Coagulation of Colloidal Particles with Ferrate(Vi)

Environmental Science Water Research & Technology PAPER Coagulation of colloidal particles with ferrateĲVI)† Cite this: Environ. Sci.: Water Res. Dongyu Lv,ab Lei Zheng,b Huiqin Zhangbc and Yang Deng *b Technol.,2018,4,701 Coagulation and chemical oxidation have long been recognized as two major mechanisms of ferrateĲVI) 2− (i.e. FeO4 , an oxyanion containing FeĲVI)) in its environmental applications. Although ferrateĲVI) oxidation of various contaminants has been extensively studied, few efforts were made to appreciate the mechanisms and behaviors of ferrateĲVI)-driven coagulation in water. In this study, coagulation of colloidal kaolin particles with ferrateĲVI) in simulated natural water was investigated under the conditions related to drinking wa- −1 −1 ter treatment (initial turbidity 25.00 NTU, 0.0–9.0 mg L FeĲVI), pH 7.5, and 0.50–10.00 mg L DOC). FeĲIII) produced from FeĲVI) reduction initiated in situ coagulation. Lower minimum effective iron doses (MEIDs) were observed for FeĲVI) coagulation than for direct FeĲIII) coagulation at pH 6.5 and 7.5 (DOC = 2.00 mg L−1), at which the colloids were captured by iron precipitates principally via sweep coagulation. A citrate– ascorbate iron extraction method was used to reveal that FeĲVI) resultant flocs were composed of both amorphous and crystalline iron with an amorphous to crystalline Fe ratio of approximately 2.3 : 1.0 (pH 7.5, −1 −1 3.0 mg L FeĲVI), and 2.00 mg L DOC). FerrateĲVI) oxidation transformed natural organic matter (NOM) preferentially into more hydrophilic compounds, which have lower affinity with colloids and thus are less adsorbed on colloids to produce a less negatively charged surface. Therefore, ferrateĲVI) oxidation poten- tially promoted the aggregation of colloids through the alleviation of electrostatic repulsion. However, NOM at a high concentration (8.00–10.00 mg L−1 DOC in this study) could prevent the agglomeration of small iron oxide particles through the formation of a negatively charged NOM coating via adsorption, Received 28th January 2018, thereby preventing the growth of flocs. FerrateĲVI) coagulation, when combined with ferrateĲVI)oxidation, Accepted 9th March 2018 provides a more viable treatment option to address multiple water pollution in raw water, e.g. the presence of colloidal algal cells and dissolved algal toxins in water during a harmful algal bloom. The dual treatment DOI: 10.1039/c8ew00048d mechanisms enable a very effective treatment design with an economical physical footprint for supporting rsc.li/es-water a sustainable municipal drinking water supply. Water impact FerrateĲVI) coagulates colloids via sweep coagulation under drinking water treatment-related conditions. Coagulation with ferrateĲVI), distinct from tradi- tional coagulation, gradually produces iron flocs containing amorphous and crystalline iron oxides in water. Meanwhile, ferrateĲVI) oxidation degrades NOM to favor particle aggregation. Effective ferrateĲVI) coagulation, combined with ferrateĲVI) oxidation, enables a dual-mechanism treatment to address multiple pollutants for supporting water supply sustainability. Introduction FeĲVI) is reduced to more active intermediate high valence iron species (i.e. FeĲV) and FeĲIV)) and immediately to stable Ĳ 2− Ĳ Ferrate VI)(i.e. FeO4 ) is acknowledged as a promising treat- ferric ions (Fe III)) through electron transfer as a result of self- ment agent for drinking water treatment due to its ability to decay and/or reactions with reductants in water.2 Accompany- perform multiple functions with minimal production of dis- ing FeĲVI) reduction, FeĲIII)isin situ produced at an equimolar 1 infection by-products. After ferrateĲVI) is added to water, amount, serving as a potential coagulant for the removal of colloidal particles in water or as a precipitating agent for the a College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, elimination of undesirable soluble chemicals (e.g. phos- Gansu Province, 730000, China phate). At a typical water treatment pH condition (6.0–8.0), b Department of Earth and Environmental Studies, Montclair State University, ironĲIII) hydroxides are produced with a potential to further Montclair, New Jersey 07043, USA. E-mail: [email protected] adsorb certain water contaminants (e.g. heavy metal cations). c School of Civil Engineering, Architecture and Environment, Hubei University of Besides, these ferrateĲVI) resultant particles are able to cata- Technology, Wuhan, Hubei Province, 430068, China Ĳ 3–5 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ lyze ferrate VI) self-decomposition. Among multiple treat- c8ew00048d ment mechanisms (i.e. chemical oxidation/disinfection, This journal is © The Royal Society of Chemistry 2018 Environ. Sci.: Water Res. Technol.,2018,4,701–710 | 701 Paper Environmental Science: Water Research & Technology coagulation, adsorption, and precipitation) sequentially oc- tificial raw water using a PDA. They measured the floccula- curring after ferrateĲVI) is dosed, chemical oxidation and coag- tion index (FI), which typically increases with the growth of ulation are recognized as the two principal ones for water flocs, and found that the FI of the FeĲIII) coagulation was treatment. Although many publications are available to report greater than that of the FeĲVI) at a specific iron dose of within the performance and mechanisms of ferrateĲVI) oxidation of 0.02–0.30 mM Fe, thus concluding that ferrateĲVI) underwent target contaminants in water, very few studies have been car- poorer coagulation. As a result, whether ferrateĲVI) may serve ried out to appreciate the coagulation behaviors of FeĲVI)in as a dual-function (i.e. chemical oxidation and coagulation) water,6 which is likely due to two major reasons. Firstly, agent in drinking water treatment is questioned.13 many previous studies were completed in a laboratory- Recently, research interests have been directed to the char- controlled phosphate or borate-buffered solution to facilitate acterization of ferrateĲVI)-induced particles in water. Goodwill 13 the investigation of FeĲVI) oxidation, because these buffer et al. compared FeĲVI) and FeĲIII) resultant particles pro- chemicals can both maintain pH and complex the produced duced in laboratory buffered water and reservoir water at pH FeĲIII) to prevent the formation of iron precipitates, thereby 6.2. The results showed that both particles had different size enabling an optical monitoring of FeĲVI) in water. However, distributions and morphologies. Moreover, the FeĲVI) resul- this experimental approach eliminates iron flocs and inhibits tant particles were composed of crystalline Fe2O3, which was the occurrence of coagulation and the ensuing flocculation. not detected in the FeĲIII) resultant particles, as well as more Secondly, it is technically difficult to separate the oxidation nanoparticles that generated a stable colloidal suspension. 14,15 and coagulation effects of ferrateĲVI) when ferrateĲVI)-induced Prucek et al. validated that the iron oxide particles from Ĳ γ – γ coagulation for the removal of dissolved natural organic mat- Fe VI) reduction exhibited a unique core ( -Fe2O3) shell ( - ter (NOM) is investigated.7 FeOOH) structure. Zheng and Deng16 also evaluated the Few efforts were made to investigate the mechanisms and settleability of FeĲVI)-induced particles in biologically treated performance of ferrateĲVI) coagulation in water. Conclusions municipal wastewater. They found that the majority of FeĲVI)- from the few available studies are inconsistent on what role induced particles was suspended in the secondary effluent, ferrateĲVI) plays in the removal of colloidal particles, particu- indicating the poor performance of coagulation and the ensu- 17 larly when compared with FeĲIII). Several groups reported that ing flocculation. Very recently, Cui et al. pointed out that ferrateĲVI) can behave better than FeĲIII) in terms of coagula- the ferrateĲVI) dose played a key role in the size distribution tion performance or can enhance particle removal in ensuing and coagulation of ferrate resultant particles. An increased 8 treatments. Jiang et al. reported that FeĲVI) was advantageous ferrateĲVI) dose augmented the fractions of soluble and colloi- − over ferric sulfate (2–12 mg L 1 as Fe) for the treatment of up- dal iron, while reducing the particulate portion, implying that land colored water, and could remove almost all turbidity at only a sufficiently high ferrateĲVI) dose could accelerate the 9,10 pH 7.5. Ma and Liu found that FeĲVI) pre-oxidation could aggregation of fine iron particles to facilitate downstream enhance a following alum coagulation for the removal of al- solid–liquid separation. Of note, the disparity among the gae and turbidity, but the coagulation driven by FeĲVI) alone aforementioned studies is likely due to different solution 11 was not investigated. Goodwill et al. reported that FeĲVI) chemistry conditions, operational conditions, and physical/ pre-oxidation could improve the effluent turbidity of a down- chemical characteristics of the target particulate matter. stream dual-media filter in continuous flow treatment experi- The objective of this study was to investigate the mecha- ments for the treatment of two U.S. Northeast surface waters. nisms and behaviors of ferrateĲVI)-driven coagulation for col- In contrast, other studies showed that FeĲVI) cannot pro- loidal particles in a typical drinking water treatment scenario. vide better coagulation than FeĲIII), a commonly used iron co-

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