon (DOC) removal in humic acid solution in the absence of particulate matter (Graham et al. 2010). Fe(VI) pre-oxidation also improved the effluent turbidity of a downstream dual-media filter in continuous flow treatment experiments (Goodwill et al. 2016). In contrast, some research showed that Fe(III) demonstrated better coagulation effect than Fe(VI) using photometric dispersion analyzer 2000 (PDA 2000, Rank Brother, Inc., Cambridge, UK) (Yu et al. 2016). The flocs resulted from Fe(VI) and Fe(III) effects were different in size distributions and morphologies. Moreover, Fe(VI) resultant particles were mainly composed of crystalline Fe₂O₃, which was not detected in the Fe(III) resultant particles, as well as more nanoparticles that generated a stable colloidal suspension (Goodwill et al. 2015). It was found that the majority of Fe(VI)-induced flocs was suspended in the secondary effluent indicating its poor settleability (Zheng & Deng 2016). Fe(VI) dose played a key role in the size distribution and coagulation (Cui et al. 2018). High Fe(VI) dose could accelerate the aggregation of fine particles and facilitate downstream solid-liquid separation.

In order to only evaluate the coagulation ability of ferrate and clarify the coagulation performance of ferrate and Fe(III) salt, kaolin was used as the modelled solid particle in water suspension since it is inert to the oxidation effect of ferrate, so the observations of particle growth are believed to be caused by the coagulation effects of ferrate hydrolysis species and the FeCl₃ (Tien et al. 2008).

It is commonly admitted that floc properties (such as floc size, structure and strength) of Fe(VI) and FeCl₃ induced flocs have significant effect on the solid-liquid separation (Sang & Xiao 2008, 2009; Sang & Englezos 2012). Large and compact floc would facilitate the sedimentation and solid-liquid separation (Hubbe 2005). Previously, either jar test or photometric dispersion analyzer were employed to evaluate such floc properties. However, both methods have limitations. Conventional jar test methods are limited in terms of sensitivity and practical convenience, though they do provide a useful visual and semi-quantitative simulation of Fe(VI) and FeCl₃ induced coagulation (Lv et al. 2018). Photometric dispersion analyzer (Rank Brother, Inc., Cambridge, UK), based on optical technique, can provide sensitive and rapid response of Fe(VI) and FeCl₃ induced coagulation. However, information about degree of coagulation and the strength of flocs is ‘relative’. The degree of coagulation/flocculation is represented by a ‘relative’ turbidity. The floc strength is expressed as strength factor which is determined by the ratio of ‘relative’ turbidity obtained before and after the sudden increase in shear rate (Tien et al. 2008). Both aforementioned methods did not provide information on the real floc size, structure and strength. Traditionally, floc structure is assessed by microscopy imaging or transmission electron microscope (TEM) (Lv et al. 2018). The result is not representative for all the floc presented in the suspension as this method is limited to the measurement of only a few hundred flocs. It is time consuming and requires the performance of highly demanding procedures due to the fragility of the flocs.

It is thus necessary to take advantage of the latest developed technology which can monitor the floc properties during Fe(VI) and FeCl₃ induced coagulation (Benn et al. 2018; Chassagne & Safar 2020). Mass fractal dimension is such a concept which describes the floc structure and the dynamic light scattering technique makes its measurement possible (Callesen et al. 2018). Therefore, the first objective of this research was to continuously monitor the floc size and floc structure change through dynamic light scattering using Mastersizer 2000 (Malvern Instruments Inc., Malvern, UK).

Strong flocs are desirable for solid-liquid separation during water treatment as good strength minimizes the floc breakage and increases the removal efficiency. The higher the floc density, the stronger the floc, the easier the removal of particles (Gregory 2004). Floc strength is therefore used to assess the floc quality. A traditional floc strength characterization is carried out by observing the floc index response to a sudden shear rate increase within a Gator reactor (Smith & Kitchener 1978). In order to measure the floc strength, micromechanical technique was developed to rupture a single floc in the tensile mode (Yeung & Pelton 1996). Because of limited measurement of flocs and highly demanding procedure due to floc fragility, micromechanical technique cannot provide the representative result for floc strength.

In order to manipulate the floc properties during Fe(VI) and FeCl₃ induced coagulation, there is a need to obtain the quantitative information of floc strength and its dependence on process parameters. It is known floc breakup would happen when the hydrodynamic force applied to the flocs is equal or greater than the floc strength (Rau et al. 2018). Through incorporating the breakage rate and the critical energy dissipation rate required for the floc breakage into the population balance model, floc strength can be extracted. This is the second objective of this research.

In order to assist the judicial selection of chemicals and optimization of operating parameters, it is desired to have the relationship between collision efficiency, restructure rate and process conditions. Establishment of such relationship is the third objective of this research.

Kaolin, obtained from Chenming Group, China, was used as the model suspended solid in the water. The inherent volumetric mean particle size of the kaolin is 2.79 μm as measured by Mastersizer 2000 (Malvern Instruments Inc., Malvern, UK). It has a wide size distribution with the lowest limit size of 0.24 μm and the largest limit size of 7.26 μm. Kaolin suspension (50 mg/L) was prepared by adding kaolin to deionised distilled water and mixing thoroughly. FeCl₃ and K₂FeO₄, both analytical regents, obtained from Chenming Group, China. Ferric chloride stock solution at a concentration of 0.01 mol/L was prepared by dissolving 2.71 g FeCl₃.6H₂O in 1 L deionised and distilled water immediately before the tests. Boric acid/NaOH (pH 9.0) buffer solution, prepared by mixing 50 mL 0.1 M boric acid/0.1 M KCl solution, 20 mL 0.1 M NaOH solution and 30 mL distilled and deionised water, was used to make ferrate solution. To maintain the reactivity of ferrate, stock K₂FeO₄ solutions were prepared immediately before use. Bicarbonate buffer solution (0.2 M) was made by dissolving NaHCO3 (Chenming Group, China) in distilled and deionised water. HCl and NaOH, analytical grade, were used to control pH values of the coagulation system.

The kaolin coagulation process was continuously monitored by Mastersizer 2000 (Malvern Instruments Inc., Malvern, UK) where a peristaltic pump was used to circulate the kaolin suspension (50 mg/L) to the instrument for particle size and structure characterization. Figure 1 shows the schematic of the kaolin coagulation setup. Water bath was used to control the experiment temperatures. pH of the coagulation system was controlled by the addition of HCl and NaOH. The temperature, shear rate, pH, dosages of Fe(VI) and FeCl₃ were adjusted accordingly to investigate their effects on the coagulation process. Kaolin is assumed to be inert to the oxidation effect of the ferrate and thus the observed floc properties (e.g. floc size, structure and strength) change could be regarded as the coagulation effects of Fe(III) from Fe(VI) self-decomposition. Experiments were conducted in the normal pH range of surface water and groundwater of 6 to 10.

The collision frequency, determined by particle radius, is defined as the number of particles passing through the capture cross-section per unit time. For particles smaller than one micron (Liu et al. 2012), the collision frequency is mainly determined by the Brownian motion if settling and inertial phenomenon are ignored (Hsu et al. 2001).

The ratio of the mass to volume of settled kaolin flocs (compactness) was calculated as the ratio of the mass to the volume of settled flocs after 30 min of settling experiments.

As shown in Figure 2, population balance modeling achieved excellent approximation of kaolin floc size and floc structure change induced by the K₂FeO₄ and FeCl₃ systems. The time required to reach steady state was about 8 min for K₂FeO₄ system and 5 min for FeCl₃ system. The observed substantial floc formation induced by the two chemicals at iron concentration of 150 μM and neutral pH is corresponding to the sweep flocculation mechanism where kaolin is enmeshed within amorphous ferric hydroxide precipitates.

The results confirmed the simultaneous occurrence of aggregation, breakage and restructure during the coagulation process. During the initial stage of the coagulation, aggregation is the dominated factor which governs the floc growth. Meanwhile, floc became more susceptible to the hydrodynamic shear which broke, compacted (restructure effect) and limited the further growth of floc. It is evident that ferric chloride demonstrated better performance over potassium ferrate in inducing kaolin coagulation. This is probably because (1) the slow ferrate decomposition for coagulation and (2) the decomposition of Fe(VI) yields nanoparticles having a core-shell nanoarchitecture with a γ-Fe₂O₃ core and a γ-FeOOH shell (Prucek et al. 2013), which has a lower coagulation ability (Yu et al. 2016). Ferric chloride produced more compact flocs than those produced by potassium ferrate, as indicated by the higher mass fractal dimension. The mass fractal dimension also increased as coagulation proceeded, suggesting more compact flocs were formed.

Figure 3 shows the positive effect of potassium ferrate(VI) and FeCl₃ dosage increase on collision efficiency and floc strength (indicated by energy dissipation rate) and negative effect on restructure rate. Increasing the coagulant dosage gives progressively larger and stronger flocs but, little further improvement was observed beyond the optimum dosage.

Compared to potassium ferrate(VI) system, FeCl₃ system demonstrated higher collision efficiency, higher floc strength and lower restructure rate. The inferior performance of ferrate system in comparison to ferric chloride is due to the slow decomposition of ferrate in water and low activity of the resulted iron nanoparticle for coagulation (Yu et al. 2016). The increase of potassium ferrate(VI) dosage would result in more decomposed iron nanoparticle for kaolin coagulation and thus higher collision efficiency. At the same time, high dosage provided more open structure from hydroxide precipitates for capturing colloidal particles and thus increased the collision efficiency and produced stronger flocs (Tran Tien 2012). It is also possible that the ‘bridging’’ of particles by precipitated hydroxide may give stronger aggregates.

Increasing Fe(VI) and FeCl₃ dosage resulted in larger and stronger floc as indicated by the higher energy dissipation rate and lower restructure rate at high dosage. Compared to Fe(VI) system, higher energy dissipation rate (Figure 3(b)) and lower restructure rate (Figure 3(c)) of FeCl₃ coagulation system is due to the larger and more compact flocs produce by the system, as shown in Figure 2.

Figure 4 shows FeCl₃ system resulted in higher collision efficiency, stronger floc and lower restructure rate than Fe(VI) system over the whole range of shear rate investigated. As shown in Figure 4(a) and 4(b), shear rate had a profoundly negative effect on collision efficiency and floc strength for both systems. Collision efficiency was lowered at high shear rate probably because the open structure and the bridge formed by hydroxide precipitates for colloidal particle capture were broken and therefore bonding point between the particles was reduced (Tran Tien 2012). Because of the reduction of bonding point, weaker floc was formed.

Restructure rate increase with the increase of shear rate was observed for both coagulants as shown in Figure 4(c). As the increase of shear rate, more force was applied to the flocs and thus more significant restructure. Restructure processes would help particles to rearrange their position within the flocs and form additional bonds to densify the flocs.

Figure 5 shows temperature increase positively affected the collision efficiency, floc strength and restructure rate. This phenomenon may be explained by the temperature effects on the particle transport processes and the coagulant chemistry. Through its effect on viscosity, and therefore on the mixing energy dissipated in water, water temperature eventually affects the particle collision rates during coagulation. Low temperature would negatively affect the fluid-particle interaction, particle-particle interaction, rate and extent of hydrolysis of metal salt coagulants, and adsorption and precipitation rates (Duan & Gregory 2003). For particles with a size greater than approximately 1 μm, orthokinetic collision dominates the coagulation process. However, orthokinetic collision caused by fluid shear diminished as the viscosity increased with decreasing temperature (Vik & Eikebrokk 1989). The poor rapid-mixing conditions at low water temperature also led to inhomogeneous coagulant distribution and resulted in poor coagulation (Vik & Eikebrokk 1989). Temperature decrease negatively affected the hydrolysis of ferrate(VI) and FeCl₃ and the precipitation and solubility of Fe(OH)₃ (Duan & Gregory 2003). High collision efficiency observed as temperature increase can be reasonably attributed to the effect of temperature on the perikinetic and orthokinetic aggregation process. High temperature has a positive effect on perikinetic collision frequency and results in more rapid Brownian motion as shown by Equation (4). Meanwhile, temperature increase also reduced the viscosity of water and facilitated fluid-particle interaction and particle-particle interaction and thus the coagulation process. Meanwhile, low viscosity of the system at high temperature would allow the shear force much easier to be transferred to flocs and restructure them. That is the possible reason for the high restructure rate observed at high temperature.

Figure 6 shows the optimum pH for both coagulation system is between 6 and 8 where higher collision efficiency and stronger flocs were observed. It is well established the decomposition rate of ferrate depends on pH (Li et al. 2005). In order to achieve better Fe(VI) coagulation performance, it is desired to have higher Fe(VI) decomposition rate to generate Fe(III) for coagulation and lower solubility of amorphous iron(III) hydroxide to capture the colloidal particle. Fe(VI) has highest decomposition rate (Li et al. 2005) and lowest solubility of amorphous iron(III) hydroxide over pH range of 6 to 8 (Amirtharajah & Melia 1990; Tran Tien 2012). Fe(III) has limited solubility at pH range of 6 to 8 and the amorphous hydroxide would enmesh colloidal particles and cause hetero-coagulation (Duan & Gregory 2003). The combination of high Fe(VI) decomposition rate and low Fe(OH)₃ solubility at pH range of 6 to 8 made the collision efficiency high and floc strong.

More significant decrease of collision efficiency and floc strength was observed for ferrate system than FeCl₃ system when pH was increased beyond 8. This is probably because, at this pH range, Fe(VI) became extremely stable and had very low (close to zero) decomposition rate (Li et al. 2005). It is reported minimum decomposition rate was observed between pH 9.2 and 9.4 that means almost no Fe(III) was produced for coagulation (Li et al. 2005). Meanwhile, pH increase also decreased the positive charge of the amorphous iron(III) hydroxide and increased its solubility which made it less effective to induce coagulation (Tran Tien 2012). The more significant restructure observed at high pH is most likely due to the low strength flocs produced which made them more susceptible to restructure.

Figure 7 shows the ratio of the mass to volume of settled kaolin flocs (compactness) after addition of ferrate or FeCl₃. As shown in the figure, FeCl₃ resulted in a larger ratio of the mass to volume of kaolin flocs (compactness) than those induced by ferrate. This result further confirmed more compact flocs formed by FeCl₃ as indicated by larger mass fractal dimension shown in Figure 2. More compact floc would facilitate the particle separation from water suspension through sedimentation process (Wilen et al. 2003). Meanwhile, the formation of more compact floc could expel water from flocs which can also facilitate drainage during compressing (Hubbe 2005). The difference in the compactness is probably because the core-shell nanoarchitecture with a γ-Fe₂O₃ core and a γ-FeOOH shell from Fe(VI) decomposition has a lower coagulation ability than FeCl₃ (Yu et al. 2016).

Population balance modeling achieved excellent approximation of kaolin floc size evolution induced by Fe(VI) and FeCl₃. Ferric chloride produced larger, stronger and more compact flocs than those produced by potassium ferrate. Potassium ferrate(VI) and FeCl₃ dosage increase had positive effect on collision efficiency and floc strength and negative effect on restructure rate. FeCl₃ system resulted in higher collision efficiency, stronger flocs and lower restructure rate than Fe(VI) system over the whole range of shear rate investigated. Temperature increase positively affected the collision efficiency, floc strength and restructure rate. Optimum pH for both coagulation system is between 6 and 8 where higher collision efficiency and stronger flocs were observed. FeCl₃ resulted in a larger ratio of the mass to volume of kaolin flocs (compactness) than those induced by ferrate which would facilitate the particle separation from water suspension through sedimentation process.

This work was supported by China Postdoctoral Science Foundation (Grant No. 2016M592271), 2016 Shandong Province Postdoctoral Innovation (Grant No. 201603007), Fundamental Research Funds for the Central Universities (Grant No. 18CX02106A), Shandong Provincial Natural Science Foundation (ZR2019MB022).

The authors declare no conflict of interest.

All relevant data are included in the paper or its Supplementary Information.