Colloids and Surfaces A: Physicochem. Eng. Aspects 416 (2013) 86 94

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l h omepa g e: www.elsev ier .com

Review

Recent

GuanghuState Key Labo

h i g h l

Hybrid hywater trea

High mecheasily recovered.

Outstanding adsorption capacity formetal ions and dyes especially.

The incorporation of clay can reducethe cost and improve the adsorptioncapacity.

a r t i c l

Article history:Received 16 MReceived in reAccepted 21 SAvailable onlin

Keywords:Hybrid hydrogAdsorptionWater treatmeHeavy metalsDyes

Contents

1. Introd2. Hybri3. The h4. The h5. The h6. Concl

Refer

CorresponE-mail add

0927-7757/$ http://dx.doi.o e i n f o

ay 2012vised form 7 September 2012eptember 2012e 28 September 2012

el

nt

a b s t r a c t

In recent years, hybrid hydrogels have gained great attention as effective adsorbents due to their highwater retention and low cost. This paper gives an overview of the principal results obtained duringthe treatment of water utilizing hybrid hydrogels for the removal of metal cations, radionuclides, dyes,anions and other miscellaneous pollutants from water. It is evident from the literature survey that hybridhydrogels have shown good potential applications for the removal of various aquatic pollutants.

2012 Elsevier B.V. All rights reserved.

uction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86d hydrogels for the removal of metal ions and radionuclides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87ybrid hydrogels for dyes removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89ybrid hydrogels for anions removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91ybrid hydrogels for other pollutants removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92usion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93ences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

ding author. Tel.: +86 571 8795 3200; fax: +86 571 8795 1612.ress: opl wl@dial.zju.edu.cn (L. Wang).

1. Introduction

With the rapid development of metal plating facilities, miningoperations, leather tanning, and pesticides industries, heavy met-als wastewater and organic compound wastewater are directly

see front matter 2012 Elsevier B.V. All rights reserved.rg/10.1016/j.colsurfa.2012.09.043 progress on study of hybrid hydrogels for water treatment

i Jing, Li Wang , Haojie Yu, Wael A. Amer, Lei Zhangratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, Peoples Republic of China

i g h t s

drogels as adsorbents fortment.anical strength, low cost,

g r a p h i c a l a b s t r a c t/ locate /co lsur fa

G. Jing et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 416 (2013) 86 94 87

or indirectly discharged into the environment increasingly. Mostof the pollutants in wastewater are known to be toxic and car-cinogenic and hence their presence in the environment shouldbe controlled. Many techniques have been used for the removalof toxic poion exchanocculation

Adsorptpriate methpollutants. cheap, effeadsorption Gels can bnon-uid cothroughoutapplicationbe dened porous nettheir structutreatment. kinetics proment.

Hybrid mof inorganicinterpenetrgels can bedistinct thrcomposed ferent polydisadvantagcomponenta part of effresources apolymers. Hfor water tinto hybridis the focusing of differhybrid hydrFor examplphotocatalyrespectivelymechanicalthis, is impoadvantagesfor water tr

Consequwork reporperspectivelutants e.g.

2. Hybrid hradionuclid

Heavy mliving organgenic [15]. Vthe major pdischarged ing, mineraforth.

Among tbe a humanalthough coessential tr

tosanpacin (Cts ofdsorhydeAA)

mpar The pH so

bea-GLA

As shovalo fo

to thds asorpt

stim-isopA) ha7]. Trk ofnt of sensce oing wraturntly

releratur

of acC and

of alt is aentiatreat

areustrls giwiths covy mentser mis a y, laty are benecial for the adsorption of heavy metals fromns. Novel chitosan-g-poly(acrylic acid)/APT (CS-g-PAA/APT)sites have been reported as adsorbents for the removal+ from aqueous solutions [20]. More than 90% equilibriumtion capacity of the hybrid hydrogel for Cu2+ occurred within, and the adsorption process would get the maximum equi-

level in 60 min. The introduction of APT clay into CS-g-PAAllutants from water such as chemical precipitation,ge, membrane ltration, adsorption, coagulation,, otation, and electrochemical treatment.ion process is often considered one of the most appro-ods for the removal of both inorganic and organicFurthermore, the adsorption process is convenient,ctive and possesses few troubles [1,2]. However, theefciency mainly depends on the type of adsorbents.e used as adsorbents for water treatment. Gel is alloidal network or polymer network that is expanded

its whole volume by a uid and it has a wide range ofs [38]. When the uid is water, this kind of gels canas hydrogel. Owing to the high water content and thework structure, which allow solute diffusion throughre, hydrogels have attracted special attention for water

However, the poor mechanical properties and the lowperties hinder their practical application in water treat-

aterial is the material composed of an intimate mixture and/or organic components. The components usuallyate on a scale of less than 1 m. So, hybrid hydro-

dened as water-swollen materials, which maintain aee-dimensional structure simultaneously and they areof polymeric material and inorganic material or dif-meric materials. Hybrid hydrogels can overcome thees of hydrogels by combining the advantages of thes and sometimes hybrid hydrogels can be prepared asorts to reduce the excessive consumption of petroleumnd the environmental impact resulting from industrialybrid hydrogels have several advantages as adsorbentsreatment. First, incorporation of the natural materials

hydrogels can lower the cost of water treatment, which of water treatment industry. Second, the incorporat-ent materials into hydrogels can form different kinds ofogels to satisfy various situations for water treatment.e, incorporating TiO2 and Fe3O4 into hydrogels can formzed hybrid hydrogels and magnetic hybrid hydrogels,

[912]. Third, most of hybrid hydrogels have good properties and tolerance of external, vigorous stirring,rtant for the reuse of adsorbents [13,14]. Owing to these, hybrid hydrogels can be potentially used as adsorbentseatment.ently, this review aims to serve as a summary of theted in the literatures on hybrid hydrogels from the

of their application for removal of various aquatic pol-metal ions, dyes, anions and so on.

ydrogels for the removal of metal ions andes

etals are not biodegradable and tend to accumulate inisms and many heavy-metal ions are toxic or carcino-arious toxic heavy-metal ions constituting one type ofollutants in water, such as Cu2+, Pb2+, As5+ and Cr6+, areinto the environment during process industries, min-l processing, leather tanning, wood preservation and so

hese metal pollutants, evidences show that copper may carcinogen and can cause harm to the aqueous fauna,pper is very important for the human beings as anace element. Recently, Bhatnagar et al. have reviewed

the chition cachitosacontenas an ataraldeacid) (PFor copared.lower CS-GLACS/PAAbeads.on remfacile tOwinggel beathe adcycles.

Thepoly(N(PNaAions [1netwopendethermopresenshrinktempesufcieIPN geltempemeansat 20

meansCos

be potwater surfacethe indminerapared requirefor heaadsorbpolym(APT) phologcapacisolutiocompoof Cu2

adsorp15 minlibriumFig. 1. The chemical structure of CS.

-based adsorbents, which have shown the high adsorp-ties for water treatment [2]. The chemical structure ofS) is shown in Fig. 1. Owing to its low cost and high

hydroxyl and amino functional groups, CS can be usedbent for water treatment. Dai et al. [16] have used glu-

(GLA) as cross-linker for the preparation of poly(acrylic blend CS (CS/PAA) hydrogel beads by one-step method.ison, CS-GLA beads without PAA have been also pre-GLA cross-linked CS/PAA beads had better stability inlutions and possessed higher mechanical strength thands without PAA. In addition, the adsorption capacity of

beads for copper ions was greater than that of CS-GLAown in Fig. 2, the efcient effects of CS/PAA-GLA beads

of Cu2+ resulted from the fact that carboxyl groups arerm bidentate carboxylates with metal ions reversibly.e efcient desorption of Cu2+ from the hybrid hydro-

t pH below 4.0 and the improved mechanical strength,ion capacity of regenerated beads had no loss up to six

uli-responsive interpenetration network (IPN) gel ofropylacrylamide) (PNIPAm) and poly(sodium acrylate)ve been synthesized for the removal of heavy metalhe structure of the IPN is depicted in Fig. 3. The rst

PNIPAm and the second network of PNaAA are inde-each other. So the IPN hydrogel of PNIPAm exhibited aitive volume-phase transition behavior in spite of thef PNaAA that is well known to prevent PNIPAm fromhen the temperature is above its lower critical solutione (LCST). As shown in Fig. 4, the IPN gel adsorbed Cu2+

below its LCST but not above the LCST. Furthermore, theased water without releasing heavy metal ions when thee is above LCST. The adsorbed Cu2+ can be desorbed byid treatment (0.01 N HCl) of the IPN gel in swollen state

the IPN gel-adsorbent has been easily regenerated bykali treatment in the swollen state.lways the focus of water treatment industry. Clay canlly used as an alternative of the activated carbon forment industry. Porous inorganic materials possess higha and have a mechanical strength that is appropriate toial use [18]. The negative charge on the structure of clayves clay the capability to attract metal ion [19]. Com-

clay, CS is more expensive in cost. Moreover, CS alsomplexing agents to improve its removal performanceetals. Therefore, in order to reduce the cost of hydrogel

many kinds of clay have been incorporated recently intoatrices matrixes to form hybrid hydrogels. Attapulgitemagnesium aluminum phyllosilicate. Its brous mor-rge specic surface area and moderate cation exchange

88 G. Jing et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 416 (2013) 86 94

Fig. 2. (a) The structure of CS-GLA and (b) the postulated mechanism of adsorption and desorption of Cu2+ by CS/PAA-GLA.

polymeric nmay accelerthe cost of t

Sodium number ofnolic hydrohumate(CS-various amremoval of ity of the Cwith increathe adsorptof the hybractual applibent from lThe adsorbtion as a dethat the intthe hybrid hybrid hydadsorption

A pectinporating (PAAmPSA)

drogropeels rel w

hyde2+ udsorp

depund tion.licat

has mation Cl) v3 na

suchnt an

theof chial besms

chosetwork generated a loose and porous surface, whichate the adsorption process and at the same time reducehe hybrid hydrogel.humate (SH) is black crystal, which contains a large

functional groups, such as carboxylates and phe-xyls. A series of CS-g-poly(acrylic acid)/APT/sodiumg-PAA/APT/SH) hybrid hydrogels incorporated withounts of APT have been prepared as adsorbents for thePb2+ from aqueous solution [21]. The adsorption capac-S-g-PAA/APT/SH hybrid hydrogels for Pb2+ increasedsing pH and concentration of Pb2+ solution, and alsoion capacity increased with decreasing the particles sizeid hydrogels. However, considering the conditions incation, such as energy use and the separation of adsor-iquid phase (40100) mesh was an appropriate choice.ed Pb2+ ion can be desorbed by using 0.05 M HCl solu-sorbing agent. The experiment results have indicatedroduced APT and SH were benecial for recycling ofhydrogel. Furthermore, the adsorption capacity of therogel was higher than 590 mg/g after ve consecutivedesorption processes.

based hybrid hydrogel has been prepared by incor-poly(2-acrylamido-2-methylpropanesulfonicacid)

the hytion phydroghydroghybridhigh Fhigh aand pHwas fogenera

Appmetals[12]. Ameriza(APTM-Fe2Ocationspigmeated byforms essentorganican be [22]. The incorporation of PAAmPSA improved tion was fo

Fig. 3. (a) The chemical structure of cross-linked PNIPAAm and (b) the schemaels water adsorption capability and the ion sorp-rties. The loading of Fe2+ ion on biopolymers-basedesults in efcient anion sorption. So, rstly the hybridas loaded with Fe2+ and then the dried Fe2+-loadedrogel was immersed in solution to uptake As5+. Theptake by this hydrogel results in the correspondingtion of As5+. The As5+ sorption process is exothermicendent. In addition, the As5+-loaded hybrid hydrogelto be biodegradable, so it causes no problem of waste

ion of polymeric hydrogels for the removal of heavybeen hampered by difculties in separation by ltrationgnetic cationic hydrogel was synthesized by poly-of (3-acrylamidopropyl)-trimethylammonium chlorideia radical polymerization in the presence of 10 nmnoparticles [12]. Chromium has many industrial appli-

as wood preservation, leather tanning, metal nishing,d refractory industries [23]. Therefore, the waste gener-se industries is rich in hexavalent Cr6+ or trivalent Cr3+

romium. Removal of Cr6+ from contaminated water iscause it is carcinogenic and more toxic to the livingthan Cr3+. The poly(APTMCl)/-Fe2O3 hybrid hydrogelen as an adsorbent for removal of Cr6+ and the adsorp-

und to be independent on the pH of solution. The Cr6+

tic structure of IPN of PNIPAAm and PNaAA.

G. Jing et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 416 (2013) 86 94 89

Fig. 4. The recadsorption.

Reprinted with

removal kinhydrogel ininto the polration. The as Cl, SO4hydrogel isRegeneratiowashing threcovery ra

Radioactmaterials, aas nuclear waste will time until years for soor thousanplants.

Polyelecion (UO22+)mide (AAm(AMPS) andof AAm/AMous solutiohydrogels i0.49 g/g. Thwas 25.23%and the urabonding.

Becauseseemed as

Fig. 5. The chemical structure of MB.

been used as adsorbent for the removal of cobalt and the modica- CS w

hydmmmetae fou

rem

hyb

primocess

coloes ar

probrge oant. ents

caure, al bel friethyl ber, tal stinclud tis

hybte (L

remas 8

Table 1The adsorption

S. no.

1 2 3 4 56 78 yclable application of stimuli-responsive IPN gel for heavy-metal ion

permission from Ref. [17], copyright 2003 John Wiley and Sons.

etics was improved drastically by grinding the bulkto powder form. The incorporation of magnetic particlesymer matrix provided a magnetic property for fast sepa-inhibition from common background electrolytes (such2, and PO43) for Cr6+ adsorption onto the magnetic

insignicant within the anions relevant concentration.n of the magnetic hydrogel can be easily achieved bye Cr6+-loaded hydrogel with 0.5 M NaCl solution with ate of about 90% [12].ive waste is a waste product containing radioactivend it is usually produced in a nuclear process suchssion [24]. Radioactivity diminishes over time, so theseriously inuence the environment for a period ofit no longer poses a hazard. This may mean hours tome common medical or industrial radioactive wastesds of years for high-level wastes from nuclear power

trolyte hybrid hydrogels used for the sorption of uranyl have been prepared by the polymerization of acryla-) with 2-acrylamido-2-methyl-1-propanesulfonic acid

clay such as bentonite (Bent) [24]. The swelling ratioPS/Bent hybrid hydrogels can reach 15,400% in aque-

tion ofhybrid

A suferent it can bfor the

3. The

Theing prhighlyvarietihealthdischaimportadsorbimpactthermoremovmenta

Meing papchemicof MB dice, antype oflaponibent toof MB wn. The adsorption capacity for UO22+ on the hybridncreased with the AMPS content increase and reachede maximum removal efciency of the hybrid hydrogel. The main interactions between the hydrogel systemsnyl ions were thought to be hydrophobic and hydrogen

of the relatively long half-life time of cobalt, it can bea radioactive waste. Magnetic CS hybrid hydrogel has

the hybrid A chitosanhydrogel wsurface metion paramebeen achiev

Montmohighly exch

capacities of hybrid hydrogels for various metal ions removal from water.

Adsorbent Adsorbate Adsorption capacity

CS/PAA-GLA Cu2+ 121.55 mg/g IPN gel of PNIPAm with PNaAA Cu2+ 0.25 mmol/g CS-g-PAA/APT Cu2+ 303.03 mg/g CS-g-PAA/SH Pb2+ 845 mg/g CS-g-PAA/APT/SH Pb2+ 810 mg/g Pectin/poly(AAmPSA) As2+ 126 mg/g Poly(APTMCl)/-Fe2O3 Cr2+ 200 mg/g Carboxymethylated pine needles Fe2+ith xanthate improved the adsorption capacity of thisrogel [25].ary of adsorption capacities of hybrid hydrogels for dif-l ions has been presented in Table 1. From the summary,nd that few smart hybrid hydrogels have been studiedoval of heavy metals.

rid hydrogels for dyes removal

ary pollution of textile efuent emerges from the dye-. Dyeing and nishing wastes in textile industry arered and have high organic content [27]. Almost all dyee toxic, carcinogenic, and mutagenic so they can causelems to human beings. Removal of dyes before thef wastewater into natural water bodies is extremelyThe clean methods with low price and biodegradable

could be good tools to minimize the environmentsed by the manufacturing and by textile efuents. Fur-the adsorption process is also widely used for colorcause of its low cost, effectiveness, simplicity, environ-ndliness, and trouble fewer [2831].lue (MB) has a wide range of applications, such as color-emporary hair colorant, dyeing cottons and wools. Theructure of MB is shown in Fig. 5. The harmful effectsde heart beat increase, vomiting, shock, cyanosis, jaun-sue necrosis in humans. Yi et al. [32] have synthesized arid hydrogels from SH, polyacrylamide, and hydrophilicP) clay. The hybrid hydrogel was used as an adsor-ove MB and the maximum adsorption concentration00 mg/L/g of hydrogel. The adsorption concentration ofhydrogel increased with increasing SH or clay content.-g-poly(acrylic acid)/halloysite (CS-g-PAA/HT) hybridith 50% HT content has been prepared and the responsethodology has been employed to optimize the adsorp-ters and a high adsorption capacity of 1336.05 mg/g hased [33].rillonite (MMT), a layered aluminum silicate withangeable cations and reactive groups on its surface,Percent uptake Sensitivity Reference

[16]Thermal sensitive [17]

[20][21][21][22][12]

99.48 [26]

90 G. Jing et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 416 (2013) 86 94

has also been widely used to improve water absorption ratio ofhydrogels. Starch-graft copolymers have great demand in industrydue to their low-cost, biodegradable properties and environmentalfriendliness. Starch-g-acrylic acid/Na-MMT (S-g-AA/MMT) hybridhydrogels have been synthesized by graft copolymerization reac-tion of starch and acrylic acid using N,N-methylenebisacrylamide(NMBA) as a cross-linker and cerium ammonium nitrate as aninitiator in the presence of Na-MMT micropowder [27]. Thishybrid hydrogel has been used to remove safranine T fromaqueous solutions. Increasing the ratio of Na-MMT/monomersup to 1% has improved the ability of water absorbency,but further increase of Na-MMT caused a decrease in waterabsorbency.

IPN hybrid hydrogel can also be used in the application of dyeremoval from water [34,35]. Anirudhan et al. have synthesizedIPN hybrid hydrogel and poly(methacrylic acid) grafted cellu-lose/bentonite (PMAA-g-Cell/Bent) by the graft copolymerizationreaction [34]. Owing to the signicant mechanical properties of cel-lulose, this hybrid hydrogel has sustainable stability over repeatedMB loading and stripping.

Among the photocatalysts, TiO2 is believed to be the mostpromising photocatalyst due to its superior photocatalytic activity,nontoxicity, long-term stability, robustness against photocorrosionand low price [36]. Incorporating TiO2 nanoparticles into hydrogelsmatrices can form hybrid hydrogels for photocatalytic removal ofdyes [9,10]. The strong binding of dyes to TiO2 surfaces is an impor-tant requirement for high efciency in solar energy conversionsystems [37

The treait can corrotion. Yun efor the fabhybrid hydr(PDMAPAAacidic wasteswelling kintion in the nof TiO2 in the hydrogeThe mechancan be absohydrogel u

Fig. 6. The PVA/PDMAPAA

Reprinted with

decomposed effectively by the photocatalytic activity of TiO2 withthe UV light.

Generally, for improving the mechanical properties andexhibiting the pH-sensitive swelling behavior of hydrogel,PVA and PPhotocatalymedium anfor TiO2 bypoly(N-isopvia seeded(NIPAAm) (MBA) whience of TiOexcellent thpotential ap

A phot(acrylic acreported fodispersed tAA]). Withoto 85% of thbining the ethe hybrid total MB incan be phcause the shybrid hyd

red el hmay

hydrbasehosenical

to teredchanctivetrix els

protion sizedr dis

(SDSniumTAB-ter suxhib

red. p to 0rthersorbeed taddit

use hydcid s

supor threpaaphtel [4c-3-le mhow].tment of acidic waste water is usually difcult becausede the purication facilities resulting in their malfunc-t al. [10] have employed the electrospining methodrication of photocatalytic nanobrous pH-sensitiveogel from poly(N,N-dimethylaminopropyl acrylamide)m), poly(vinyl alcohol) (PVA) and TiO2 for treatment of

water. TiO2 did not inuence the swelling rate and theetics of the hybrid hydrogel signicantly. The sonica-anobers preparation improved the dispersion degreethe hydrogel. The good dispersion degree of TiO2 inl matrix lead to high efciency of dye decompositionism of dyes removal was suggested in Fig. 6, the dyesrbed into the highly swollen PVA/PDMAPAAm hybridnder the acidic conditions and then the dyes can be

suggested mechanism of organic compound degradation bym hybrid hydrogel.

permission from Ref. [9], copyright 2010 Elesvier.

Compahydrogwhich hybrid

CS-than tmechaimentsconsidthe mean effegel mahydrogfor theadsorpsynthegies fosulfatelammowith Cthe outhey eCongoratio ubut fured abimprov

In can behybridand aHybridbents fhave p3-(2-nhydrogof fmoavailabgel is sAA were selected as the components, respectively.tic activity was heightened greatly in the basicd a large surface area of hydrogel was available

using nanober supports [38]. The hybrid hydrogelropylacrylamide)/TiO2(PNIPAAm/TiO2) was prepared

emulsion polymerization of N-isopropylacrylamidein the presence of N,N-methylene-bis(acrylamide)ch acts as the cross-linking monomer [39]. The pres-2 nanoparticles in the composite did not inuence theermosensitivity of PNIPAAm. The hybrid hydrogel hasplication in the remediation of wastewater streams.ocatalytically degradable TiO2/poly[acrylamide-co-id)] (TiO2/poly[AAm-co-AA]) hybrid hydrogel wasr textile dye degradation [40]. The TiO2 particles werehroughout the hybrid hydrogel (TiO2/poly[AAm-co-ut photodegradation, the hybrid hydrogel adsorbed upe MB from a 5 mg L1 MB solution within 15 min. Com-ffect of photodegradation and the effect of adsorption,hydrogel can photocatalytically degrade 91% of the

aqueous solution. In addition, the hydrogel compositeotodegradable under UV irradiation, so it does notecond pollution into the water. TiO2 (P25)-graphenerogel can be also used as adsorbent to remove MB [41].to the bare P25 hydrogel, TiO2 (P25)-graphene hybridas a higher reaction rate in the decomposition of MB,

be attributed to the three-dimensional structure of theogel that improves the attachment of MB to TiO2.d hydrogels have higher adsorption for various dyes

of commercial activated carbon. However, the poor properties of CS-based hydrogels are serious imped-heir practical application. Carbon nanotubes (CNT) is

one of the most promising nano-llers used to enhanceical properties of polymer matrices. Moreover, CNT is

adsorbent. Interpenetrating CNT into the CS hydro-can improve the mechanical properties as well as theadsorption capacity [13,42]. The addition mode of CNTduction of CS/CNT hybrid hydrogel beads affects theirfor anionic dyes. For example, Chatterjee et al. [43] have

CNT-impregnated CS beads by four different strate-persing CNT: (a) in CS solution, (b) in sodium dodecyl) solution, (c) in CS solution containing cetyltrimethy-

bromide (CTAB), and (d) in SDS solution for gelationcontaining CS solution. Due to the presence of CNT onrface of the CS/CNT beads synthesized by strategy (d),

ited maximum adsorption capacity (375.94 mg/g) forIncreasing the CNT content in CS/CNT hybrid hydrogel.01 wt% causes an increment in Congo red absorbency,

increase of CNT content caused a decrease in Congoncy. Moreover, the sulfur (%) in the CS/CNT beads hashe adsorption property for Congo red [13].ion, coreshell membrane structural CS/SDS beadsd to remove Congo red from wastewater [44]. Therogel CS/SDS beads had higher mechanical strengthtability than CS beads formed by alkali gelation.ramolecular hydrogels can also be chosen as adsor-e removal of dyes form aqueous solutions. Wang et al.red a hybrid hydrogel through incorporating fmoc-hyl)-d-alanine supramolecular hydrogel into agarose5]. Fig. 7(a) and (b) shows the chemical structures(2-naphthyl)-d-alanine and agarose respectively. Theechanism for the removal of MB by the hybrid hydro-n in Fig. 7(c). The fracture stress of the hybrid hydrogel

G. Jing et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 416 (2013) 86 94 91

Fig. 7. T c) the

was found of any singlgel. The higfrom pollutthe interactthat MB haalanine.

A summdifferent va

4. The hyb

Inorganilutants anddrinking waing water shybrid hydremoval fro

Nitrogennisms. Nevlead to accoxygen depnumber of hand animaltion and is adsorption ride)/polya

ed bhe initial tion sorpttil 5lculan streegatontendsor

achlutioegenorbeial ae of ion p

waswo ms.uhanlluloby nlene

Table 2The adsorption

S. no.

1 2 3 4 5 6 7 8 9

10 1112 1314 he chemical structures of (a) fmoc-3-(-2-naphthyl)-d-alanine and (b) agarose and (

to be 12.9 kPa which is higher than the fracture stresse hydrogel, supramolecular hydrogel or agarose hydro-h efciency of the hybrid hydrogel for removing MBed water relies on its hydrophobic part. The analysis ofions between MB with the hybrid hydrogel indicatedd a strong interaction with fmoc-3-(2-naphthyl)-d-

ary of the adsorption capacities of hybrid hydrogels forrious dyes is presented in Table 2.

rid hydrogels for anions removal

c anions are one of the element classes of aquatic pol- many inorganic anions possess harmful effects onter sources. The removal of these pollutants from drink-upplies is an emerging issue. In recent years, manyrogels have been successfully utilized for some anionsm water [4648].

compounds are necessary elements for living orga-ertheless, when they are more than needed, they canelerated eutrophication of lakes and rivers, dissolvedletion and sh toxicity in receiving water, leading aealth problems involving living species such as humanss [49]. Nitrate is an inorganic form of nitrogen pollu-one of the limiting factors causing eutrophication. The

examinwith tthe inadsorpthe adtent unThe caThe iohave nhigh cgood acan beion sogood rbe despotentincreasattractcess. Itwere tproces

Chalignoceforms methyof nitrate by poly(dimethyl diallyl ammonium chlo-crylamide (PDMAAC/PAAm) hybrid hydrogel has been

functionalizhybrid hyd

capacities of hybrid hydrogels for dyes removal from water.

Adsorbent Adsorbate Ad

CS-g-PAA/HT MB 13S-g-AA/MMT MB 22TiO2/PVA/PDMAPAAm MB TiO2/poly[AAm-co-AA] MB Fmoc-3-(2-naphthyl)-l-alanine/agarose MB PVA/TiO2 Procion blue PVA/TiO2 Acridine orange TiO2/PVA/PAA Coomassie brilliant blue TiO2 (P25)-graphene MB 87CS/CNT beads Congo red 42magnetic CS/PVA hybrid hydrogel Congo red 47CS/CNT beads Congo red 45CNT-impregnated CS beads Congo red 37CS/SDS beads Congo red 20 available mechanism of removal of MB by the hybrid hydrogel.

y Zheng et al. [48]. The adsorption capacity increasedcrease of contact time up to several minutes. Afterincrease, the adsorption capacity decreased and theprocess reached its equilibrium at 60 min. Additionally,ion capacity increased with increasing PDMDAAC con-0 wt% beyond which a sudden decrease was observed.ted maximum adsorption capacity was 19.6 mg N/g.ngth and the presence of competitive phosphate ionsive effects on the adsorption of hybrid hydrogel witht of PDMDAAC. However, this cationic hydrogel hadption capacities of 5.46 mg N/g and 4.9 mg N/g thatieved at high saline concentration and competitivens (10 mmol/L). This hybrid hydrogel also exhibitederation ability because the adsorbed nitrate ions cand efciently in strong acidic or basic media. The zetand conductivity of the hydrogels increased with thePDMDAAC content which testied that the electrostaticlays an important role during the whole adsorption pro-

considered that swelling and electrostatic attractionain adsorption mechanisms for the whole adsorption

et al. prepared cation and anion adsorbents fromsics using pine needles and their carboxymethylatedetwork/hydrogel formation with acrylamide and N,N-bisacrylamide. The hybrid hydrogels were further

ed by partial alkaline hydrolysis with 0.5 M NaOH. Thisrogels had the maximum adsorption capacity when it

sorption capacity Removal efciency Reference

36.5 mg/g [33]37 mg/g [27]

>90% [28]90% [31]97.7% [35]92% [27]92% [27]

>95% [38].63 mg/g [41]3.34 mg/g [42]0.1 mg/g [31]0.4 mg/g [13]5.94 mg/g [43]8.3 mg/g [44]

92 G. Jing et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 416 (2013) 86 94

Table 3The adsorption capacities of hybrid hydrogels for anions removal from water.

S. no. Adsorbent Adsorbate Adsorption capacity Removal efciency Reference

1 Cu2+-loaded CS hydrogel beads Phosphate 28.86 mg/g [47]2 PDMAAC/PAAm Nitrate 19.6 mg N/g [48]3 Carboxymethylated pine needles Nitrate 78.9% [26]

was loaded with Fe2+ and could be used as novel anionic adsorbentfor nitrate [26].

Over the years, the presence of excess amounts of phosphateanions in water has drawn substantial research attention becauseit is believed to be one of the main reasons for eutrophicationand water quality problems. The presence of trace amounts ofphosphate anions, in the treated wastewater from municipalitiesand industries, often leads to eutrophication (excess nutrients andassociated hyacinth-likenvironmenwater bodie

Traditionrecovery fohydrogel beof many heafter the rephate fromhad a greatthe results i28.86 mg/g)

A summis presented

5. The hyb

Much ofwater comefood-procesable. Howeexhaust theanimals in tion is seve(MCH) by methylamothe removaof this remoa dosage ofa 100% remcomparableural organicbe regenera

The hybrused to remforms a onbonding [51

ppear

d wit

atingt calhybrium the d

alg hybnt drid htion.mon

At thicaten used for the removal of ammonium from aqueous solu-52]. Although the adsorption capacity decreased with thee of content of rectorite in the hydrogel composite, theoration of rectorite into the polymer matrix can reduce thetion cost.rporating HT particles into CS-g-PAA can enhance theels adsorption capacity for NH4+ because the introduc-

the HT particles improved the hydrogels porosity, whichhance the contact of the hydrogel with NH4+ [53]. The

hydrogel CS-g-PAA/HT had a particle size of 4080 mesh.ell-dened three-dimensional structure and the presence

hydrophilic anionic groups on the chains were consid-e governed mechanism for the higher adsorption capacity

Table 4The adsorption

S. no. Adsorption capacity Removal efciency Reference

1 About 400 mg/g About 100% [11]2 About 300 mg/g [11]3 About 450 mg/g [11]4 >80% [51]56 78 effects such as rapid growth of blue green algae ande plants), which results in short-term and long-termtal problems in lakes, coastal areas, and other conneds [50].ally, the treatment for disused adsorbents is usually

r recycling or direct discarding. As discussed before, CSads (CS) are effective bio-adsorbents for the removalavy metal ions from aqueous solutions. The disused CSmoval of Cu2+ can be directly applied to adsorb phos-

the aqueous solutions [47]. The pH value of solution impact on the adsorption capacity of phosphate andndicated that the maximum adsorption capacity (about

was achieved at around pH 5.0.ary of adsorption capacities of hybrid hydrogels anions

in Table 3.

rid hydrogels for other pollutants removal

the natural organic materials found as pollutants in from domestic sewage and the efuents of farms andsing industries. These organic materials are biodegrad-ver, the decomposing process of organisms would

oxygen in water. This brings about the death of manythe water, and few plants thrive when organic pollu-re. Rao et al. synthesized magnetic cationic hydrogelradical polymerization of (3-Acrylamidopropyl)tri-nium chloride (APTMACl) [11]. This MCH can be used forl of natural organic materials and the main mechanismval was considered to be electrostatic adsorption. With

37.5 mg L1, the hybrid hydrogel could almost achieveoval of Aldrich humic acid. The MCH dosage applied is

with the dosage of coagulants used to remove the nat- materials from drinking water. However, the MCH canted by immersing in NaCl solution.id hydrogel of PNIPAm/TBP (tributylphosphate) can beove the phenol from aqueous solutions because TBP

e-to-one complex with phenol molecule by hydrogen]. Pan et al. [14] reinforced the hybrid hydrogel with a

Fig. 8. A

Reprinte

thin cobivalenof the of calcdered calciumforcedsufciethe hybdesorp

Amlution.eutrophas betions [increasincorpproduc

Incohydrogtion ofcan enhybridThe wof theered th

capacities of hybrid hydrogels for other pollutants removals from water.

Adsorbent Adsorbate

Poly(APTMACl)/-Fe2O3 Aldrich humic acid Poly(APTMACl)/-Fe2O3 Suwannee River humic acid Poly(APTMACl)/-Fe2O3 Suwannee River fulvic acid PNIPAm/TBP/PVA Phenol

PNIPAm/TBP/calcium alginate Phenol PAA/rectorite Ammonium 401CS-g-PAA/HT Ammonium 40.9 mPAA/BT Ammonium 32.87ance of NIPAM/TBP hybrid gel, composite gel and used composite gel.

h permission from Ref. [14], copyright 2008 Elsevier.

layer of calcium alginate gel, which was cross-linked bycium ions. As shown in Fig. 8, the mechanical strengthid hydrogel was apparently improved by the coatingalginate gel. Meanwhile, the thin coating hardly hin-iffusion of phenol due to the superior hydrophilicity ofinate gel. The phenol can be desorbed from the rein-rid hydrogel in basic aqueous solution. Owing to theurability provided by the coating of calcium alginate,ydrogel can carry out repeated cycles of adsorption and

ium (NH4+) also is an inorganic ion form of nitrogen pol-he same time, it is one of the limiting factors causingion. A hybrid hydrogel composite CS-g-PAA/rectorite>80% [14]10 mg N/g [52]g N/g [53]

344.47 mg N/g [54]

G. Jing et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 416 (2013) 86 94 93

for NH4+ removal. Meanwhile, the hybrid hydrogel showedpH-independence within the range of 4.07.0. The adsorptionequilibrium can be achieved within 5 min with the equilibriumadsorption capacity of 27.7 mg N/g.

As a goohigh adsorpity for multbiotite (BT)perature vipersulfate aNaOH, the hremoval of gel composmaximum aadsorptionin the adsor

A summother pollu

6. Conclus

This revhybrid hydreported thhigh adsorpadsorption would provtype of polvarious polattractive fespecially frials are lohybrid hydrbe regenerathe water trjust been telutants fromLast but notit is easy todemands.

There artreatment: materials; hydrogels; them envirooffer a lot future.

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Recent progress on study of hybrid hydrogels for water treatment1 Introduction2 Hybrid hydrogels for the removal of metal ions and radionuclides3 The hybrid hydrogels for dyes removal4 The hybrid hydrogels for anions removal5 The hybrid hydrogels for other pollutants removal6 ConclusionReferences