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w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 2 2 5 e2 3 3
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journal homepage: www.elsevier.com/locate/watres
Star-shaped polymers of bio-inspired algae core
and poly(acrylamide) and poly(acrylic acid) as arms
in dissolution of silica/silicate
Kalpana Chauhan a,*, Priyanka Patiyal a, Ghanshyam S. Chauhan b,
Praveen Sharma c
a
Department of Chemistry, Shoolini University, Solan 173229, India
Department of Chemistry, Himachal Pradesh University, Shimla 171005, India
c
Himachal Pradesh State Pollution Control Board, Shimla 171009, India
b
article info
abstract
Article history:
Silica, in natural waters (due to weathering of rocks) decreases system performance in
Received 3 June 2013
water processing industry due to scaling. In view of that, the present work involves the
Received in revised form
synthesis of novel green star shaped additives of algae core (a bio-inspired material as
28 December 2013
diatom maintains silicic acid equilibrium in sea water) as silica polymerization inhibitors.
Accepted 5 March 2014
Star shaped materials with bio-inspired core and poly(acrylamide) [poly(AAm)] and poly(-
Available online 16 March 2014
acrylic acid) [poly(AAc)] arms were synthesized by economical green approach. The pro-
Keywords:
and APAAc (Algae-g-poly(AAc)) dendrimers (star shaped) in colloidal silica mitigation/in-
Silica inhibition
hibition at 35 C and 55 C. Synthesized dendrimers were equally proficient in silica inhi-
ficiency was evaluated in ‘mini lab’ scale for the synthesized APAAm (Algae-g-poly(AAm))
Green dendrimers
bition at 12 h and maintains 450 ppm soluble silica. However, APAAm dendrimers of
Algae
generation 0 confirmed better results (z300 ppm) in contrast to APAAc dendrimers in silica
Heterogeneous
inhibition at 55 C. Additionally, dendrimers also worked as a nucleator for heterogeneous
polymerization to inhibit silica homo-polymerization. APAAm dendrimer test set showed
no silica deposit for more than 10 days of inhibition. EDX characterization results support
nucleator mechanism with Si content of 6.97%e10.98% by weight in silica deposits
(SiO2-APAAm dendrimer composites).
ª 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
Silicon is the second most abundant element (as silicate
minerals) in the earth’s crust. Natural weathering process of
silicate minerals increases silica/silicates concentration in
natural water. Moreover, water resource conservation and
environmental concerns have forced water processing industry to reuse “spent” water. But, efficient reuse of water
in water processing industry is limited due to the low solubility of silica (30e120 mg/L) (Sheikholeslami and Bright,
2002). Silica in super-saturated concentration (120 mg/L)
results hard and persistent deposits, which is a major
operational obstacle for industrial processes (Neofotistou
* Corresponding author. Tel.: þ91 9459368088; fax: þ91 01792226364/308000.
E-mail address: kalpana13chauhan@gmail.com (K. Chauhan).
http://dx.doi.org/10.1016/j.watres.2014.03.009
0043-1354/ª 2014 Elsevier Ltd. All rights reserved.
226
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 2 2 5 e2 3 3
Fig. 1 e Proposed structure for APAAm dendrimer.
and Demadis, 2004; Javaherdashti, 2000). Silica polymer, an
amorphous compound once formed is very difficult to
remove because conventional anionic inhibitors are not
sufficiently proficient to prevent silica polymerization.
Conventional technologies (filtration and chemical precipitation) have been reported moderately successful in silica mitigation (Sheikholeslami and Bright, 2002; Mavredaki et al., 2007;
Ueda et al., 2003; Greenlee et al., 2010). In therapeutic
approach, chemical cleaning for silica deposits is a corrosive
technique which can pose health hazards and environmental
concerns (Demadis et al., 2012). However, in protective measures
the additives (anionic, non-ionic and cationic) have been reported of potential in silica polymerization inhibition (Demadis
et al., 2011; Roussy et al., 2005). The potential commercial silica
inhibitors have primary and secondary amine as a structural
unit (Demadis and Stathoulopoulou, 2006). Polyaminoamido
STARBURST dendrimers (PAMAM) have also been reported
proficient for silica inhibition, but the efficiency is in synergism
with anionic polyelectrolytes (Mavredaki et al., 2005; Zhang et al.,
2012). Moreover, commercial additives in silica mitigation, costs
high and have an environmental concern (Demadis et al., 2005;
Stathoulopoulou and Demadis, 2008). In view of that, there is
an utmost need for an efficient, green and cost effective alternative in silica mitigation. It is evident from the literature that
dendrimers with amine and amide functional groups are useful
additives for silica polymerization mitigation. Knowledge of
proficient commercial additive structures can be used as a guide
for the synthesis of novel environmental friendly alternatives in
silica inhibition (Lowenstam and Weiner, 1989).
Algae, rich in amino acids can be exploited as a core for green
bio-inspired material to mitigate silica polymerization (Brown
et al., 1997; Falkowski and Raven, 1997; Wang and Brown,
2013). Moreover, algae (Diatom) have polyamine chains and
phosphate groups, which maintains naturally silicic acid equilibrium (bio-mineralization) in the sea (Kroger et al., 2000). Algae
biomass has already been extensively exploited as a commercial source for biofuels and polyphosphate accumulation
(Aresta et al., 2005; Powell et al., 2009). But, the algae remnant
(rich in protein) of the biofuels industry is a big challenge for
disposal and utilization. Algae are generally microscopic organisms, which can act as an economical core for competent
dendrites (Bhatanagar et al., 2002; Sigee, 2005). Demadis et al.
(2009) have reported bioinspired control of colloidal silica, but
in the presence of synthetic polymers. Furthermore, no previous works are reported in literature where waste protein rich
biomass of algae was used as a core for dendrimers synthesis. In
view of that, the aim of the present work is to design algae based
promising new green dendrimers that are more efficient and
cost-effective for homogeneous silicification inhibition. A suitable modification can enhance the desirable technological
properties of the synthesized materials. In the present article,
the algae based additives were synthesized via green protocol
and evaluated as potential inhibitors for silica and silicates in
simulated water solutions.
2.
Experimental
2.1.
Materials
Sodium silicate (CDH, India), acrylic acid (AAc, CDH, India),
acrylamide (AAm, CDH, India), ammonium molybdate (CDH,
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 2 2 5 e2 3 3
227
Fig. 2 e a(i). Microscopic photo image for green algae, SEM result for a(ii) microalgae showing granular structure, a(iii)
microalgae with fibril structure in insat (b) APAAm at (i) 1000 magnifications, (ii) 2000 magnification (c) APAAc at 2000
magnification. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article.)
India), ammonium persulfate (APS, CDH, India), oxalic acid
(CDH, India) and all other reagents were of analytical grade.
The weights were taken on Denver Balance having minimum
readability of 0.01 mg.
2.2.
Microalgae modification
Green microalgae were collected from nearby fresh water
resource (Shown in Fig. 2a(i)). Micro-algae work as a core in the
dendrimers synthesis because the cell wall of the same is
difficult to disintegrate (Chen and Oswald, 1998). Microalgae
were copolymerized with AAm and AAc in the molar ratio of
1:2 using APS as initiator (0.5% w/v), respectively. The resulting reaction mixture was stirred well to make it uniform and
then kept undisturbed for 3 h at 65 C. Finally, the product was
collected, washed with water and acetone for 30 min, separately and then dried at 50 C. The efficiency of the protocol
was calculated as follows:
%efficiency ¼ WS =WR 100
(1)
228
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 2 2 5 e2 3 3
Table 1 e EDX Results for APAAm and APAAc
dendrimers.
Algae
Algae-g-poly
(AAm)
Algae-g-poly
(AAc)
Element Wt% N:C ratio Wt% N:C ratio Wt% N:C ratio
CK
NK
OK
38.26
04.46
36.88
1:8.6
28.16
13.83
54.95
1:2.04
51.94
06.82
33.40
1:7.62
where WS is the weight of the synthesized product and WR is
the weight of all reacting species including algae, AAm, APS
and AAc, respectively.
2.3.
Silicomolybdate spectrophotometric determination
SiO2 solution (500 ppm) was prepared from Na2SiO3$5H2O in
polyethylene containers to avoid silica leach out from borosilicate beakers. In control set, 200 ml of the SiO2 stock solution was placed in a plastic beaker. The pH of this solution was
adjusted to 7.00 with HCl and NaOH. The beaker was covered
with plastic membrane and set aside without stirring. Silica
concentrations were checked at 12, 24, 48 and 72 h in the inhibition test sets. To the test solutions, the synthesized green
inhibitors were also added to achieve desirable inhibitor
concentration i.e., 20 ppm, 60 ppm and 80 ppm. Soluble silica
measured by silicomolybdate method (Coradin et al., 2004). To
the 20 ml of the sample solution, 2.0 ml ammonium molybdate (10% w/v, pH 7.0), 1.0 ml oxalic acid (7.0% w/v) and 1.0 ml
HCl (6 M) were added in the sample tube. Reactive silica (soluble silica) reacts with ammonium molybdate to form silicomolybdate (a yellow colored compound). Oxalic acid is added
to destroy any color interference from phosphate as molybdophosphoric acid. The solution was mixed well after addition
of 2.0 ml oxalic acid solution. The solution was kept as such
for 2 min. The spectrophotometer (Systronics 117 UVevis
spectrophotometer) was set at a zero absorbance with water
as the blank. Finally, the sample absorbance was measured at
452 nm. The standard curve was generated to calculate the
unknown concentration of SiO2.
2.4.
Characterization
The synthesized green inhibitors were characterized for
dendrimer structure by FTIR and SEM-EDX studies. FTIR was
taken on a Perkin Elmer using KBr pellets. The morphology of
the synthesized dendrimer and silica deposits was examined
through SEM-EDX studies using Joel JSM 6100.
3.
Results and discussion
3.1.
Microalga dendrimers
APS, a free radical initiator generates reactive sites on the
surface of the microalgae core, which has carbohydrate as
main constituents (Cronshaw and Preston, 1958). The reactive
sites on microalgae cell wall initiate radical sites on AAm or
AAc (reactive functionality) and thus lead to the multivalent
functionalized products, respectively (Chauhan et al., 2012).
Poly(AAm) or poly(AAc) chains protrude from algae cell wall as
arms and provides the star like structure to the synthesized
APAAm-0G (Algae-g-poly(AAm)-0 generation) and APAAc-0G
(Algae-g-poly(AAc)-0 generation) dendrimers (Zhang et al.,
2012). The resultant star shaped graft copolymer can be presented as depicted in Fig. 1.
3.2.
SEM-EDX characterization
Table 1 shows results for the elemental analysis. The weight%
ratio of nitrogen (N) to carbon (C) in the fresh water green
microalgae is 1:8.6. Elser et al. (2000) have reported almost
same results elsewhere. Furthermore, algae grafting reactions
to APAAm dendrimer structure results increase in the nitrogen to carbon wt% ratio i.e., 1:2.04, which is closer to AAm
monomer unit (1:3). The results can be accounted for more
poly(AAm) modification of algae surface, which results star
like structure or dendrimer of generation 0 for APAAm. The
synthesized APAAc dendrimer (shorter chain length) also
shows a smaller change in nitrogen to carbon ratio (1:7.62),
which is due to the fast chain transfer in case of AAc acid. The
results can be explained on the basis of reactivity ratio of the
two monomer units, which causes the longer homopolymeric
chains in case of AAm in contrast to AAc. Seymour and
Carraher (1988) have reported 1.38 and 0.36 reactivity ratios
for AAm and AAc, at 60 C, respectively.
SEM images results of microalgae show an almost granular
structure and layers of parallel microfibrils (Fig. 2a(ii and iii)).
SEM results for APAAc and APAAm support the modification
as the surface appears rough after grafting reaction (Fig. 2b
and c). SEM results also support clearly the star like structure
for APAAm at different magnifications (Fig. 2b(i) and (ii). Images show protruded chains or structure from the surface of
microalgae. But, SEM results for APAAc show no such clear
characteristic for dendrimers structure (Fig. 2c). In conclusion,
the distinction in SEM results can be accounted for the difference in reactivity ratios and chain transfer reactions for
AAc and AAm. Additionally, APAAc retains almost structure
of algae, which is also supported by EDX results (Table 1). SEM
results characterize the size for synthesized dendrimers,
which ranges from 10 to 50 mm for APAAm in contrast to
1e20 mm for APAAc.
3.3.
Silica inhibition
The evaluation of synthesized dendrimers efficiency as a silica
polymerization inhibitor was carried out at 500 ppm SiO2 and
pH 7.0 (Sjoberg, 1997). Solution is supersaturated and has
value greater than 2.0 (Supersaturated ¼ c/c*, where, c is the
solution concentration and c* is the equilibrium saturation at
room temperature). The silica inhibition results show the
decreases in soluble silica concentration with time, even if
synthesized dendrimers are present. Therefore, green additives can only retard or extend silica polymerization. Control
set shows maximum decrease (202 ppm) in soluble silica
concentration at 72 h and 35 C (Fig. 3). Test sets also confirm
decrease in soluble SiO2 concentration with time. However,
decrease is very small in 24 h of polymerization inhibition. In
dosage dependent inhibitory activity, the three polymeric
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 2 2 5 e2 3 3
229
Fig. 3 e a. Effect of structure on silica solubility at 35 C and 20 ppm. b. Effect of structure on silica solubility at 35 C and
60 ppm. c. Effect of structure on silica solubility at 35 C and 80 ppm. d. Effect of temperature on the solubility of silica with
APAAm dendrimers at different concentration.
inhibitors (algae, APAAm dendrimers and APAAc dendrimer)
show better silica inhibition efficacy at higher concentrations
(60 ppm and 80 ppm) (Fig. 3). APAAm dendrimer at 60 ppm
dosage shows z71% inhibition in 72 h and retains 355 ppm
soluble silica (Fig. 3c). APAAc shows highest inhibitory activity
at 80 ppm and 12 h with 474 ppm reactive silica (95%) in
contrast to 442.7 ppm (z89%) in APAAm (Fig. 3b). APAAm
dendrimers of generation 0 shows preferential results in
comparison to earlier cite of most efficient PAMAM-0G inhibitor (Zhang et al., 2012). Furthermore, silica inhibition efficiency (by dissolution) of APAAm (320 ppm) and APAAc
(263.3 ppm) appears lower at 72 h, in contrast to microalgae
(370 ppm) at 80 ppm dosage (Fig. 3a).
The effect of temperature shows increase in silica polymerization tendency with temperature as concentration for
soluble silica decreases with temperature (Fig. 3d). Same results for temperature effect are reported elsewhere by Bradley
(1993). Control set shows more decrease in soluble silica
(212 ppm) in 12 h and 55 C in contrast to 35 C (347.3 ppm),
while the further decrease is slight to 166.6 ppm at 55 C in 72 h.
APAAm, 20 ppm dosage retains 290 ppm (z60%) and 442.7 ppm
soluble silica in 12 h at 55 C and 35 C, respectively. After 72 h,
soluble silica level drops further and is almost identical to
control set (187 ppm) at 55 C. Lower temperature retains
higher soluble silica (290 ppm) till 72 h. Higher dosages of
60 ppm and 80 ppm, are more efficient in silica polymerization
230
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 2 2 5 e2 3 3
1981; Iler, 1979). Firstly, heterogeneous nucleation mechanism allows solubilisation of silica, which may not be accessible for detection by silicomolybdate test. Actually,
silicomolybdic acid is a cage-like structure and can yield a
yellow compound for characterization only with monomeric
silicic acid (Coradin et al., 2004). The inhibitory interaction of
dendrimers and silica can prevent the yellow complex formation for concentration detection. The results can be supported
in the absence of silica fluffy deposits till 10 days of inhibition
test. Inhibitors can disrupt silicate polymerization by distorting
nucleophilic attack of silicate ions among themselves. Dendrimers can also react with silicic acid by nucleophilic substitution or condensation reaction with active hydrogen of
dendrimers to inhibit silica polymerization (Ning, 2002). The
highest potential of APAAm polymers in silica inhibition can be
credited to the hydrogen bond association of eNHCOe group of
polymer with silanol groups (Sengupta et al., 2005, 2006).
3.4.
Fig. 4 e FTIR spectra of (a) algae (b) APAAm (c) APAAmsilica deposits.
inhibition to retain 252 ppm and 300.4 ppm (z60%) of soluble
silica at 55 C and 72 h, respectively. APAAm shows almost
comparable results for soluble silica (320.4) at 35 C and 80 ppm.
The commercially available dendrimers has also been reported
to maintain 300 ppm soluble silica, but the results are in synergism with polyelectrolytes (Mavredaki et al., 2005).
Polymerization of silicic acid in colloidal silica can occur in
supersaturated silica solutions through SN2 nucleophilic
mechanism between silicate ions or silicic acid (Zhang et al.,
2012). The green additives from waste biomass act as interference with the silica polymerization reaction and potentiated
the silica solubility limit by keeping silicic acid or oligomer
silica dispersed for the extended period of time. APAAm and
APAC dendrimers can associate with silicate ions or silica
oligomers, thus preventing further polymerization. Furthermore, the lower soluble silica results can be credited to the
nucleation mechanism of dendrimers to floc by condensation
or nucleophilic substitution (Euvrard et al., 2007; Weres et al.,
FTIR characterization
FTIR spectrum of microalgae shows characteristic peaks at
3350.3 cm1 (polymeric associated OeH broad intense
stretching or overlie eNH2 stretchings), 2923.4 cm1 (eCH2
stretchings), 1654.8 cm1 (eC]O of peptide), 1560.3 and
1425.4 cm1 (eNH2 bendings) and 1050-1250 cm1 (eCOCe
stretchings) (Fig. 4a). APAAm dendrimer shows additional
broad and strong peak at 1662.2 cm1 due to C]O of amide
(Fig. 4b). FTIR spectrum of APAAm also shows distinction in
the values and intensity of characteristic peaks of microalgae,
which can be accounted for modification approval. FTIR results also support silica inhibition in test sets. FTIR spectrum
of SiO2 deposit shows characteristic IR adsorption signals for
bending vibration, symmetrical and asymmetrical stretching
vibrations of bridged SieOeSi bonds at 463.2 cme1, 879.9 cme1
and 1015.9 cme1, respectively (Fig. 4c) (Lin, 1997). The deposits
also show broadening and shifting in eOH stretching
(3322.9 cme1), due to interaction of dendrimers through eOH
group with silica in silica inhibition. Additionally, FTIR spectrum of silica deposit also shows an additional weak sharp
signal at 2494.7 cme1 for quaternized amine, which can be
accounted for protonation and nucleophilic reactions of eNH2
group in dendrimers (DeTar and Novak, 1970).
3.5.
SEM-EDX analysis of silica deposits
SEM results for silica deposits in APAAm dendrimer inhibition
test set at 20 ppm show almost continuous silica layer (Fig. 5a)
in contrast to earlier “rougher” surface of the dendrimers
(Fig. 2b). At 80 ppm, APAAm dendrimer shows much more
pronounced tendency for silica compact aggregation (Fig. 5b
and c). SEM result also shows diffusion of silica in the pore
sites or in microfibril structure of the dendrimers at higher
magnification. The diffusion of silica in pore sites confirms the
higher efficiency of synthesized dendrimers in silica inhibition. Dendrimers are acting as nucleators for silica polymerization. The silica particles exhibit severe aggregation and
form continuous films by further depositions on the surface of
the dendrimers. SEM image of silica inhibition in real conditions (with APAAm) show biomorphs with hairy sphere
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 2 2 5 e2 3 3
231
Fig. 5 e SEM image of APAAm-silica deposit at (a) 20 ppm (b) 80 ppm and 1000 magnifications (c) 80 ppm and 3000
magnifications (d) in real conditions at 80 ppm and 3000 magnifications.
clusters composed of radially aligned crystals (Fig. 5d) (Kniep
and Busch, 1996).
EDX characterization results show high amounts of silica in
silica deposits i.e., 10.98% and 6.97% by weight at 20 ppm and
80 ppm APAAm, respectively (Fig. 6). From the spectrum, it is
clear that the precipitates consist essentially of silicon and oxygen. The synthesized dendrimers show the silica inhibition
efficiency and acts as silica nucleators forming SiO2-algae composites, which are accountable through high Si content in
deposits.
Fig. 6 e EDX spectra for APAAm-silica deposits at (a) 20 ppm (b) 80 ppm.
232
4.
w a t e r r e s e a r c h 5 6 ( 2 0 1 4 ) 2 2 5 e2 3 3
Conclusions
The area of research on scale inhibitor chemistry is making
new efforts for green alternate. This work exploited a mean to
convert spent biomass of biofuels to value added products.
APAAm dendrimers of generation 0 showed maximum silica
inhibition efficiency and retains 300 ppm soluble silica by
dissolution at 55 C, which is even superior to commercial
dendrimers. APAAm showed preferential competence in
contrast to PAMAM-0G inhibitor at 12 h, as the green additive
retard silica polymerization and maintains more than
450 ppm silica solubility. Secondly, the observed polymerization time or induction time was more than 10 days with
APAAm dendrimers as no flocs were observed in test set. EDX
results of silica deposits support heterogeneous nucleator
mechanism of APAAm dendrimers with high amounts of silica (6.97%e10.98%) by weight. In conclusion, the study has
contributed significantly and attractively to the green
replacement of commercial additives. Algae as a core in the
synthesis of dendrimers have not been exploited yet. Moreover, the processing cost in conventional dendrimers is high
and they are not green.
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