Science of the Total Environment 711 (2020) 135120

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Science of the Total Environment

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

https://doi.org/10.1016/j.scitotenv.2019.1351200048-9697/� 2019 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: zhanjun.li@foxmail.com (Z. Li).

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

h i g h l i g h t s

� MgO performs abnormally high NOMremoval capacity.

� MgO serves as a two-in-one coagulantand adsorbent.

� Dissolved Mg2+ removes 92% NOM viacoagulation.

� Mg(OH)2 is responsible for theadsorption removal of residue NOM.

� MgO can be regenerated for morethan 10 times without generating anysolid waste.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 October 2019Received in revised form 19 October 2019Accepted 21 October 2019Available online 21 November 2019

Editor: Huu Hao Ngo

Keywords:Natural organic matterMgOAdsorptionMembrane separationAnnealing regeneration

a b s t r a c t

Excess natural organic matter (NOM) in water not only lead to unpleasant black color and dissolved oxy-gen depletion in wastewater and natural water body but also causes carcinogenic chlorinated organicbyproduct during drinking water chlorine disinfection. We try to develop a novel cost-effective and greentechnology for water NOM removal. In our simulated NOM removal process using humic acid (HA) as typ-ical organic matter, we find that mesoporous nano-MgO performs an abnormally high NOM removalcapacity (1260 mg-HA/g-MgO, or 446 mgC/g-MgO) when coupled with microfiltration membrane sepa-ration, 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 ishighly pH sensitive and weak acidic condition is favored for high NOM removal efficiency. MgO can beregenerated for more than 10 circulations by annealing Mg(OH)2/Mg-NOM composite at 500 �C, so thatour MgO recycling process will be sustainable without the need of continuous chemical purchase. Moreimportantly, no solid waste is generated in this novel process. This MgO-recycling NOM-removal processis simple, efficient, and sustainable for water NOM removal and will be significant in promoting novelsustainable technologies for NOM- or HA-related water remediation and treatment while minimizingthe generation of solid waste.

� 2019 Elsevier B.V. All rights reserved.

1. Introduction

Natural organic matter (NOM) widely exists in aquatic environ-ment. NOM can generate chlorinated organic byproducts duringchlorine disinfection which lead to toxic or even carcinogenicthreats to human health (Bhatnagar and Sillanpaa, 2017; Gan

Fig. 1. Schematic illustration of NOM removal process using renewable MgO.

2 J. Zhou et al. / Science of the Total Environment 711 (2020) 135120

et al., 2019; Wan et al., 2019; Young et al., 2018). NOM contamina-tion is also an important issue in some advanced water treatmentprocesses, such as membrane separation (Zhao et al., 2018; Zhuet al., 2018), adsorption (Li et al., 2003; Zheng et al., 2019), and cat-alytic oxidation (Cai et al., 2019; Chen et al., 2019; Ren et al., 2018).Humic acid (HA) represents one of the most popular NOM. Anthro-pogenic pollutant discharge may release extra amount of HA intoaquatic systems. For example, landfill leachate contains a largeportion of HA (Aftab and Hur, 2019; Iskander et al., 2019; Yeet al., 2019). As HA-related NOM mainly consists of biomass resi-due after biodegradation, it is not easy to be efficiently degradedby traditional activate sludge treatment. Although some new tech-nologies, such as membrane separation process (Abdullah et al.,2018) and advanced oxidation process (Cui et al., 2019; Kimet al., 2018) have been explored, its removal mainly relies on coag-ulation process in which people have to continuously purchasecoagulants (mostly aluminate chloride, ferric chloride, and polyaluminum chloride etc.) (Jin et al., 2018; Song et al., 2019; Wanget al., 2014b; Wu et al., 2016). Furthermore, NOM-coagulant sludgeforms extra NOM solid waste that needs further disposal (Junget al., 2015; Xu et al., 2016). Both the coagulant purchase andNOM sludge disposal impose increasing costs on NOM water treat-ment. New technologies are still needed for water NOM removal.

Adsorption is a simple and effective technology that can removewater HA and HA-related NOM (Augusto et al., 2019; Qin et al.,2015; Wang et al., 2016; Wen et al., 2017). For example, magneticnanoparticles were developed for HA removal from water byadsorption and magnetic separation. Layered double hydroxides/hollow carbon microsphere composites were developed to simul-taneously remove HA and Pb(II) (Huang et al., 2017). Yet, theadsorption capacities, no matter high or low, of all adsorbentsare limited and regeneration is needed after adsorbents are satu-rated. Generally, adsorption is only suitable to remove low concen-tration NOM or frequent regeneration or replacement ofadsorbents will be needed (Gueu et al., 2019; Kamranifar et al.,2019; Qiu et al., 2019). Thus, adsorption is usually combined withother technologies in NOM removal. For example, Xu et al. foundthat combining the alum coagulation and magnetic chitosannanoparticle adsorption could significantly improve flocs settle-ment performance and increase the floc sizes (Wang et al., 2018).Jung et al. used biochar as NOM adsorbent and combined adsorp-tion with coagulation by using polyaluminum chloride to realizeefficient removal of humic acid and tannic acid (Jung et al.,2015). Yet, the existing adsorption/coagulation processes rely onthe combined usage of two kind of chemicals, adsorbents and coag-ulants. To the best of our knowledge, no single material wasreported 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 canhydrolyze and form Mg(OH)2 nanosheet, which can release authi-genic Mg2+ at room temperature in the presence of water and turnback to MgO at relatively low temperatures higher than 350 �C. Wepropose to use MgO as adsorbent and authigenic Mg2+ as coagulant toremove NOM from water and recycle MgO by annealing Mg(OH)2/NOM sludge at 500 �C (Fig. 1). In this way, no extra adsorbent orcoagulant purchase is needed and all the MgO reagent can be recy-cled while maintaining a very high removal efficiency.

2. Experimental

2.1. Materials

Magnesium chloride and ammonium water (28 wt.%) are A.R.grade and used as received. Sodium humate was purchased fromShanghai Aladdin biochemical technology co., LTD. HA solution

was prepared by dissolving sodium humate in D.I. water andadjusted to desired pH by adding hydrochloride acid (1 mol/L).Microfiltration membranes (PES, 0.45 lm) were purchased fromBeijing North TZ-Biotech Develop, Co. Ltd.

2.2. Synthesis of nano-MgO

Mg2+ solution (1 M, 100 mL) was prepared by dissolving magne-sium chloride in water. 15 mL ammonium water (28 wt.%) wasadded quickly into Mg2+ solution with vigorous stirring. Mg(OH)2was formed immediately and the mixture seemed milky. The com-posite solution was heated to boiling to let water evaporate andobtain dried Mg(OH)2/NH4Cl mixture. The mixture was thenannealed at 450 �C for 2 h and cooled to room temperature. Loosewhite 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 sodiumhumate into 0.5 L of D.I. water and filtered through rapid filterpaper and its TOC value was tested. HA solutions with exact TOCconcentrations were prepared by water dilution of the stock solu-tion. MgO was added into 500 mL of HA solution (100 mgC/L,pH = 6) under stirring. 5 mL aliquots of the MgO/HA mixture solu-tion were taken at predetermined time intervals and filteredthrough micro-filtration film (0.45 lm). Filtrates were tested toobtain residue TOC values in the solution.

2.4. Adsorption isotherms of HA by using MgO

MgO (0.2 g/L) were added to 20 mL of HA solution with diverseHA concentrations and shaken for 24 h in a thermostatic oscillator.5 mL aliquots of the MgO/HA mixture solution were taken and fil-tered through micro-filtration film (0.45 lm). Filtrates were testedto obtain residue HA TOC values in the solution. The HA removalefficiency (g) and equilibrium adsorption capacity (qe, mg-TOC/g-MgO) were calculated using eq. (1) and eq. (2), respectively, asfollows:

g ¼ C0 � Ceð ÞC0

� 100% ð1Þ

qe ¼C0 � Ceð Þ � V

mð2Þ

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

J. Zhou et al. / Science of the Total Environment 711 (2020) 135120 3

1qe

¼ 1qmklCe

þ 1qm

ð3Þ

lnqe ¼1nlnCe þ lnkf ð4Þ

where qe and qm (mg-TOC/g-MgO) are the equilibrium and maxadsorption capacity, respectively; Ce (mg/L) is the equilibrium HAconcentration and kl and kf, n are constants.

2.5. HA removal by magnesium chloride coagulation

Magnesium chloride was dissolved into D.I. water to prepare acoagulant solution (CMg = 0.1 mol/L), which was added into 20 mLof HA solution (100 mg-TOC/L, pH = 6) and shaken for 1 h in a ther-mostatic oscillator. 5 mL aliquots of the MgO/HA mixture solutionwere taken and filtered through micro-filtration film (0.45 lm).Filtrates were tested to obtain residue HA TOC values in thesolution.

2.6. Characterization

The morphology of MgO was observed by using a field emissionscanning electron microscope (SEM) with an accelerating voltageof 5 kV (S-4800, Hitachi, Japan). The x-ray diffraction patterns(XRD) were acquired by using a powder diffractometer with CuKa radiation (k = 1.5418 Å) (D2 PHASER, AXS, Germany). The N2

adsorption/desorption isotherm was acquired using an automatedgas sorption analyzer (Autosorb, Quantachrome, USA). Totalorganic carbon (TOC) of HA solution was tested by using a TOCanalyzer (Vario, Elementar, Germany). The concentrations of theHA solution during MgO treatment were detected according tothe TOC value (mgC/L).

Fig. 2. Properties of synthesized nano-MgO. (a) SEM, (b) XRD, (c) N2 ad

3. Results and discussion

3.1. Properties of the as-synthesized MgO

The synthesis of nano-MgO can be realized easily via heatdecomposition of Mg(OH)2 at temperatures higher than 350 �C.Mg(OH)2 is generally prepared by a precipitation reaction betweenMgCl2 and base reagents such as sodium hydroxide, calciumhydroxide, and ammonium. Ammonium was used here becausethe byproduct, NH4Cl, can be removed easily during the heatdecomposition of Mg(OH)2 into MgO at 450 �C. The as-synthesized MgO has a loose white powder outlook. The XRD pat-tern indicates a pure MgO phase. The SEM image indicates that theas-synthesized MgO has a porous sheet-like nanostructure (Fig. 2).The thickness of the nanosheet is as thin as ca. 10 nm while thelength and width are ca. 1000 nm. The porous structure might becaused by the gas byproducts (NH3, HCl, and H2O) during theannealing process. To further verify the porous structure of theas-synthesized MgO. N2 adsorption/desorption isotherm wasacquired. The results indicate that the nano-MgO has a specific sur-face area as high as 66.83 m2/g and a mesopore volume of0.135 cm3/g with pore diameters peaking at 1.4 nm with a rangfrom ca. 1 nm to ca. 6 nm. The high specific surface area and porousstructure imply possible good NOM removal properties.

3.2. NOM removal capacity of MgO

Humic acid (HA) solution was used to simulate NOM water. Totest the NOM removal 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 mix-ture were taken and filtered through a microfiltration membrane(pore size, 0.45 lm). The filtrate was taken to measure residue

sorption/desorption isotherm, and (d) mesopore size distribution.

Fig. 3. Removal of humic acid (TOC concentration, 100 mgC/L, pH = 6) from waterby MgO.

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

Sample Langmuir Freundlich

qmax*(mgC/g)

kl (L/mg) R2 1/n kf (L mg�1)(L mg�1)1/n

R2

MgO 446 0.0421 0.605 0.569 106 0.968AC 17.0 0.0384 0.910 0.566 1.48 0.976

Table 2Comparison of the Langmuir maximum HA adsorption capacity of MgO with existingresports.

Adsorbents qmax, mg-HA/g Ref.

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 nanotubes 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 1260a This work

a The HA adsorption capacity was calibrated according to the carbon content ofHA (35.4%)

4 J. Zhou et al. / Science of the Total Environment 711 (2020) 135120

HA concentration. The results indicate that HA can be removedquickly 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 effi-ciency can reach �93.4% after extending the stirring time to 2 h.However, the efficiency can’t be further increased significantlyafter extending the mixing time to 3 h (94.5%). The color of theHA water turns from deep black to slight yellowish after MgOtreatment. The removal rate can be significantly accelerated ifthe MgO dosage is increased to 1.0 g/L. Over 97.3% removal effi-ciency can be reached within 0.5 h. After that, the removal efficien-cies increased slowly to 98.0% and 99.7% at 1 h and 3 h,respectively. Or, put it another way, a high removal capacitys canreach 99.7 mgC/g-MgO at a rather low equilibrium HA concentra-tion of 0.3 mgC/L, which can hardly be realized by traditionaladsorption methods.

MgO was considered as an efficient adsorbent in many existingreports(Cui et al., 2018; Kiani et al., 2019; Yi et al., 2019). Thus, weperformed the adsorption isotherm to reveal its HA adsorptioncapacity (Fig. 4). Interestingly, we find out that the adsorptioncapacity increases almost linearly along with the equilibrium HAconcentration. The HA/MgO adsorption isotherm can be fittedmuch better by using Freundlich model (R2 = 0.968) than Langmuir

Fig. 4. HA removal isotherm using MgO in comparison with activated carbon. Theinset is the enlarged isotherm curve of activated carbon.

model (R2 = 0.605) (Table 1). The Langmuir maximum adsorptioncapacity is � 446 mgC/g-MgO or 1579 mg-HA/g-MgO which ismuch higher than existing HA adsorbents, such as layered doublehydroxides (LDHs), carbon nanotubes, porous carbon shells, lay-ered double hydroxides/hollow carbon microsphere composites,as shown in Table 2. As a comparison, adsorption isotherm wasalso obtained by using activated carbon powder (AC) as adsorbent.Although the HA/MgO isotherm fails to be fitted by Langmuirmodel, the raw maximum adsorption capacity of MgO is about76 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 ofMgO (Fig. 5). After that, the removal efficiency increases veryslowly. 99.7% HA can be removed by using 1.0 g/L of MgO. Thishigh removal capacity can hardly be realized by simple adsorptioneven by using activated carbon power. It seems more like a reac-tion rather than adsorption between HA and MgO.

3.3. Possible reason for the high HA removal capacity

We hypothesize that dissolved Mg2+ may play an important rolein HA removal using MgO treatment. At first, we analyzed the

Fig. 5. Influence of MgO dosage on HA removal (pH = 6.0, reaction time = 3 h).

J. Zhou et al. / Science of the Total Environment 711 (2020) 135120 5

hydrolysis reaction of MgO into Mg(OH)2 (eq. (5)) by monitoringthe released hydroxyl anions (pH variation) after MgO was addedinto water (Fig. 6a). The results indicate that MgO hydrolyzed veryquickly with water within 10 min. After 20 min, the pH valuereached a value of ca. 10.6. To further explore the possible reasonfor the extremely high HA removal capacity of MgO, we analyzedthe theoretical solubility of Mg(OH)2 (MgO exists as Mg(OH)2 inthe presence of water) vs. pH (Fig. 6b). Although Mg(OH)2 is con-sidered insoluble in pure water, one can see that the solubility ofMg(OH)2 is highly pH sensitive (eq. (5)–(7)). Mg(OH)2 is actuallyhighly soluble in water under neutral or acidic conditions. Its insol-uble attribute actually comes from the strong alkaline property(pH � 10.6, 25 �C) of saturated Mg(OH)2 aqueous solution. HA isa mixture of many organic weak acids which can be consideredas 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 andserve as a multivalent cationic coagulant which can be facilitatedby the hydrolysis of HA (eq. (8)). We observed weak alkaline efflu-ent (pH � 9.4) after MgO treatment (3 g/L) of HA water (initial pH,� 6.0) and the treated water contained 0.009 mol/L of dissolvedMg2+. Therefore, we propose that dissolved Mg2+ plays an impor-tant role in HA removal.

Mg OHð Þ2 ¼ Mg2þ þ 2OH� ð5Þ

Ksp ¼ Mg2þh i

OH�½ �2 ð6Þ

Log Mg2þh i

¼ Log Ksp� �þ 28� 2pH ð7Þ

HAþ OH� ¼ A� þH2O ð8Þ

Fig. 6. Influence of pH and dissolved Mg2+. (a) pH change due to hydrolysis of MgO aconditions. Log[Mg2+] is the logarithmic equilibrium Mg2+ concentration (mol/L) in MgInfluence of pH on HA removal after MgO treatment. (Initial HA concentration is 100 m

To quantify the contribution of dissolved Mg2+, a coagulationexperiment was performed by adding MgCl2 into HA solution.The results indicate that HA can be removed by coagulation(Fig. 6c). More than 80% of HA can be removed by adding3.75 mmol/L of MgCl2. However, the HA removal efficiencyincreases very slowly even after doubling the MgCl2 dose from3.75 to 7.50 mmol/L and remains nearly unchanged, � 92%, athigher dose. Mg(OH)2 can theoretically release more than8.91 mmol/L Mg2+ at pH lower than 9.4 in Mg(OH)2/water binarysystem. Thus, dissolved Mg2+ contributes to ca. 92% of the HAremoval capacity in the MgO treatment process while Mg(OH)2contributes to the residue ca. 8%.

To further verify our hypothesis, we studied the influence of ini-tial pH on the HA removal efficiency as lower pH will release moreMg2+ into water. The results indicate that the HA removal processby using MgO is highly pH-sensitive (Fig. 6d). Weak acidic condi-tions can facilitate HA removal while alkaline conditions can hin-der the process. More than 90% HA can be removed when theinitial pH is lower than 6.5. However, less than 30% HA can beremoved when the initial pH is higher than 7.2. Good HA removalperformance can be expected as natural HA water and wastewaterusually has weak acidic pH value, such as landfill leachate and pol-luted black odor river.

Based on the aformentioned results, a possible mechanism isproposed to illustrate the efficient HA removal capacity of theMgO treatment process (Fig. 7). 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 releasesmore Mg2+. Coagulation occurs immediately between HA and dis-solved Mg2+, forming HA-Mg coagulum. Residue HA is adsorbedonto Mg(OH)2 nanoparticles. HA-Mg and HA-adsorbed Mg(OH)2

fter MgO is added into water, (b) theoretical solubility of Mg(OH)2 at various pH(OH)2/H2O binary system at 25 �C, (c) coagulation removal of HA using MgCl2, (d)gC/L, C(MgO) = 1 g/L, time = 3 h).

Fig. 7. Schematic illustration of adsorption/coagulation removal of HA using MgO.

Fig. 8. X-ray diffraction patterns of MgO, Mg(OH)2-HA sludge, and regeneratedMgO after 1 (MgO-R1) and 10 (MgO-R10) HA removal circulations.

Fig. 9. HA removal using regenerated MgO in multiple cycles. (Initial HA concen-tration = 100 mgC/L, m(MgO) = 1.0 g/L, pH = 6.0, time = 3 h). The inset pictures areHA water before and after MgO treatment.

6 J. Zhou et al. / Science of the Total Environment 711 (2020) 135120

forms bigger composite coagulum and is removed bymicrofiltration.

3.4. Recovery of MgO by annealing

As most of the organic matter can be degraded by combustionand Mg(OH)2 can transform to MgO at temperatures higher than

350 �C. We tried to regenerate MgO at 500 �C. The results indicatethat the precipitate in the MgO treatment process contains most ofhexagonal brucite Mg(OH)2 (JCPDS no. 96–100-0055) and theimpurity peaks ranging from 20 � to 35 � can be assigned to theMg-HA coagulum formed by coagulation between dissolved Mg2+

and HA (Fig. 8). The MgO raw material has a cubic periclase phase(JCPDS no. 96–100-0054). After annealing at 500 �C for 2 h, onlyMgO peaks (MgO-R1) can be observed in the X-ray diffraction pat-tern while the Mg-HA peaks disappear because of high tempera-ture 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 cir-culations (Fig. 9). And periclase MgO are successfully recycled forat least 10 times and no obvious impurity peaks can be observed(Fig. 8) in the regenerated MgO (MgO-R10). MgO is slight solublein the effluent. Thus, possible mass loss might occur. Interestingly,we didn’t observe apparent mass loss in the recycled MgO. On thecontrary, a slight mass increase (0.67%) was observed in the recy-cled MgO after 10 HA removal cycles. Possible reason might be thatsome metal ions coexisted with HA and were recycled togetherwith MgO. The color of the recycled MgO turned from white toslight brown, which indicate the existence of inorganic impurities.The good recycle property of MgO will greatly alleviate the reagentcost so that we don’t have to keep buying chemicals like traditionalcoagulation HA removal process.

4. Conclusion

HA can be efficiently removed by MgO treatment, in which MgOserves as two-in-one coagulant and adsorbent. The MgO treatmentprocess is highly pH sensitive and weak acidic condition is favoredfor high HA removal efficiency. Dissolved Mg2+ removes ca.92% ofthe HA via coagulation while Mg(OH)2 is responsible for theadsorption removal of residue 8%. MgO can be regenerated formulticycles via annealing Mg(OH)2/Mg-HA composite at 500 �C,so that we can save money from adsorbent and coagulant purchaseand avoid the generation of adsorbent waste. This MgO treatmentprocess is simple, efficient, and cost effective for water HA andNOM removal. Although MgO regeneration needs high tempera-ture and consumes thermal energy, this NOM removal processmight still work well if it is applied together with some heat recy-cle processes such as photothermal conversion of solar energy andenergy recycle from anaerobic methane production and wasteincineration. This work will be significant in promoting new tech-nologies for HA- or NOM-related drinking water purification, natu-ral water remediation, and industrial black wastewater treatment.

Declaration of Competing Interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.

Acknowledgements

This research was supported by the National Natural ScienceFoundation of China (21701052), the Science and Technology Pro-ject of Guangzhou (201804010401), the Natural Science Founda-tion of Guangdong Province of China (32217073), the SpecialFunds for Basic Scientific Research Operations of Central Universi-ties of China (11617323).

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