Efficient natural organic matter removal from water using nano-MgO coupled with microfiltration membrane separation

Efficient natural organic matter removal from water using nano-MgO coupled with microfiltration membrane separation

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Journal Pre-proofs Efficient natural organic matter removal from water using nano-MgO coupled with microfiltration membrane separation Juanjuan Zhou, Yan Xia, Yanyan Gong, Wanbin Li, Zhanjun Li PII: DOI: Reference:

S0048-9697(19)35112-5 https://doi.org/10.1016/j.scitotenv.2019.135120 STOTEN 135120

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Science of the Total Environment

Received Date: Revised Date: Accepted Date:

8 October 2019 19 October 2019 21 October 2019

Please cite this article as: J. Zhou, Y. Xia, Y. Gong, W. Li, Z. Li, Efficient natural organic matter removal from water using nano-MgO coupled with microfiltration membrane separation, Science of the Total Environment (2019), doi: https://doi.org/10.1016/j.scitotenv.2019.135120

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Efficient natural organic matter removal from water using nano-MgO coupled with microfiltration membrane separation Juanjuan Zhou, Yan Xia, Yanyan Gong, Wanbin Li, Zhanjun Li* Guangdong Key Laboratory of Environmental Pollution and Health, School of Environment, Jinan University, Guangzhou 510632, China Corresponding author: Dr. Zhanjun Li, email: [email protected]

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Abstract Excess natural organic matter (NOM) in water not only lead to unpleasant black color and dissolved oxygen depletion in wastewater and natural water body but also causes carcinogenic chlorinated organic byproduct during drinking water chlorine disinfection. We try to develop a novel cost-effective and green technology for water NOM removal. In our simulated NOM removal process using humic acid (HA) as typical organic matter, we find that mesoporous nano-MgO performs an abnormally high NOM removal capacity (1260 mg-HA/g-MgO, or 446 mgC/g-MgO) when coupled with microfiltration membrane separation, which can’t be illustrated by traditional adsorption mechanism. Actually, Mg2+ from dissolved Mg(OH)2 contributes ~92% NOM removal via coagulation while Mg(OH)2 is responsible for the residue ~8% via adsorption. MgO serves as a two-in-one coagulant and adsorbent. The MgO treatment process is highly pH sensitive and weak acidic condition is favored for high NOM removal efficiency. MgO can be regenerated for more than 10 circulations by annealing Mg(OH)2/Mg-NOM composite at 500 oC, so that our MgO recycling process will be sustainable without the need of continuous chemical purchase. More importantly, no solid waste is generated in this novel process. This MgO-recycling NOM-removal process is simple, efficient, and sustainable for water NOM removal and will be significant in promoting novel sustainable technologies for NOM- or HA-related water remediation and treatment while minimizing the generation of solid waste. Keywords: Natural organic matter; MgO; adsorption; membrane separation; annealing regeneration.

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1 Introduction Natural organic matter (NOM) widely exists in aquatic environment. NOM can generate chlorinated organic byproducts during chlorine disinfection which lead to toxic or even carcinogenic threats to human health.(Bhatnagar and Sillanpaa, 2017; Gan et al., 2019; Wan et al., 2019; Young et al., 2018) NOM contamination is also an important issue in some advanced water treatment processes, such as membrane separation(Zhao et al., 2018; Zhu et al., 2018), adsorption(Li et al., 2003; Zheng et al., 2019), and catalytic oxidation(Cai et al., 2019; Chen et al., 2019; Ren et al., 2018). Humic acid (HA) represents one of the most popular NOM. Anthropogenic pollutant discharge may release extra amount of HA into aquatic systems. For example, landfill leachate contains a large portion of HA.(Aftab and Hur, 2019; Iskander et al., 2019; Ye et al., 2019) As HA-related NOM mainly consists of biomass residue after biodegradation, it is not easy to be efficiently degraded by traditional activate sludge treatment. Although some new technologies, such as membrane separation process(Abdullah et al., 2018) and advanced oxidation process(Cui et al., 2019; Kim et al., 2018) have been explored, its removal mainly relies on coagulation process in which people have to continuously purchase coagulants (mostly aluminate chloride, ferric chloride, and poly aluminum chloride etc.).(Jin et al., 2018; Song et al., 2019; Wang et al., 2014b; Wu et al., 2016) Furthermore, NOM-coagulant sludge forms extra NOM solid waste that needs further disposal.(Jung et al., 2015; Xu et al., 2016) Both the coagulant purchase and NOM sludge disposal impose increasing costs on NOM water treatment. New technologies are still needed for water NOM removal. Adsorption is a simple and effective technology that can remove water HA and HA-related NOM.(Augusto et al., 2019; Qin et al., 2015; Wang et al., 2016; Wen et al., 2017) For example, 3

magnetic nanoparticles were developed for HA removal from water by adsorption and magnetic separation. Layered double hydroxides/hollow carbon microsphere composites were developed to simultaneously remove HA and Pb(II).(Huang et al., 2017) Yet, the adsorption capacities, no matter high or low, of all adsorbents are limited and regeneration is needed after adsorbents are saturated. Generally, adsorption is only suitable to remove low concentration NOM or frequent regeneration or replacement of adsorbents will be needed.(Gueu et al., 2019; Kamranifar et al., 2019; Qiu et al., 2019) Thus, adsorption is usually combined with other technologies in NOM removal. For example, Xu et al. found that combining the alum coagulation and magnetic chitosan nanoparticle adsorption could significantly improve flocs settlement performance and increase the floc sizes.(Wang et al., 2018) Jung et al. used biochar as NOM adsorbent and combined adsorption with coagulation by using polyaluminum chloride to realize efficient removal of humic acid and tannic acid.(Jung et al., 2015) Yet, the existing adsorption/coagulation processes rely on the combined usage of two kind of chemicals, adsorbents and coagulants. To the best of our knowledge, no single material was reported to possess both adsorption and coagulation properties. MgO is an efficient and cheap inorganic adsorbent.(Cui et al., 2018; Kiani et al., 2019; Wang et al., 2013) Interestingly, it can hydrolyze and form Mg(OH)2 nanosheet, which can release authigenic Mg2+ at room temperature in the presence of water and turn back to MgO at relatively low temperatures higher than 350 oC. We propose to use MgO as adsorbent and authigenic Mg2+ as coagulant to remove NOM from water and recycle MgO by annealing Mg(OH)2/NOM sludge at 500 oC (Fig.1). In this way, no extra adsorbent or coagulant purchase is needed and all the MgO reagent can be recycled while maintaining a very high removal efficiency.

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Fig.1 Schematic illustration of NOM removal process using renewable MgO. 2 Experimental 2.1 Materials Magnesium chloride and ammonium water (28 wt.%) are A.R. grade and used as received. Sodium humate was purchased from Shanghai Aladdin biochemical technology co., LTD. HA solution was prepared by dissolving sodium humate in D.I. water and adjusted to desired pH by adding hydrochloride acid (1 mol/L). Microfiltration membranes (PES, 0.45 μm) were purchased from Beijing North TZ-Biotech Develop, Co. Ltd. 2.2 Synthesis of nano-MgO Mg2+ solution (1 M, 100 mL) was prepared by dissolving magnesium chloride in water. 15 mL ammonium water (28 wt.%) was added quickly into Mg2+ solution with vigorous stirring. Mg(OH)2 was formed immediately and the mixture seemed milky. The composite solution was heated to boiling to let water evaporate and obtain dried Mg(OH)2/NH4Cl mixture. The mixture was then annealed at 450 oC for 2 h and cooled to room temperature. Loose white powder, Nano-MgO, can be obtained after simple grinding. 2.3 HA removal by MgO treatment HA stock solution was prepared by dispersing 1.0 g of sodium humate into 0.5 L of D.I. 5

water and filtered through rapid filter paper and its TOC value was tested. HA solutions with exact TOC concentrations were prepared by water dilution of the stock solution. MgO was added into 500 mL of HA solution (100 mgC/L, pH = 6) under stirring. 5 mL aliquots of the MgO/HA mixture solution were taken at predetermined time intervals and filtered through micro-filtration film (0.45 μm). Filtrates were tested to obtain residue TOC values in the solution. 2.3 Adsorption isotherms of HA by using MgO MgO (0.2 g/L) were added to 20 mL of HA solution with diverse HA concentrations and shaken for 24 h in a thermostatic oscillator. 5 mL aliquots of the MgO/HA mixture solution were taken and filtered through micro-filtration film (0.45 μm). Filtrates were tested to obtain residue HA TOC values in the solution. The HA removal efficiency (η) and equilibrium adsorption capacity (qe, mg-TOC/g-MgO) were calculated using eq. (1) and eq. (2), respectively, as follows:

η

C0  Ce  100%

qe 

C0

C

0



 Ce  V m

(1) (2)

where C0 (mg/L) is the initial HA concentration, Ce (mg/L) is the equilibrium HA concentration, m (g) is the MgO mass, and V (L) is the HA water volume. The adsorption isotherms were fitted using Langmuir model (eq. 3) and Freundlich model (eq. 4):

1 1 1   qe q m k l C e q m ln q e 

1 ln C e  ln k f n

(3)

(4)

where qe and qm (mg-TOC/g-MgO) are the equilibrium and max adsorption capacity, respectively; Ce (mg/L) is the equilibrium HA concentration and kl and kf, n are constants. 2.4 HA removal by magnesium chloride coagulation 6

Magnesium chloride was dissolved into D.I. water to prepare a coagulant solution (CMg = 0.1 mol/L), which was added into 20 mL of HA solution (100 mg-TOC/L, pH = 6) and shaken for 1 h in a thermostatic oscillator. 5 mL aliquots of the MgO/HA mixture solution were taken and filtered through micro-filtration film (0.45 μm). Filtrates were tested to obtain residue HA TOC values in the solution. 2.5 Characterization The morphology of MgO was observed by using a field emission scanning electron microscope (SEM) with an accelerating voltage of 5 kV (S-4800, Hitachi, Japan). The x-ray diffraction patterns (XRD) were acquired by using a powder diffractometer with Cu Kα radiation (λ = 1.5418 Å) (D2 PHASER, AXS, Germany). The N2 adsorption/desorption isotherm was acquired using an automated gas sorption analyzer (Autosorb, Quantachrome, USA). Total organic carbon (TOC) of HA solution was tested by using a TOC analyzer (Vario, Elementar, Germany). The concentrations of the HA solution during MgO treatment were detected according to the TOC value (mgC/L). 3 Results and discussion 3.1 Properties of the as-synthesized MgO The synthesis of nano-MgO can be realized easily via heat decomposition of Mg(OH)2 at temperatures higher than 350 oC. Mg(OH)2 is generally prepared by a precipitation reaction between MgCl2 and base reagents such as sodium hydroxide, calcium hydroxide, and ammonium. Ammonium was used here because the byproduct, NH4Cl, can be removed easily during the heat decomposition of Mg(OH)2 into MgO at 450 oC. The as-synthesized MgO has a loose white powder outlook. The XRD pattern indicates a pure MgO phase. The SEM image indicates that the 7

as-synthesized MgO has a porous sheet-like nanostructure (Fig. 2). The thickness of the nanosheet is as thin as ca. 10 nm while the length and width are ca. 1000 nm. The porous structure might be caused by the gas byproducts (NH3, HCl, and H2O) during the annealing process. To further verify the porous structure of the as-synthesized MgO. N2 adsorption/desorption isotherm was acquired. The results indicate that the nano-MgO has a specific surface area as high as 66.83 m2/g and a mesopore volume of 0.135 cm3/g with pore diameters peaking at 1.4 nm with a rang from ca. 1 nm to ca. 6 nm. The high specific surface area and porous structure imply possible good NOM removal properties.

Fig.2 Properties of synthesized nano-MgO. (a) SEM, (b) XRD, (c) N2 adsorption/desorption isotherm, and (d) mesopore size distribution. 3.2 NOM removal capacity of MgO Humic acid (HA) solution was used to simulate NOM water. To test the NOM removal 8

property of MgO, the as-synthesized nano-MgO powder was added into HA solution (100 mgC/L or 354 mg-HA/L, pH = 6.0) under stirring. 5 mL aliquots of the HA/MgO mixture were taken and filtered through a microfiltration membrane (pore size, 0.45 μm). The filtrate was taken to measure residue HA concentration. The results indicate that HA can be removed quickly after adding 0.50 g/L MgO into HA solution (Fig. 3). 84.3% of HA can be removed after stirring for 1 h. And the removal efficiency can reach ~ 93.4% after extending the stirring time to 2 h. However, the efficiency can’t be further increased significantly after extending the mixing time to 3 h (94.5%). The color of the HA water turns from deep black to slight yellowish after MgO treatment. The removal rate can be significantly accelerated if the MgO dosage is increased to 1.0 g/L. Over 97.3% removal efficiency can be reached within 0.5 h. After that, the removal efficiencies increased slowly to 98.0% and 99.7% at 1 h and 3 h, respectively. Or, put it another way, a high removal capacitys can reach 99.7 mgC/g-MgO at a rather low equilibrium HA concentration of 0.3 mgC/L, which can hardly be realized by traditional adsorption methods.

Fig. 3 Removal of humic acid (TOC concentration, 100 mgC/L, pH = 6) from water by MgO. MgO was considered as an efficient adsorbent in many existing reports(Cui et al., 2018; Kiani et al., 2019; Yi et al., 2019). Thus, we performed the adsorption isotherm to reveal its HA 9

adsorption capacity (Fig. 3). Interestingly, we find out that the adsorption capacity increases almost linearly along with the equilibrium HA concentration. The HA/MgO adsorption isotherm can be fitted much better by using Freundlich model (R2 = 0.968) than Langmuir model (R2 = 0.605) (Table 1). The Langmuir maximum adsorption capacity is ~ 446 mgC/g-MgO or 1579 mg-HA/g-MgO which is much higher than existing HA adsorbents, such as layered double hydroxides (LDHs), carbon nanotubes, porous carbon shells, layered double hydroxides/hollow carbon microsphere composites, as shown in Table 2. As a comparison, adsorption isotherm was also obtained by using activated carbon powder (AC) as adsorbent. Although the HA/MgO isotherm fails to be fitted by Langmuir model, the raw maximum adsorption capacity of MgO is about 76 times as that of AC. In the case of our simulated HA water (100 mgC/L), 96.4% HA can be removed by adding 0.50 g/L of MgO (Fig. 4). After that, the removal efficiency increases very slowly. 99.7% HA can be removed by using 1.0 g/L of MgO. This high removal capacity can hardly be realized by simple adsorption even by using activated carbon power. It seems more like a reaction rather than adsorption between HA and MgO.

Fig. 3 HA removal isotherm using MgO in comparison with activated carbon. The inset is the enlarged isotherm curve of activated carbon. 10

Table 1 Langmuir and Freundlich fitting of the HA removal isotherms. Sample

Langmuir

Freundlich kf

*

qmax

kl R2

(mgC/g)

(L mg-1)(L

1/n

R2

(L/mg) mg-1)1/n

MgO

446

0.0421

0.605

0.569

106

0.968

AC

17.0

0.0384

0.910

0.566

1.48

0.976

Table 2 Comparison of the Langmuir maximum HA adsorption capacity of MgO with existing resports. Adsorbents

qmax,

Ref.

mg-HA/g zinc oxide-coated zeolite

120

(Wang et al., 2016)

LDHs/HCMSs

300.5

(Huang et al., 2017)

LDHs

225.6

(Wang et al., 2014a)

Multiwalled carbon

82

(Wang et al., 2009)

Dual-pore carbon shells

99.27

(Yu et al., 2017)

Activated carbon

6.9

(Ferro-García et al., 1998)

MgO

1260*

This work

nanotubes

*The HA adsorption capacity was calibrated according to the carbon content of HA (35.4%)

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Fig. 4 Influence of MgO dosage on HA removal (pH = 6.0, reaction time = 3 h). 3.3Possible reason for the high HA removal capacity We hypothesize that dissolved Mg2+ may play an important role in HA removal using MgO treatment. At first, we analyzed the hydrolysis reaction of MgO into Mg(OH)2 (eq.3) by monitoring the released hydroxyl anions (pH variation) after MgO was added into water (Fig. 5a). The results indicate that MgO hydrolyzed very quickly with water within 10 min. After 20 min, the pH value reached a value of ca. 10.6. To further explore the possible reason for the extremely high HA removal capacity of MgO, we analyzed the theoretical solubility of Mg(OH)2 (MgO exists as Mg(OH)2 in the presence of water) vs. pH (Fig. 5b). Although Mg(OH)2 is considered insoluble in pure water, one can see that the solubility of Mg(OH)2 is highly pH sensitive (eq.3-5). Mg(OH)2 is actually highly soluble in water under neutral or acidic conditions. Its insoluble attribute actually comes from the strong alkaline property (pH ~10.6, 25 oC) of saturated Mg(OH)2 aqueous solution. HA is a mixture of many organic weak acids which can be considered as a kind of buffer solution. When MgO was added into HA water, a part of Mg(OH)2 may be dissolved to release Mg2+ into water and serve as a multivalent cationic coagulant which can be facilitated by the hydrolysis of HA (eq.6). We observed weak alkaline effluent (pH ~ 9.4) after MgO treatment 12

(3 g/L) of HA water (initial pH, ~ 6.0) and the treated water contained 0.009 mol/L of dissolved Mg2+. Therefore, we propose that dissolved Mg2+ plays an important role in HA removal. Mg(OH)2 = Mg2+ + 2 OH-

(3)

Ksp = [Mg2+][OH-]2

(4)

Log[Mg2+] = Log(Ksp) + 28 - 2pH HA + OH- = A- + H2O

(5) (6)

Fig.5 Influence of pH and dissolved Mg2+. (a) pH change due to hydrolysis of MgO after MgO is added into water, (b) theoretical solubility of Mg(OH)2 at various pH conditions. Log[Mg2+] is the logarithmic equilibrium Mg2+ concentration (mol/L) in Mg(OH)2/H2O binary system at 25 oC, (c) coagulation removal of HA using MgCl2, (d) Influence of pH on HA removal after MgO treatment. (Initial HA concentration is 100 mgC/L, C(MgO) = 1 g/L, time = 3 h). To quantify the contribution of dissolved Mg2+, a coagulation experiment was performed by 13

adding MgCl2 into HA solution. The results indicate that HA can be removed by coagulation (Fig. 5c). More than 80% of HA can be removed by adding 3.75 mmol/L of MgCl2. However, the HA removal efficiency increases very slowly even after doubling the MgCl2 dose from 3.75 to 7.50 mmol/L and remains nearly unchanged, ~ 92%, at higher dose. Mg(OH)2 can theoretically release more than 8.91 mmol/L Mg2+ at pH lower than 9.4 in Mg(OH)2/water binary system. Thus, dissolved Mg2+ contributes to ca. 92% of the HA removal capacity in the MgO treatment process while Mg(OH)2 contributes to the residue ca. 8%. To further verify our hypothesis, we studied the influence of initial pH on the HA removal efficiency as lower pH will release more Mg2+ into water. The results indicate that the HA removal process by using MgO is highly pH-sensitive (Fig. 5d). Weak acidic conditions can facilitate HA removal while alkaline conditions can hinder the process. More than 90% HA can be removed when the initial pH is lower than 6.5. However, less than 30% HA can be removed when the initial pH is higher than 7.2. Good HA removal performance can be expected as natural HA water and wastewater usually has weak acidic pH value, such as landfill leachate and polluted black odor river. Based on the aformentioned results, a possible mechanism is proposed to illustrate the efficient HA removal capacity of the MgO treatment process (Fig.6). When MgO is added into HA water, MgO hydrolyzes to form Mg(OH)2 and releases Mg2+ into water. Hydrolysis of HA facilitates the hydrolysis reaction and releases more Mg2+. Coagulation occurs immediately between HA and dissolved Mg2+, forming HA-Mg coagulum. Residue HA is adsorbed onto Mg(OH)2 nanoparticles. HA-Mg and HA-adsorbed Mg(OH)2 forms bigger composite coagulum and is removed by microfiltration. 14

Fig.6 Schematic illustration of adsorption/coagulation removal of HA using MgO. 3.4 Recovery of MgO by annealing As most of the organic matter can be degraded by combustion and Mg(OH)2 can transform to MgO at temperatures higher than 350 oC. We tried to regenerate MgO at 500 oC. The results indicate that the precipitate in the MgO treatment process contains most of hexagonal brucite Mg(OH)2 (JCPDS no. 96-100-0055) and the impurity peaks ranging from 20

o

to 35

o

can be

assigned to the Mg-HA coagulum formed by coagulation between dissolved Mg2+ and HA (Fig. 7). The MgO raw material has a cubic periclase phase (JCPDS no. 96-100-0054). After annealing at 500 oC for 2 h, only MgO peaks (MgO-R1) can be observed in the X-ray diffraction pattern while the Mg-HA peaks disappear because of high temperature degradation. The regenerated MgO was used in multiple HA-removal/MgO-regeneration cycles. High removal efficiencies (~ 99.7%) can be successfully obtained for 10 treatment/recycle circulations (Fig. 8). And periclase MgO are successfully recycled for at least 10 times and no obvious impurity peaks can be observed (Fig. 7) in the regenerated MgO (MgO-R10). MgO is slight soluble in the effluent. Thus, possible mass loss might occur. Interestingly, we didn’t observe apparent mass loss in the recycled MgO. On the contrary, a slight mass increase (0.67%) was observed in the recycled MgO after 10 HA removal cycles. Possible reason might be that some metal ions coexisted with HA and were recycled together with MgO. The color of the recycled MgO turned from white to slight brown, which 15

indicate the existence of inorganic impurities. The good recycle property of MgO will greatly alleviate the reagent cost so that we don’t have to keep buying chemicals like traditional coagulation HA removal process.

Fig.7 X-ray diffraction patterns of MgO, Mg(OH)2-HA sludge, and regenerated MgO after 1 (MgO-R1) and 10 (MgO-R10) HA removal circulations.

Fig.8 HA removal using regenerated MgO in multiple cycles. (Initial HA concentration = 100 mgC/L, m(MgO) = 1.0 g/L, pH = 6.0, time = 3 h). The inset pictures are HA water before and after MgO treatment. 4 Conclusion HA can be efficiently removed by MgO treatment, in which MgO serves as two-in-one 16

coagulant and adsorbent. The MgO treatment process is highly pH sensitive and weak acidic condition is favored for high HA removal efficiency. Dissolved Mg2+ removes ca.92% of the HA via coagulation while Mg(OH)2 is responsible for the adsorption removal of residue 8%. MgO can be regenerated for multicycles via annealing Mg(OH)2/Mg-HA composite at 500 oC, so that we can save money from adsorbent and coagulant purchase and avoid the generation of adsorbent waste. This MgO treatment process is simple, efficient, and cost effective for water HA and NOM removal. Although MgO regeneration needs high temperature and consumes thermal energy, this NOM removal process might still work well if it is applied together with some heat recycle processes such as photothermal conversion of solar energy and energy recycle from anaerobic methane production and waste incineration. This work will be significant in promoting new technologies for HA- or NOM-related drinking water purification, natural water remediation, and industrial black wastewater treatment. Acknowledgements This research was supported by the National Natural Science Foundation of China (21701052), the Science and Technology Project of Guangzhou (201804010401), the Natural Science Foundation of Guangdong Province of China (32217073), the Special Funds for Basic Scientific Research Operations of Central Universities of China (11617323). References Abdullah N, Rahman MA, Othman MHD, Jaafar J, Abd Aziz A. Preparation, characterizations and performance evaluations of alumina hollow fiber membrane incorporated with UiO-66 particles for humic acid removal. Journal of Membrane Science 2018; 563: 162-174. Aftab B, Hur J. Unraveling complex removal behavior of landfill leachate upon the treatments of Fenton oxidation and MIEX (R) via two-dimensional correlation size exclusion chromatography (2D-CoSEC). Journal of Hazardous Materials 2019; 362: 36-44. Augusto PA, Castelo-Grande T, Merchan L, Estevez AM, Quintero X, Barbosa D. Landfill leachate treatment by sorption in magnetic particles: preliminary study. Science of the Total 17

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Graphical abstract

Highlights (1) MgO performs abnormally high NOM removal capacity. (2) MgO serves as a two-in-one coagulant and adsorbent. (3) Dissolved Mg2+ removes 92% NOM via coagulation. (4) Mg(OH)2 is responsible for the adsorption removal of residue NOM.

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(5) MgO can be regenerated for more than 10 times without generating any solid waste.

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