Membrane Fouling and Rejection of Organics during Algae-Laden

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Membrane fouling and rejection of organics during algaeladen water treatment using ultrafiltration: a comparison between in situ pretreatment with Fe(II)/persulfate and ozone Bin Liu, Fangshu Qu, Huarong Yu, Jiayu Tian, Wei Chen, Heng Liang, Guibai Li, and Bart Van der Bruggen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03819 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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Environmental Science & Technology

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Revised Manuscript for Environmental Science & Technology

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Date: Oct 23rd 2017

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Membrane fouling and rejection of organics during algae-laden

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water treatment using ultrafiltration: a comparison between in situ

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pretreatment with Fe(II)/persulfate and ozone Bin Liu1,2, Fangshu Qu1*, HuarongYu1, Jiayu Tian1, Wei Chen3,4, Heng Liang1, Guibai Li1, Bart Van der

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Bruggen2,5*

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State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), Harbin Institute of Technology,

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Harbin, 150090, P.R. China 2

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Department of Chemical Engineering, Process Engineering for Sustainable Systems (ProcESS), KU Leuven,

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Celestijnenlaan200F, B-3001 Leuven, Belgium 3

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Ministry of Education Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes,

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Hohai University, Nanjing 210098, PR China 4

16 17 18

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College of Environment, Hohai University, Nanjing 210098, PR China

Faculty of Engineering and the Built Environment, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa

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*Corresponding author.

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Tel.: +86 451 86282252; Fax: +86 451 86282252.

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E-mail address: [email protected] (Fangshu Qu)

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[email protected] (Bart Van der Bruggen)

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ACS Paragon Plus Environment


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Abstract:

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In this study, in situ pretreatments with ozone and Fe(II)/persulfate were employed to suppress membrane fouling

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during filtration of algae-laden water and to improve the rejection of metabolites. Both ozonation and

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Fe(II)/persulfate pretreatment negatively impact the cell integrity, especially ozonation. Fe(II)/persulfate

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pretreatment improved the removal of dissolved organic carbon and microcystin-LR, but ozonation resulted in a

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deterioration in the quality of the filtered water. This suggests that the Fe(II)/persulfate oxidation is selective for

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organic degradation over cell damage. With ozonation, 2-methylisoborneol and geosmin were detected in the

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filtered water, and the irreversible fouling increased. The intracellular organic release and generation of small

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organic compounds with ozonation may be the reason for the increased membrane fouling. Fe(II)/persulfate

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oxidation substantially mitigated the membrane fouling resistance at concentrations over 0.2 mM compared to the

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membrane fouling resistance without oxidation. The combined effect of oxidation and coagulation is likely the

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reason for the excellent fouling control with Fe(II)/persulfate pretreatment. Membrane fouling during the filtration

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of algae-laden water is successively governed by complete blocking and cake filtration mechanisms. Ozonation

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caused a shift in the initial major mechanism to intermediate blocking, and the Fe(II)/persulfate pretreatment (> 0.2

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mM) converted the dominant mechanism into single standard blocking.

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Keywords: algae; in situ pretreatment; Fe(II)/persulfate; ozone; UF.

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Introduction

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In recent decades, algae blooms in coastal waters, reservoirs and lakes have been frequently reported as a

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consequence of eutrophication.

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threats and challenges for water treatment plants (WTP). 2 Algae-laden water causes two types of problems. First,

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Algae blooms, especially during warm periods, have become one of the largest

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the apparent color of the algae cells has a serious visual impact. Additionally, algal metabolites, such as

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microcystins and odor components, compromise the production of safe drinking water and threaten public health. 4

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Ultrafiltration (UF) shows great potential for algae-laden water purification due to its high efficiency for solid and

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liquid separations. 5-7 The membrane retains the algae cells using nano-scale pores, which are smaller than the size

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of the algae cells by at least one order of magnitude.

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metabolites could be an obstacle for UF applications, and previous studies have reported that UF of algae-laden

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water can lead to a severe decline in the flux. 4-8 A further challenge of UF is the limited removal of low molecular

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weight organic compounds.

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(GSM), easily penetrate through the membrane pores due to their low molecular weight. 10

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Many studies have employed an oxidative treatment for handling algae-laden water and concluded that oxidation is

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a possible method for solving toxin and odor issues.

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with different pre-oxidation methods and determined that oxidation has benefits, including changes in the cell

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architecture and extracellular organic matter (EOM) degradation, and drawbacks, including cell lysis and toxin

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release with overdosing. Ozone, a strong oxidant species, can remove organic matter with a high efficiency. Chang

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et al

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effective for MCLR degradation. Coral et al

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performance of algae-laden water treatment, but a high concentration of ozone can lead to release of intracellular

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organic matter (IOM). Potassium persulfate is also an oxidant with a high oxidation potential, high selectivity and

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high adaptability in neutral or alkaline aqueous environments. 16 Activation is necessary when employing persulfate

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as an oxidant. Previous studies have reported that persulfate can be activated by transition metal ions, heat and

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ultraviolet radiation to generate sulfate radicals, which have a higher oxidation potential (2.5-3.1 V) than persulfate

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(2.01 V).

14

9

7

However, the fouling produced by the algae cells and

Algal metabolites, such as microcystins, 2-methylisoborneol (2-MIB) and geosmin

8, 11-18

Henderson et al

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examined the inactivation of algae

reported the ozonation degradation pathway for microcystin-LR (MCLR) and determined that ozone is

20

12

reported that an appropriate concentration of ozone enhances the

Moreover, when persulfate is activated by ferrous ions, the generated ferric ions can act as in situ

3



21

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coagulants, which might improve the cake layer structure.

The potential for cell breakage and IOM release

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deserves specific attention. 22 With an improper treatment, the algal cells can easily break, and this leads to a rapid

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increase in the intracellular metabolites, such as microcystins, 2-MIB and GSM, of several orders of magnitude. 23

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Moreover, based on the ability to degrade algae metabolites, the cell removal efficiency via oxidation is not

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comparable to that obtained with membrane technology.

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Both UF and oxidation have advantages and disadvantages for treating algae-laden water. Oxidation as a

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pretreatment for UF may result in a synergistic effect between oxidation and UF. Oxidation has been reported to be

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an effective method for mitigating membrane fouling and degrading algal metabolites that easily penetrate through

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the membrane pores. 24 In contrast, UF membranes effectively retain algal cells. To the best of our knowledge, the

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performance of persulfate activated by ferrous ions (Fe(II)/persulfate) has not been reported as a pretreatment for

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UF to treat algae-laden water. Therefore, Fe(II)/persulfate and ozone (reported as an effective oxidant) were

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investigated as in situ pretreatments for UF applications. The viability of the algal cells and the effectiveness of the

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in situ oxidation for organic removal are discussed in this paper. Furthermore, the variation in the special algal

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metabolites in the filtered water, such as MCLR, 2-MIB and GSM, was investigated. The impact of the in situ

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pretreatments on the membrane flux, fouling reversibility and mechanisms are discussed. In addition, confocal laser

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scanning microscopy was applied to detect the cake layer composition and cell viability.

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Materials and methods

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Algae culture and oxidant preparation. To simulate the algae-laden surface water, Microcystis aeruginos,

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which is one of the most commonly occurring and problematic algae species in fresh water, was cultivated under lab

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conditions using seeds (PCC7820) purchased from the Institute of Hydrobiology, Chinese Academy of Sciences.

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The algal cells were axenically cultivated using BG-11 medium. The solution was stored at 25 °C in a biological

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incubator (HPG-280HX, Donglian, China), and an intermittent illumination at 5000 lx was provided for 14 h every

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day. For the oxidation experiments and filtration tests, the harvested algae samples were diluted to a concentration

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of 2.0×106 cells mL-1. The diluent contained 0.5 mM CaCl2, 1.0 mM NaHCO3, and 15.0 mM NaClO4 to simulate

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the natural ionic strength of an aquatic environment.

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Experimental protocol. Ozone and Fe(II)/persulfate were used as the oxidants for the in situ pretreatments

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during UF. The ozone was generated using an ozone generator (LAB2B, Ozonia, England) with high-purity oxygen

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gas. The generated ozone gas was first pumped into milli-Q water at 4 °C at a rate of 100 mL/min for at least 1 h to

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obtain a saturated ozone solution. 8 The ozone concentration of the saturated ozone solution was measured using a

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titration method.

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peroxymonosulfate and ferrous chloride were freshly prepared.

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potassium persulfate and ferrous chloride were used at equal S2O82-/ Fe2+ molar ratios and denoted Fe(II)/persulfate.

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The oxidation reactions and standard oxidation potentials of ozone and Fe(II)/persulfate are shown in Table S1. 25

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The simulated algae-laden water was treated by ozone and Fe(II)/persulfate with concentrations of 0-0.06 mM and

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0-0.4 mM, respectively. To determine the impact of the in situ oxidation on the UF performance, the UF test began

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within 30 s after the oxidant was added.

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Polyvinylidene fluoride (PVDF) flat sheet membranes (Tianchuang, China), (The characteristics are specified in the

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Supporting Information.), were employed in this study. The new membranes were rinsed with Milli-Q water until the

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DOC (dissolved organic carbon) concentration of the filtered water was lower than 0.2 mg/L. The membrane first

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underwent an anti-oxidation test with ozone (0.06 mM) and Fe(II)/persulfate (0.4 mM) using milli-Q water as the

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feed water, and no significant impact on the permeability (Fig. S1) and membrane pore size (Fig. S2) was observed

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after the oxidant exposure.

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Ferrous chloride was employed as the activator to generate persulfate radicals. Potassium 16

During the Fe(II)/persulfate oxidation test, the

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The UF system contained a filtration cell (Amicon 8400, Millipore, USA), a nitrogen gas cylinder to provide a

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constant pressure, and an electronic balance linked to a computer to automatically log the weight data every 5 seconds.

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The prepared membrane was placed in the bottom of the cell, and the feed water was driven through the membrane

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for the filtration by a constant pressure (100 kPa). No stirring was applied in this protocol. All the experiments were

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conducted in triplicate.

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Membrane fouling assessment. The fouling resistances were classified as three types, i.e., the intrinsic

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membrane resistance, reversible resistance and irreversible resistance. The reversible and irreversible resistances

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were considered to be associated with the cake layer formation due to foulant deposition and pore blocking due to

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the foulant adsorption, respectively. The resistance–in-series model (Eq. 1) was used to calculate the different types

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of fouling resistance: J=

µRi +Rc +Rb

(1)

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where J is the permeate flux (L/m2·h), µ is the dynamic viscosity (Pa·s), ∆P is the operating pressure (Pa), Ri is the

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intrinsic membrane resistance (m-1), Rc is the reversible resistance (m-1), and Rb is the irreversible resistance (m-1).

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To determine the intrinsic membrane resistance, 300 mL of milli-Q water was filtered through the new membranes

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to obtain a stable flux, which was denoted as J0. During the filtration of the algae-laden water, the flux was

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monitored, and the stable flux at the end of the filtration was denoted as J1. After the filtration test, the fouled

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membranes were carefully wiped with a wet sponge to remove the reversible fouling. Subsequently, clean water was

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filtered again to determine the stable flux (J2). Finally, the reversible and irreversible resistances were obtained via Eq.

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2, 3 and 4. Ri =

∆P µJ0

Rb =

µJ2 µJ0

Rc =

µJ1 µJ2

6


(2)

(3)

(4)

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To analyze the impact of the in situ oxidation on the fouling mechanisms, a combined model proposed by Ho and

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Zydney 26 was used to analyze the filtration data. The J/J0 data versus time, t, plot was fitted to Eq. 1 via a non-linear

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optimization using the curve fitting tool in MATLAB. To determine the dominant mechanism in the different

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filtration phases, d2t/dV2 versus dt/dV curves of all the filtration data were plotted, and the dominant mechanism

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was determined according to the following equation. 27 n

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d 2t  dt  =k  2 dV  dV 

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where t is the filtration time, V is the total filtered volume, and n characterizes the filtration mechanism, with n=0 for

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cake filtration, n=1 for intermediate blocking, n=1.5 for standard blocking, and n=2 for complete pore blocking 28.

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The required derivatives in Eq. 5 were evaluated in terms of the filtrate flux.

(5)

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1 dt = dV JA

(6)

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d 2t 1 dJ =− 3 2 2 dV J A dt

(7)

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where dJ/dt was numerically evaluated by differentiating the flux versus the time data using the curve fit toolbox in

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Matlab® 2014 (The Mathworks, Inc.) to obtain the derivative of a series of piece-wise cubic polynomials that were fit

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to the raw data. The exponent n in Eq. 5 was analytically evaluated by differentiating the logarithm of d2t/dV2 with

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respect to the logarithm of dt/dV .28

  d t  d log    dV    n=   dt   d log      dV   2

2

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(8)

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The required derivatives were evaluated using Eq. 6 and 7, and the n values were evaluated in different phases in the

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filtration.

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Analytical methods. The release of potassium was employed to evaluate the breakage of the algal cells caused 7



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by the oxidation and hydraulic shear during the filtration.

First, algae solutions were filtered with a 0.45 µm

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microfilter, and the concentration of potassium in the filtrate was determined and denoted C0. Subsequently, the

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algae samples were ultrasonically treated for 30 min to break the cells and allow the total concentration of

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potassium (Ct) to be measured. Finally, the concentration of potassium in the permeate of each UF test (C) was

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measured, and the potassium release was calculated via Eq. 9. The potassium concentration was measured using

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inductively coupled plasma-atomic emission spectrometry (ICP-AES, 5300DV, PerkinElmer, USA). Cell breakage (%)=

c- -

(9)

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The DOC concentrations were measured by a TOC analyzer (Multi N/C 2100, JENA, Germany). Before the

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measurements, the samples were first filtered through a 0.45 µm cellulose filter to reject the suspended matter. The

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concentration of MCLR, which is a representative microcystin in algae-laden water, was determined via the Elisa

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kit method.

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chromatography-mass spectrometry (AGILENT-6890 GC/5973N MS); the analytes were detected in SIM mode. A

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fluorescence spectrophotometer (F7000, Hitachi, Japan) was utilized to determine the fluorescence characteristics

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of the permeated solution.

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Confocal laser scanning microscopy (CLSM, LeicaSP5, Wetzlar, Germany) was employed to investigate the cake

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layer composition, structure and cell viability on the membrane surface. The membrane samples were first fixed with

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a formaldehyde solution and then washed three times with milli-Q water. To investigate the cake layer composition

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and structure, concanavalin A was used to target the polysaccharides, and sypro orange was used to mark the proteins.

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For the algae cell viability analysis, SYTOX Green (Invitrogen, Life Technologies, Grand Island, USA) was used to

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distinguish the integrity and rupture of the algae cells.

30

For the 2-MIB and GSM detection, quantitative analyses were performed using gas

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Results and discussion

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Effects of in situ pretreatment on algal integrity. Fig. 1 shows the ratios of the algal cell breakage caused

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by the oxidation and permeate shear. Only 3% of the cells ruptured during direct UF. The minor cell breakage

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during filtration was attributed to the hydraulic shear and is consistent with the literature.

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substantially increased to 58% and 81% when ozone was used at concentrations of 0.015 and 0.06 mM,

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respectively. Fan et al

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within 5 min at ozone concentrations of 4 mg/L and 6 mg/L, respectively. When Fe(II)/persulfate was applied, the

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cell breakage was minor (5%) at a low concentration (0.05 mM), but the breakage was substantially aggravated to

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30 and 38% at concentrations of 0.2 and 0.4 mM, respectively. The CLSM images of the UF membranes fouled by

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algal cells and EOM are shown in Fig. 2 and Fig. S3. Because SYTOX green permeates compromise the cell

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integrity and stain nucleic acids, the intensity of the green fluorescence implies the cell breakage degree.

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The green fluorescence in the control trial was very weak, and the fluorescence intensity substantially increased

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when ozonation was applied. For Fe(II)/persulfate, the fluorescence intensity increased with the concentration but

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was generally weaker than that for ozonation, which indicated fewer cells break with Fe(II)/persulfate. Gu et al

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investigated the effect of Fe(II)/persulfate oxidation for treating algae-laden water and determined that the cell

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breakage was 70% at an oxidant concentration of 25 mg/L. In another work, a combination of persulfate and

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ultraviolet irradiation was applied to treat algae-laden water at an extremely high concentration (500 mg/L), which

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resulted in complete breakage of the algal cells. 16, 31 Despite its higher oxidation potential, Fe(II)/persulfate caused

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less cell oxidation-induced algal cell breakage than ozone, indicating that the destructive impact of the oxidants on

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algal cells is not strictly related to their oxidation potential. Overall, both ozone and Fe(II)/persulfate cause algal

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cell breakage and a release of intracellular organics and toxins, which might impact the permeate quality.

13

5, 7

The cell breakage

also demonstrated that ozonation induced 70% and 90% cyanobacterial cell breakage

9


13, 17, 29

31


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K+ release （ %）

O3 80

Fe(Ⅱ)/Persulfate 60 40 20 0

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None

0.015

0.06

0.05

0.2

0.4

oxidant concentration (mmol/L)

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Fig. 1 Potassium release during filtration of Microcystis aeruginosa solutions with different in situ oxidation-UF

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conditions. Error bars indicate the standard deviation (n=3).

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Fig. 2 CLSM images of the cake layers formed by algal cells treated with different in situ pretreatments: (a) None, (b)

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0.015 mM O3, (c) 0.06 mM O3, (d) 0.05 mM Fe(II)/persulfate, (e) 0.2 mM Fe(II)/persulfate, and (f) 0.4 mM

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Fe(II)/persulfate.

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Effects of in situ pretreatment on organic rejection. Fig. 3 shows the removals of DOC and MCLR

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during the algae-laden water treatment using in situ pretreatment-UF. The DOC and MCLR rejections were 44%

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and 32% during direct UF. The result is consistent with the reported ratios of macromolecular fractions in algal

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EOM. 32 Although MCLR is much smaller than the membrane pores, the membrane adsorption and cake retention

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contribute to the MCLR removal.

201

decreased to 36% and -3%, respectively. This is attributed to the ozonation-induced cell breakage and release of

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IOM. In addition, the macromolecules in EOM and IOM can be broken up into smaller molecules, resulting in

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additional penetration of organic compounds through the membrane pores and a lower rejection. Similarly, the

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MCLR rejections were also hindered by ozonation, and the removal rates decreased to 16% and 12% at oxidant

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concentrations of 0.015 and 0.06 mM, respectively. Although ozonation was demonstrated to be capable of

206

degrading MCLR, the rejection performance might be compromised by the release of intracellular toxins. 14 Wei et

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al

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ozonation increased the MCLR concentration in the permeate, especially at higher ozone concentrations.

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When Fe(II)/persulfate was applied, the rejection of DOC and MCLR increased to 60-69% and 43-57%,

210

respectively, even though Fe(II)/persulfate also causes cell breakage. Gu et al. 31 demonstrated that Fe(II)/persulfate

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oxidation degraded 90% of the total soluble proteins released by M. aeruginosa cells. Another study on algae-laden

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water treatment using persulfate activated by ultraviolet reported that the algal organics were mineralized by 80% at

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an extremely high oxidant concentration (1500 mg/L). 16

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There are two interpretations for the improved rejection results associated with the in situ oxidation with

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Fe(II)/persulfate. First, the cell breakage due to Fe(II)/persulfate was less than that due to ozonation, which resulted

216

in fewer negative impacts on the rejection of organic compounds. Additionally, it is likely that mechanisms other

217

than membrane rejection and oxidation were involved in the removal of the organics. In this work, the activation of

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Fe(II)/persulfate using ferrous ions produced ferric ions, which promoted the agglomeration of algal organics. Ma

219

et al

8

4, 5

When ozone was applied at 0.015 and 0.06 mM, the DOC rejections

investigated an algae-laden water treatment using ceramic UF membranes and determined that an in situ

24

performed a study on algae removal by a KMnO4-Fe(II) process and determined that the in situ-formed

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Fe(III) facilitated a higher removal of algal cells than the pre-formed Fe(III) due to the higher reactive surface area.

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Moreover, the ferric ions can improve the adsorption of MCLR onto the membrane and cake layer by reducing the

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electrostatic repulsions. Hence, although cell breakage and the release of intracellular organics occurred, the in situ

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pretreatment using Fe(II)/persulfate still improved the rejection of DOC and MCLR. Fig. S4 presents the EEM

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spectra of the different pretreated samples, and the removal efficiency of the fluorescent organic components highly

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correlated with the DOC rejection. 80

Fe(II)/Persulfate

DOC MCLR

70

Removal rate (%)

60

O3

50 40 30 20 10 0 -10

None

226

0.015

0.06

0.05

0.2

0.4

oxidant concentration (mmol/L)

227

Fig. 3 Removal efficiencies of DOC and MCLR during the filtration of algal-laden water with different in situ

228

pretreatments.

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In this work, 2-MIB and GSM were selected as representative odor compounds to investigate the performance of

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the combined in situ pretreatment and UF for odor control. Table 1 shows the concentrations of 2-MIB and GSM in

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the UF permeate. The concentration of MIB in the UF permeate was 23.0 ng/L, and the concentration of GSM was

232

below the detection limit. When the in situ ozonation was applied, the concentrations of 2-MIB increased to 109.0

233

ng/L at a concentration of 0.015 mM and decreased to 4.9 ng/L at an oxidant concentration of 0.06 mM. The

234

concentrations of 2-MIB in the UF permeate likely increased and then decreased because of the release of

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intracellular 2-MIB due to cell breakage and then the subsequent degradation by ozone. However, the

236

concentrations of GSM considerably increased to 201.6 and 311.0 ng/L at ozone concentrations of 0.015 and 0.06

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mM, respectively. This implies that the release of intracellular GSM into the aqueous solution that was induced by

238

the in situ ozonation was higher than the degradation of the GSM. This result is consistent with the conclusion

239

reported in the literature that most GSM and 2-MIB are intracellular 33. When Fe(II)/persulfate was applied for the

240

in situ oxidation, the concentrations of 2-MIB in the UF permeate decreased to 3.6, 3.4 and 7.1 ng/L. The

241

concentrations of GSM in the UF permeate were all below the detection limit. These results indicated that the

242

combined process of in situ oxidation by Fe(II)/persulfate and UF was efficient for removing odor compounds.

243

Both the oxidation by Fe(II)/persulfate and coagulation by the in situ-formed ferric ions contributed to the

244

improved rejection of the odor compounds. 16, 34

245

Table 1 Effects of the in situ oxidation on the concentrations of 2-MIB and GSM in the UF permeate (ND: not

246

detected) Oxidants

Concentration (mM)

2-MIB (ng/L)

GSM (ng/L)

None

/

23.0

ND

0.015

109.0

201.6

0.06

4.9

311.0

0.05

3.6

ND

0.2

3.4

ND

0.4

7.1

ND

Ozone

Fe(II)/persulfate

247 248

Effects of the in situ pretreatment on membrane fouling. Fig. 4(a) shows the flux curve profiles during

249

the filtration of the simulated algae-laden water. During the direct UF, the flux substantially decreased in the initial

250

filtration phase and subsequently reached a plateau, and the specific flux was finally reduced to 0.049. When ozone

251

was added at a concentration of 0.015 or 0.06 mM, the flux decline during the filtration of the algae-laden water

252

was not alleviated, and the specific flux was reduced to 0.055 and 0.061, respectively. As shown in section 3.1,

253

ozonation causes severe cell breakage, which likely results in reduced compression of the cake layer. However, the

254

occurrence of cell debris and IOM, which have great fouling potentials, has been demonstrated to offset the

13



4

255

contribution of ozonation to the fouling control.

When ozone was applied at much lower concentrations (< 0.6

256

μM), the cell damage was reduced (Fig. S5), but the flux decline was aggravated rather than alleviated, particularly

257

in the initial filtration phase (Fig. S6(a)). Wei et al 8 performed a study on an algae-laden water treatment using in

258

situ ozonation and ceramic UF and determined that the membrane fouling decreased with the increasing ozone

259

concentration. This discrepancy is because a ceramic UF membrane was used in their work, which has a much

260

higher fouling resistance. When Fe(II)/persulfate was added to the feed water at a low concentration (0.05 mM), the

261

flux decline during the filtration was slightly alleviated, and a specific flux of 0.096 was obtained at the end of the

262

filtration. However, when the Fe(II)/persulfate concentration increased to 0.2 and 0.4 mM, the initial flux decline

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considerably decreased, which allowed the specific flux in the plateau phase to increase to 0.572 and 0.701,

264

respectively. To verify the effect of Fe(II)/persulfate, flux profiles were obtained during filtration of the treated

265

algae-laden water using only persulfate and only Fe(II) (Fig. 5). The pretreatment using only persulfate did not cause

266

a significant difference in the flux decline. However, the Fe(II) substantially retarded the flux decline caused by the

267

algae, and the final specific fluxes were reduced to 0.158, 0.348 and 0.410 at concentrations of 0.05, 0.2 and 0.4 mM,

268

respectively. In contrast, Fe(II)/persulfate reduced the flux decline due to algae more than persulfate and Fe(II)

269

alone. Because ferric ions were generated during the in situ Fe(II)/persulfate oxidation, the coagulation effect of the

270

ferric ions could also be utilized for membrane fouling control. To verify this hypothesis, 0.2 Mm ferric chloride

271

was added into raw and ozone-treated algae-laden water. As shown in Fig. S7 in the Supplementary Information, the

272

flux decline in the algae-laden water was notably alleviated in the presence of ferric chloride, particularly for the

273

ozone-treated samples. Although Fe(II)/persulfate oxidation also leads to cell breakage, the ferric salt produced

274

during the Fe(II)/persulfate oxidation can promote the aggregation of algal foulants and induce the formation of a

275

more porous cake layer.

276

Fig. 4(b) shows the reversible and irreversible fouling resistances caused by the Microcystis suspensions. The

14


Page 14 of 27

Page 15 of 27


277

reversible and irreversible fouling resistances caused by the raw Microcystis suspensions were 82.7 and 1.4×1011

278

m-1, respectively, which revealed the predominance of the reversible fouling associated with the deposition of cells

279

and macromolecular fractions of EOM in the fouling formation. When ozonation was applied, the reversible

280

fouling was alleviated, which was probably due to the decomposition of the macromolecular fractions of EOM and

281

IOM. However, the irreversible fouling was considerably aggravated, and the resistances increased to 68 and

282

87×1011 m-1. When ozone was added at lower concentrations (< 6 μM), neither the reversible or irreversible

283

resistance was significantly affected (Fig. S6(b)) due to the lower release of IOM. There are two interpretations of

284

the severe irreversible fouling associated with the in situ ozonation. First, the in situ ozonation induces severe cell

285

breakage, resulting in a release of IOM and the occurrence of cell debris. IOM and cell debris have been

286

demonstrated to have much higher potentials for irreversible fouling. 4 Additionally, macromolecular biopolymers

287

can be degraded by ozonation into small and hydrophilic fractions that can penetrate the membrane pores and cause

288

pore blockages. When 0.05 mM Fe(II)/persulfate was added, the reversible and irreversible resistances due to the

289

Microcystis suspension slightly decreased to 4.45×1011 m-1 and 0.95×1011 m-1, respectively. As the Fe(II)/persulfate

290

dosage increased, the reversible fouling due to the Microcystis cells and organics further decreased. When only

291

persulfate was used, no significant differences were observed in the reversible (P=0.109) and irreversible resistances

292

(P=0.371) during the filtration (Fig. 5(b)). For only Fe(II), both the irreversible and reversible resistances decreased

293

(Fig. 5(d)), which confirmed that coagulation plays a role in improving the fouling reversibility during the filtration

294

of algae-laden water. As demonstrated above, the cell breakage induced by the Fe(II)/persulfate exposure was

295

severe. Hence, it is reasonable to assume that the reversible fouling was aggravated because more organics fill the

296

voids inside the cake layer formed by the cells and debris. However, the presence of the in situ-formed Fe(III)

297

might enhance the agglomeration of the cells and debris and strengthen the adhesion of the organics onto the flocs,

298

resulting in the formation of a more porous cake layer and less reversible fouling. Yu et al 35 demonstrated that the

15



Page 16 of 27

299

in situ-formed Fe(III) caused NOM to aggregate and form large flocs, lowering the thickness and density of the

300

cake layer during the filtration. The addition of ferric chloride into the raw and ozone-treated algae-laden water also

301

improved the reversibility of the fouling caused by algae-laden water (Fig. S7(b)), but the improvements were less

302

than those obtained with Fe(II)/persulfate, which implied that the in situ-formed Fe(III) is more effective for fouling

303

control than ferric chloride. 36 However, at a Fe(II)/persulfate concentration of 0.4 mM, the irreversible fouling due

304

to the algal cells and organics was apparently aggravated, and the irreversible resistance increased to 1.24×1011 m-1.

305

The interpretation is that the presence of the in situ-formed Fe(III) decreases the electrostatic repulsion between the

306

organics and the membrane surface, leading to more small molecules adhering inside the membrane pores.

307

Overall, in situ pretreatment by ozone is ineffective for alleviating flux decline and aggravates the irreversible

308

fouling, and the Fe(II)/persulfate pretreatment can improve both the permeability and fouling reversibility via in

309

situ oxidation, especially at 0.2 mM. None; O3 (mM):

(a)

0.015

Fe(Ⅱ )/Persulfate (mM):

1.0

0.06;

0.05

0.2

0.4

J/J0

0.8 0.6 0.4 0.2 0.0 0

310

50

100

150

200

250

Permeate volume (mL)

16


300

350

24

Page 17 of 27


100 90

(b)

Rir

-1

70

11

Resistance (10 m )

80 60

Rre

Fe(II)/Persulfate

Ozone

50 15 10 5 0

None

0.06

0.015

0.05

0.2

0.4

oxidant concentration (mM)

311 312

Fig. 4 Membrane fouling during the filtration of algae-laden water with different pretreatments: Flux curve profile (a)

313

and membrane resistance (b). Error bars indicate the standard deviation (n=3). The abscissa “None” represents no

314

oxidant. 1.0

0.40 mM

(b)

Rir

J/J0

0.6 0.4 0.2

80 60 40 20

0.0 0

50

100

150

200

250

300

0

350

0.00

1.0

(c) Fe(II) doses:

0.05

0.20

0.40

persulfate doses (mM)


315

0 mM 0.20 mM

0.05 mM 0.40 mM

100

(d)

Rir

0.8

Rre Rir

J/J0 0.4 0.2

6

-1

Resistance (m )

80

11

-1

Resistance (10 m )

8

0.6

60

4 2 0 0.00

40

0.05

0.20

0.40

Fe(II) doses (mM)

20

0.0 0

316

Rre

11

-1

0.8

100

persulfate doses: 0 mM 0.05 mM 0.20 mM

Resistance (10 m )

(a)

50

100

150

200

250

300

0

350

0.00

0.05

0.20

0.40

Fe(II) doses (mM)


317

Fig. 5 Effects of only persulfate and only Fe(II) on the fouling caused by algae: Flux decline (a) and fouling reversibility (b) at different

318

persulfate concentrations; Flux decline (c) and fouling reversibility (d) at different Fe(II) concentrations. Error bars indicate the standard

17



319

deviation (n=3).

320 321

Effects of the in situ pretreatment on the membrane fouling mechanisms. Fig. 6 shows the results

322

from fitting the experimental data to the combined blocking and cake filtration model. The filtration data were

323

replotted as d2t/dV2 versus dt/dV as suggested by Eq. 6, and the n value represents the dominant mechanism and its

324

transition during the filtration. In addition, the filtration volume was also plotted versus dt/dV to determine the

325

filtration volume at which the fouling mechanism changed (i.e., the n value changed). For the raw algae-laden

326

water, the fouling formation was successively governed by complete blocking and cake filtration mechanisms with

327

n values of 2.071 and 0.043, respectively. An obvious transition in the fouling mechanism occurred at a permeate

328

volume of approximately 100 mL. This implies that the fouling during the filtration of the raw algae-laden water is

329

initially dominated by pore blocking by algal cells and macromolecular EOM and is followed by the buildup and

330

compression of a cake layer due to the deposition of the cells (Fig. S8). The dual fouling mechanism of initial pore

331

blocking and successive cake filtration was also observed in the treatment of a secondary effluent by UF. 37

332

When ozone was used to pretreat the algae-laden water, the fouling was still successively dominated by the pore

333

blocking and cake filtration mechanisms, but the n value in the initial phase decreased to approximately 1, which

334

indicated the intermediate blocking mechanism dominated over the complete blocking mechanism. This is likely

335

due to the increased in the IOM, which has been demonstrated to cause intermediate blocking of the UF membrane

336

pores.

337

degraded by ozone into less compressible debris. For 0.05 mM Fe(II)/persulfate, the fouling mechanism was

338

similar to that without pretreatment. However, at higher Fe(II)/persulfate concentrations (0.2 and 0.4 mM), the pore

339

blocking mechanism governed the fouling formation throughout the entire filtration test, and cake filtration did not

340

occur. This is consistent with the substantially reduced flux decline observed when Fe(II)/persulfate was applied

38

The transition in the fouling mechanisms was delayed to some extent because the algal cells were

18


Page 18 of 27

Page 19 of 27


341

(Fig. 4). The algal cells and EOM likely coagulated into large flocs in the presence of the in situ-formed Fe(III),

342

which resulted in a very loose deposition layer (Fig. S8) that has less impact on the membrane permeability.

343

Moreover, the n values were all approximately 1.5 in the Fe(II)/persulfate-involved cases, which implied that

344

standard blocking is the major mechanism. Because the in situ-formed Fe(III) can reduce the electrostatic repulsion

345

between the membrane and the foulants, the adhesion of small EOMs inside the membrane pores is enhanced and

346

contributes to the increased role of the standard blocking mechanism.

347

decreased fouling reversibility observed when 0.4 Mm Fe(II)/persulfate was applied (Fig. 4(b)). Hence, an

348

optimization of the concentration should be performed for the application of Fe(II)/persulfate to prevent

349

aggravation of the irreversible fouling.

19


40

39

This is consistent with the slightly


Data

2.0x1010

0.0 0 0.0 5.0x106 1.0x107 1.5x107 2.0x107

n

4.0x10

0.0 0.0

2 2

V (mL)

1

V (mL)

100 0

0.0 2.0x106 4.0x106 6.0x106 8.0x106

1.6x109

6

1.8x10

d t/dV (s/m )

dt/dV (s/m3)

(f)

2

n1 = 1.414

9

1.4x109

n1 = 1.467

1.2x109 1.0x109 8.0x108

9

350

200

0 5.0x106 1.0x107 1.5x107

2x109 1x10 1.0x106

400 300

8.0x109

2

d2t/dV2 (s/m6)

3x10

1.2x1010

100

4x109 9

1.6x1010

200

n2 = -0.073

2.0x1010 n1 = 1.523

6

=0 .98 2

300

dt/dV (s/m3)

(e)

(d) d t/dV (s/m )

400

2.0x1010 0.0 0.0

0 5.0x106 1.0x107 1.5x107

dt/dV (s/m3)

n2 = 0.074

n

d2t/dV2 (s/m6)

4.0x1010

200 100

dt/dV (s/m3)

(c) 6.0x1010

300

V (mL)

10

=1 .02 2

6.0x1010

1

6

100

2.0x1010

400

n2 = 0.019

2

200

Permeate volume (V)

10

2

4.0x1010

300

d t/dV (s/m )

6.0x10

10

V (mL)

8.0x10

10

Model

(b) 8.0x10

400

n2 = 0.043

n1 = 2.0 71

d2t/dV2 (s/m6)

(a)1.0x10

11

Page 20 of 27

1.5x106

8.0x105

2.0x106

1.0x106

1.2x106

1.4x106

3

dt/dV (s/m3)

dt/dV (s/m )

351

Fig. 6 d2t/dV2 versus dt/dV curves for the filtration of algae-laden water pretreated by different pretreatments: (a)

352

None, (b) 0.015 mM ozone, (c) 0.06 mM ozone, (d) 0.05 mM Fe(II)/persulfate (e) 0.2 mM Fe(II)/persulfate and (e)

353

0.4 mM Fe(II)/persulfate. Values of n in Eq. 5 are evaluated and shown. Filtration volume versus dt/dV curves for the

354

filtrations are also shown except for the Fe(II) pretreatment at concentrations of 0.2 and 0.4 M because cake filtration

355

did not occur.

356 357

Implications. The removal of algal cells and their metabolites is significant in algae-laden water treatment to

358

create potable water. Coagulation pretreatments using alum, aluminum chlorohydrate, ferric sulfate, and ferric

20


Page 21 of 27


359

chloride have been intensively investigated for algae removal. 41-43 Coagulation has been demonstrated to be efficient

360

for the removal of algae, but EOM increases the coagulant demand.

361

favorable than iron-based coagulants due to a lower impact on the pH value of water despite their similar

362

performances and costs. 42 In addition, most experiments on algae removal in previous studies were performed using

363

static sedimentation, and the performance may be compromised in real applications. Owing to the low gravidity of

364

the flocs formed by algal cells, algae removal can be easily impacted by hydraulic factors in water plants, which

365

facilitates the inhibition of algal cells using strong oxidants such as chlorine, ozone, permanganate, ferrate and

366

persulfates. 31, 45-47 Chlorination is extensively applied to address the clogging of sand filters caused by algae, but the

367

increased formation of DBPs may pose a threat to human health. 48, 49 Ozone, which has a strong oxidation potential,

368

can induce extensive cell lysis even at a low concentrations

369

membrane fouling in an oxidation-membrane system, which is attributed to the release of IOM.

370

degradation effect on algae metabolites,

371

based on the results in this work. Permanganate pre-oxidation has also been extensively studied for algae-laden

372

water pretreatment due to its low impact on cell integrity.

373

dioxide was observed to have an important role in controlling algae-associated membrane fouling via adhering to

374

the cell surface, absorbing EOM and contributing to the formation of a more porous cake layer.

375

permanganate pre-oxidation was demonstrated to be effective in removing both toxins

376

However, permanganate pre-oxidation is ineffective for degrading intracellular toxins and odors at acceptable

377

concentrations (< 2 mg/L) for potable purposes.

378

impart a purple color to the treated water.

379

Compared to the aforementioned oxidants, Fe(II)/persulfate can simultaneously improve the removal of algal

380

metabolites (toxins and odor compounds) and retard the membrane fouling caused by algal cells and organics as

53, 54

46, 50

44

Aluminum-based coagulants are more

and is ineffective or adversely affects irreversible 51, 52

Despite its

ozonation is not applicable when algae cells and metabolites coexist

55

45, 55

In our previous studies, in situ-formed manganese

58

56, 57

Moreover,

and odor compounds 59.

Moreover, an improper concentrations of permanganate can

21



381

illustrated in this work. An in situ Fe(II)/persulfate pretreatment efficiently improves organic removal despite cell

382

breakage. For the Fe(II)/persulfate pretreatment, the ferrous ions act as the activator and generate ferric ions, which

383

can result in a less dense cake layer. 31, 60 The porous structure outside the algae cells is probably the reason for the

384

anti-fouling property of the in situ Fe(II)/persulfate pretreatment-UF. Moreover, the irreversible fouling caused by

385

pore blocking can be mitigated by the Fe(II)/persulfate oxidation. This work was performed on a lab scale using only

386

Microcystis aeruginosa and two types of oxidants. Hence, more extensive research is still warranted on real

387

applications of Fe(II)/persulfate-UF for algae-laden water treatment.

388 389

Associated content

390

Supporting Information. Eight appendices are provided reporting characteristics of PVDF membranes used,

391

standard oxidation potentials of ozone and Fe(II)/persulfate, permeability and pore sizes of the membranes under

392

extreme oxidant exposure, CLSM images of the fouled membranes , fluorescence EEM spectra of UF permeate,

393

cell breakage and fouling characteristics under low-dose ozonation, effects of ferric coagulation on membrane

394

fouling performance and topological SEM images of fouled membranes by raw and pre-oxidized algae-laden water.

395

396

Acknowledgements

397

This work was financially supported by the Natural Science Foundation of China (Grants 51308146), Open Project of

398

State Key Laboratory of Urban Water Resource and Environment (ES201511-02), Fund from China Postdoctoral

399

Science Foundation (Grants 2015T80360), Heilongjiang Postdoctoral Fund and China scholarship council, and HIT

400

Environment and Ecology Innovation Special Funds (HSCJ201603).

22


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236x118mm (150 x 150 DPI)


Membrane Fouling and Rejection of Organics during Algae-Laden

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