FTIR analysis and potentiometric titration in identification of functional groups on the surface of

new biosorbents – strawberry and raspberry seeds

Mateusz Samoraj1, Izabela Michalak1, Katarzyna Chojnacka1

1 Department of Advanced Material Technologies, Faculty of Chemistry, Wrocław University of

Technology, Smoluchowskiego 25, 50-372 Wrocław, Poland

Presenting author email: Katarzyna Chojnacka, katarzyna.chojnacka@pwr.edu.pl

Corresponding author: Mateusz Samoraj, mateusz.samoraj@pwr.edu.pl

Abstract

In the present work, post-extraction residues of strawberry and raspberry seeds were used as biosorbents

of micronutrients. Taking into account physical form of strawberry and raspberry seeds and relative

homogeneity, they are interesting biosorbents. Their sorption propertiesare unknown,so far.

In this study, the surface chemical functional groups of new biosorbents were identified. The

biomass of post-extraction residues of strawberry and raspberry seeds was enriched with Cu(II), Mn(II)

and Zn(II) ions via biosorption in stirred tank reactor (40 L) at 25 oC, pH=5. About 40 g of each

micronutrient-enriched product was prepared. The role of the surface chemistry of the biomass in

biosorption process was investigatedby ICP–OES,FTIR and potentiometric titration. The comparison of

the obtained results was performed in order to investigate the functional groups thatparticipate in

biosorption. Results obtainedby ICP-OES analysis showed exchange of naturally bound alkali cations

with micronutrients. Important functional groups present on the biomass of residues of berries seeds were

carboxyl, amino and hydroxyl groups. There was found a correlation between cation exchange capacity

assessed with potentiometric titration and sorpion capacity of copper ions by ICP-OES.The use of FTIR

analysis, potentiometric titration and ICP-OES is a useful approach in the elaboration of biosorption

phenomenon.

Keywords: biosorption, berries seeds, functional groups, FTIR, ICP-OES

1. Introduction

Waste management became more importantdue to the availabilityand lower cost of usingwaste materials

(Schaub and Leonard 1996). An exemple are strawberry and raspberry seeds which are particularly

interesting waste in fruits industry. About 200 000 Mg of strawberries and 120 000 Mg (GUS 2014) of

raspberries are produced annually in Poland with the total global production of 1 600 000 and 460 000

Mg (Novagrim 2010;Freshplaza 2014 ). Constituents of berries:anthocyanins and polyphenols such as

ellagic acid, are beneficial in protecting cells from various health injuries, such as ageing and different

forms of cancer,as it was shown in in vitro and in vivo tests (Juranic et al., 2005).

In the processing of raspberry and strawberry fruits, the seeds become a by-product. Raspberry

and strawberry seeds constitute up to 50% of the waste (Samoraj et al., 2014). Seeds of raspberries and

strawberries may be subjected to supercritical extraction (Nakonieczna et al., 2014). The obtained oil is

rich in tocopherols, vitamin E, lipids, phospholipids, free fatty acids (Oomah et al., 2000). Seed oil has

superior antioxidant potential as compared to vitamins C, E, and β-carotene (Zafra‐Stone et al., 2007).

Natural antioxidants (including phenolic compounds) may prevent lipid peroxidation in edible oils as in

the case of synthetic antioxidants such as butylated hydroxyanisole and butylated hydroxytoluene (Parry

and Yu 2004). It is very important because oxidation of lipids has been gaining a great concern because of

its influence on a quality and nutritive value decrease of fat and fat-containing food, as well as the diverse

biological activity of the products formed (Małecka et al., 2003).

Utilization of berries seeds by the supercritical extraction of oils generates post-extraction

residues. In the last time, special attention is paid to the use of process wastes from biomass processing

which provides an opportunity to substitute chemical fertilizers. There are many advantages of this type

of residues for example: slow release of micronutrients, growth stimulants, improvement of soil structure

(Hue and Silva 2000; Tuhy et al., 2014). The new idea involves the use of supercritical post-extraction

residues of berries seeds as components of fertilizers (Chojnacka et al., 2014).

Taking into account physical form of strawberry and raspberry seeds, post-extraction residues

and relative homogeneity, they are interesting biosorbents. The quality of biosorbents depends on strong

molecular interactions (i.e. electrostatic attractions), chemical state and composition of surface functional

groups. The state - ionization or protonation of the functional groups present on the surface depends

directly on medium acidity (pH) and dissociation constant. Usually, value of this constant for chemical

groups presented by biomolecules closely corresponds to pKa value of appropriate inorganic acid or

base,because of the presence of different types of functional groups (commonly): amine, carboxyl,

hydroxyl, phosphoryl and sulfone groups on the biomass surfaces with amphoteric properties (Dmytryk et

al., 2014). In general, surface chemical reactions between mentioned functional groups allow (mainly by

complexation) for selective binding of micronutrients (Pagnanelli et al., 2008)

On the surface of the strawberry and raspberry seeds polysaccharides, fatty acids, proteins,

amino acids are found(Parry and Yu 2004). These molecules contain a variety of functional groups (e.g.

carboxyl, hydroxyl etc.) which can participate in the biosorption of microelement ions (Michalak et al.,

2013). Their sorption propertiesare unknown,so far. Due to biosorption, there is a possibility to increase

the content of essential micronutrients such as zinc, manganese and copper in the biomass. As it was

reported previously, micronutrients after biosorption are bound to the biomass matrices in bioavailable

form (Tuhy et al., 2014; Chojnacka et al., 2014).

In present work, post-extraction residues of strawberry and raspberry seeds were examined as a

new biosorbent. The surface chemical functional groups were identified. The role of the surface chemistry

of the biomass on biosorption was investigated. The biomass of strawberry and raspberry post-extraction

seeds residues was enriched with Zn(II), Cu(II) and Mn(II) which are important in plant feeding. In order

to explain the mechanism of the biosorption, different analytical techniques were applied. The surface

functional groups of the biomass were identified by Fourier Transform Infrared (FTIR) analytical technique

and potentiometric titration. These techniques are reported as useful methods in the determination of the

presence of functional groups on the biosorbents surface (Yun et al., 2001). Surface characterization by

titration modeling have been applied also by other researchers to different heterogeneous natural materials

(soils, agricultural wastes, bacterial biomasses) (Pagnanelli et al., 2000, 2003, 2004, 2006). The content of

elements in metal loaded biomass was examined by Inductively Coupled Plasma –Optical Emission

Spectroscopy (ICP–OES).

2. Material and methods

2.1. Biosorbent

The biomass of post-extraction residues of raspberry and strawberry seeds was obtained from the New

Chemical Syntheses Institute – Supercritical Extraction Department in Puławy.

2.2. Biosorption process

The biomass was enriched with Zn(II), Cu(II) and Mn(II) ions via biosorption in stirred tank reactor (40

L) at 25 oC, pH=5 (Tuhy et al., 2014). Aqueoussolution of microelement ions (300 mg/L) was prepared

by dissolving ZnSO4·7H2O, CuSO4·5H2O, MnSO4·H2O in deionized water.

2.3. ICP-OES analysis

For the ICP–OESmultielemental analysis of samples known mass of each material was digested in

microwave system Milestone Start D (USA) with spectrally pure nitric acid – 69% m/m (Suprapur,

Merck, USA) in teflon bombs. All mineralization parameters were matched to assure complete digestion

of samples. After mineralization, samples were analyzed with ICP–OES (Varian–Vista MPX, Australia).

Samples were introduced with ultrasonic nebulizer CETAC U5000AT+. The analyses were carried

out in Laboratory Accredited by Polish Centre of Accreditation (PCA) according to PN-EN ISO/IEC

17025:2005. Quality assurance of the test results was achieved by using Combined Quality Control

Standard from ULTRA SCIENTIFIC, USA. All samples were analyzed in three repeats (the relative

standard deviation was <5%).

2.4. FTIR analysis

Functional groups presented on the surface of the biomass were identified by FTIRspectroscopy.

FTIRabsorptionspectrawere performedin thewave number range4000-400cm-1. Approximately 2 mg of

biological material sample was prepared and compressed with 200 mg of dry KBr. Analysis was

performed with Spectrum System 2000 FT–IR (Perkin Elmer). Studies were carried out the Central

Laboratory of Instrumental Analysis at Wrocław University of Technology.

2.5. Potentiometric titration

Second method used for the identification offunctional groups on the biomass was potentiometric

titration.For the analysis,1.0 g of the biomass was suspended in 100 mL of ultrapure deionized water.

Titration was performed in two steps with automatic titrator (T50, Mettler Toledo). In first step,pH was

lowered to 2.5. After that the biomass was titrated with NaOH (0.1 M). All probable pKa points (acidic

dissociation constants) on titration curve were assigned using graphical method with Origin Pro 2015

software. From 1st derivative curve, all peaks were determined. From 2nd derivative curve, all crossing

points with x-axis were chosen from population of Xadd values (molar ratio of added titrant – mmol/g of

biosorbent), determined in first step. These points are probable Xadd values for all measured pH at pKa

points.

3. Results and discussion

Biosorption is a complex process. Biomass can bind metal ions by (I)chemical exchange of cations (ion

exchange) orprotons (proton displacement), chelation orcomplexation and (II) physical (van der Waals

forces and electrostatic interaction) (Davis et al., 2003; Michalak et al., 2013). The identification of

biosorption mechanism might be evaluatedby: potentiometric titration, XRF,FTIR, SEM-EDX(Michalak

et al., 2013). Main role in the biosorption process play functional groups (especially carboxyl) which are

present on the biomass surface.

In the present work,qualitative determination of functional groups on the surface of the biomass was

conducted with FTIR analytical technique and potentiometric titration (Yun et al., 2001). FTIR allows

identification of different functional groups on the cell wall structure and provides important information

related to the nature of the bonds (Michalak et al., 2013; Murphy et al., 2007). The potentiometric titration is

also useful in description of aqueous biomass suspension. However, post-extraction residues of strawberry

and raspberry seeds have not been studied in this respect, so far. Prepared materials were also subjected to

ICP-OES analysis in order to determine which metal ions were exchanged during the process.

3.1. Multielemental analysis

In the present study, both tested biosorbents had the highest biosorption capacity for copper ions, then

zinc and finally for manganese.This phenomenon can be explained by characteristics of the metal ionic

properties. Taking into account several parameters (Can and Jianlong 2007) listed in Table 1.It may be

assumed that AR, IP andXm are responsiblefor the highest biosorption capacity of the examined

biomasses for Cu(II) ions.

Table 1.Metal ionic properties

Micronutrient

Biosorption capacity (mg/g)

Oxidation

number

(OX)

Atomic

weight

(AW) (u)

Atomic

radius (AR)

(Å)

Ionization

potential

(IP) (eV)

Electronegativity(Xm)

(Pauling Units)

Post-extraction

residue of

strawberries

seeds

Post-extraction

residue of

raspberries seeds

Cu 12.6 9.58

2

63.5 1.35 20.3 1.90

Zn 5.13 4.78 65.4 1.31 18.0 1.65

Mn 5.03 2.50 54.9 1.12 15.6 1.65

Basing upon data from Table 2, the content of K(I) in the enriched with Zn(II), Cu(II) and Mn(II)

ions biomass of strawberry seeds decreased by 80.0%, 83.4% and 48.9%, respectively, when compared to

the natural biomass.Similarly for raspberry seeds decreases were 89.5%, 94.4%, 89.3% for Zn(II), Cu(II)

and Mn(II) enriched biomass, respectively.In case of Mg,decrease was 62.9% (81.2%),67.9%(90.6%),

79.5% (80.4%) for Zn(II), Cu(II) and Mn(II) enriched biomass, respectively, for strawberry seeds (and

raspberry).For Ca decrease after biosorption was smaller – 8.42% (55.1%), 12.4% (76.6%), 3.0% (39.2%)

for strawberry seeds (and raspberry).Content of S was 32.6% (22.5%), 34.1% (2.63%), 38.2% (28.3%)

lower for Zn(II), Cu(II) and Mn(II) enriched biomass, for strawberry seeds (and raspberry), respectively.

During biosorption process, mainly light metal cations (K(I), Mg(II), Ca(II)) which are bound

with functional groups (presented on the surface), were exchanged with micronutrient cations (Davis et

al., 2003). In conducted biosorption process of Zn(II), Cu(II) and Mn(II) on strawberry and raspberry

seeds, mainly K, Mg, Ca and S ions took part in ion exchange.

Table 2. Multielemental composition of natural and enriched witch micronutrients strawberry and

raspberry seeds biomass

Element S - natural S + Zn S + Cu S + Mn R - natural R + Zn R + Cu R + Mn

(mg/kg)

Mic

ronu

trie

nts

Zn 43.6 5134 151 59.1 34.6 4776 171 26.1

Cu 13.5 12.9 12611 87.3 8.96 9.09 9578 71.5

Mn 85.8 32.8 25.6 5027 75.9 18.8 14.0 2484

Fe 152 131 118 117 122 101 184 113

Mo 2.34 1.57 2.11 2.37 6.54 2.14 4.41 3.37

Ni 1.63 2.34 <0.03 0.04 <0.03 0.46 <0.03 1.23

Mac

ronu

trie

nts

P 2780 1434 1298 1784 1551 757 797 1056

K 3353 672 555 1715 2767 290 156 295

S 2199 1482 1450 1360 1407 1090 1370 1009

Ca 5594 5123 4899 5424 2502 1123 585 1521

Mg 2464 915 791 505 1802 339 170 354

Na 615 674 333 349 472 622 630 787

Oth

er e

lem

ents

Al 40.0 50.4 51.0 61.8 57.8 28.4 44.2 35.9

Se 19.4 8.85 10.0 6.08 7.40 <0.5 1.14 <0.5

Si 253 289 365 364 130 153 170 175

Ti 2.26 2.60 3.66 2.88 8.59 9.63 3.60 3.35

V 3.28 1.69 1.54 2.05 1.50 1.88 0.894 2.59

Tox

ic e

lem

enrs

Cd 0.574 0.772 0.799 0.412 0.830 0.245 0.532 0.407

Ni 1.63 2.34 <0.03 0.0414 <0.03 0.465 <0.03 1.23

As <0.3 <0.3 <0.3 8.87 <0.3 <0.3 <0.3 <0.3

Pb 4.50 8.59 4.14 7.63 <0.5 0.629 19.0 2.50

Cr 1.16 5.63 0.423 1.82 0.117 1.87 0.616 0.596

S- post-extraction residues of strawberry seeds

R- post-extraction residues of raspberry seeds

3.2. FTIR analysis

FTIR spectra for natural and enriched with Zn(II), Mn(II) and Cu(II) ions post-extraction residues of

strawberry and raspberry seeds are presented in Figure1 and 3, respectively. SinceFTIR curves have

similar shape, we assumed that the same functional groups were involved in the biosorption process. In

Figure2 and 3,FTIR spectra for each biomass were presented, separately. Nature of the surface of

analyzed biosorbentsis very complex because number of absorption peaks was observed.

In the literature, different wavenumbers are assigned to cellular molecules, for example: lipid

1780–1708 cm−1, amide I: 1705–1575 cm−1, phosphoryl 1350–1190 cm−1, amide II 1575–1480 cm−1,

carboxyl 1064–880 cm−1(Stehfest et al., 2005). The biomass of raspberry and strawberry seeds has a

number of functional groups from macromolecules such as: polysaccharides, fatty acid, amino acids,

phenols etc. (Parry and Yu2004;Oomach et al., 2000).

The comparison of FTIR peaks for natural and Mn, Cu and Zn loaded biomass with literature

data is presented in Table 3. In comparison between natural and metal loaded strawberry and raspberry

seeds, there are visible spectra transitions, which allow for identification of functional groups (Figure2

and Figure4). In this study, minor changes in the wavenumber for both investigated materialswere

detected for amine group. Smaller shifts were observed for carboxyl group, amide I band, phosphoryl and

phosphine. This shows which functional groups were actually involved in the binding of microelement

ions from the aqueous solution. Most of the peaks in FTIR spectra of Cu, Zn and Mn enriched biomass

appeared at the similar wavelength.

Table 3.Comparison of FTIR peaks for natural, Mn, Cu and Zn loaded biomass

Strawberry seeds Raspberry seeds Assigned group Literature -* Zn Cu Mn -* Zn Cu Mn

Peak at 333

9 331

4

3422

3411

3418

3398

3390 3321 3307

3420

3416

3426

Amine

3200-3500 (Lambert et al., 1987;Feride et al., 2012)

2958

2929

2927

2928

2928

2928 2926

2926

2927

Carboxylic

2900-3300 (Michalak et al., 2013)

2858

2856

- 2855

2856 2857

2857

2856

CH2 of saturated CH primarily from lipids

3000-2800 (Stehfest et al., 2005)

2339

2363

2376

2386

2341 2255

2339

2348

P-H phosphines 2280-2410 (Lambert et al., 1987)

- 1739

1734

1739

1733 1739

1738

1739

C═O strech of COOH, C=O of fatty acids 1740(Fourest and Volesky 1996), 1780-1708 (Stehfest et al., 2005)

1656

1654

1653

1652

1656 1655

1656

1653

Amide I band (with proteins) 1705-1575 (Stehfest et al., 2005; Lambert et al., 1987;Feride et al.,

2012) 154

0 1517

1522

1517

15311519

1515

1515

1515

N-H of amides associated with proteins amide II band

1575-1480 (Stehfest et al., 2005;Feride et al., 2012)

1451

1452

1444

1451

1450 1452

1451

1452

CH3 of proteins 1455 (Stehfest et al., 2005)

1397

1373

1376

1374

1368 1371

1428

1372

1371

C-O of COO- carboxylic group ~1400 (Dmytryk et al., 2014; Chojnacka et al., 2005)

1317

1238

1319

1235

1323

1241

1318

1237

1318 1235

1319

1234

1323

1235

1319

1233

P=O of phosphoryl

1350-1188 (Stehfest et al., 2005;Feride et al., 2012)

- 1154

- 1151

1160 1057

1161

1055

1160

1057

1160

1055

C-O-C polysaccharides 1200-900 (Stehfest et al., 2005)

1068

902

1061

897

1063

1061

899

1039 897

897 1039

897 898

C-O of COO- carboxylic group C-O of COO- carboxylic group

1064-880 (Stehfest et al., 2005;Lambert et al., 1987)

- 809 - 780 810 810 810 809 NH2 of primary amines 760-860 (Lambert et al., 1987) 661 - - - 662 661 662 - C-OH of alcohols 620-680(Lambert et al., 1987) 597 593 599 603 602 606 602 603 N-C=O of amides 570-630 (Lambert et al., 1987)

* natural biomass

Fig. 1.Superposition of FTIR spectra for natural (black), enriched with Zn(II) (green), Cu(II) (pink) and

Mn(II) ions (blue) strawberry seeds

Fig. 2.Peak designation for strawberry seeds FTIR spectra (natural – black, enriched with Zn(II) – green,

Cu(II) ions – pink and Mn(II) ions – blue)

Fig.3.Superposition of FTIR spectra for natural (black), enriched with Zn(II) (green), Cu(II) (pink) and

Mn(II) ions (blue) raspberry seeds

Fig. 4.Peak designation for strawberry seeds FTIR spectra (natural – black, enriched with Zn(II) – green,

Cu(II) ions – pink and Mn(II) ions – blue)

3.3. Potentiometric titration

Biomass, after protonation, was titrated with 0.1 M NaOH. Titration curves were plotted for strawberry

(Figure 5) and raspberry (Figure8) seeds to identify the functional groups capable of cation exchange.1st

and 2nd derivative curves were analyzed – Fig. 6,9 and 7,10, respectively. The first derivative plots

(Figures 6 and 9) consist of the midpoint of successive amounts of NaOH added (Xadd) versus

ΔpH/ΔXadd. Reading the location of each peak on the x-axis gave the number of acidic groups on the

biomass surface (Murphy et al., 2007;Yalçın et al., 2012). From 2nd derivative curve, all crossing points

with x-axis were chosen from the population of Xadd values determined from 1st derivative. These points

are probable Xadd values for all measured pH at pKa points. The titration curve with probable pKa

alignment, assumed from 1st and 2nd derivative, is presented inFigure5 and 8 for strawberry and raspberry

seeds, respectively. The pKa value can directly give the approximate charged state of that functional

group at a specific solution pH (Satyanarayana et al., 2012). Functional groups present on cell surface

determined by potentiometric titration are presented in Table 4. Different functional groups participated

in metal ions binding, depending on pH: 2–5 carboxyl group, pH 5–9 carboxyl and phosphate group, pH

9–12 phosphate and hydroxyl (or amine) group (Chojnacka et al., 2005). In this study, post-extraction

residue of strawberry and raspberry seeds possessed several groups with distinguishable pKa. Dominating

groups were carboxyl, amine and hydroxyl.The biosorption process of Cu(II), Mn(II) and Zn(II) ions was

carried out at pH 5. Under these conditions carboxyl groups were deprotonated and therefore available for

metal ions. Hydroxyl and amino groups can play a significant role in metal binding at higher pH values

(Yalçın et al., 2012).Concentration of available groups at pH 5 was 0.397 and 0.265 meq/g for raspberry

and strawberry seeds, respectively. These values stays in correlation with biosorption capacity (q) of

tested biomass for Zn(II)(R2=0.923), Cu(II)(R2=0.989) and Mn(II)(R2=0.958) ions – Table 5.

Table 4. Functional group assigned to assumed pKa from 1st and 2nd derivative

Possible pKa

(raspberry seeds)

Possible pKa

(strawberry seeds)

Literature

(Chojnacka 2005;Fourest and Volesky

1996;Satyanarayana et al., 2012)

Functional group

3.481 3.337 2–4 Sulphate, nitrate

- 3.152 3.1 Terminal carboxyl

4.056

4.601

4.232

4.454

5.083

4–5 Carboxyl

5.697 5.834

6.778 6–7 Thiol

7.279 7.764 7 Phosphoryl

9.187

9.687

9.899

10.144

10.865

10.936

11.227

9.479

9.755

9.98

10.241

10.382

10.499

10.809

11.023

11.351

9–12 Amine and hydroxyl

Table 5.Comparison of biosorption capacity and Xad d at pH 5

Parameter Xadd at pH 5

(meq/g)

q based on Zn(II)

(meq/g)

q based on Mn(II)

(meq/g)

q based on Cu(II)

(meq/g)

Strawberry 0.397 0.157 0.183 0.397

Raspberry 0.265 0.146 0.090 0.302

R2 0.923 0.958 0.989

0.0 0.5 1.02

4

6

8

10

12

pH

Xadd

pH

0.16779, 3.1520.19427, 3.337

0.31789, 4.232

0.33555, 4.454

0.39738, 5.083

0.42387, 5.834

0.45036, 6.7780.46801, 7.764

0.52982, 9.4790.54748, 9.755

0.56514, 9.980.59163, 10.241

0.60929, 10.3820.62695, 10.499

0.68876, 10.809

0.75056, 11.023

0.90067, 11.351

Fig. 5. Titration curve of protonated strawberry seeds with probable pKa alignment, assumed from 1st and

2nd derivative of results

0.0 0.5 1.005

10152025303540455055606570

ΔpH/ΔXd

d

Xadd

1st deriv.

0.16779

0.19427

0.468010.45036

0.42387

0.397380.54748

0.565140.59163

0.609290.62695

0.688760.75056 0.90067

0.33555

0.31789

Fig. 6. First derivative potentiometric titration curve of protonated strawberry seeds

-2000

-1500

-1000

-500

0

500

1000

1500

2000

2500

3000

3500

4000

0.900670.75056

0.688760.62695

0.609290.59163

0.565140.54748

0.46801

0.45036

0.42387

0.39738

0.33555

Δ(Δp

H/ΔXd

d)/ΔXa

dd

Xadd

2nd deriv.

0.167790.19427

0.31789

Fig. 7. Second derivative potentiometric titration curve of protonated strawberry seeds

0.0 0.5 1.02

4

6

8

10

12

pH

Xadd

pH

0.18649, 3.481

0.22379, 4.056

0.25178, 4.601

0.27975, 5.697

0.31705, 7.279

0.39164, 9.187

0.43825, 9.6870.46623, 9.899

0.50352, 10.144 0.69931, 10.8650.73661, 10.936

0.89511, 11.227

Fig.8. Titration curve of protonated raspberry seeds with probable pKa alignment, assumed from 1st and

2nd derivative of results

0.0 0.5 1.00

5

10

15

20

25

30

35

40

45

50

ΔpH/ΔXd

d

Xadd

1st deriv.

0.22379

0.18649

0.25178

0.279750.31705

0.391640.43825

0.699310.73661

0.50352

0.46623

0.89511

Fig. 9. First derivative potentiometric titration curve of protonated raspberry seeds

-1000

-500

0

500

1000

1500

2000

2500

3000

0.89511

0.25178

Δ(Δp

H/ΔXd

d)/ΔXa

dd

Xadd

2nd deriv.

0.18649

0.22379

0.27975

0.31705

0.39164

0.438250.46623

0.50352

0.699310.73661

Fig.10. Second derivative potentiometric titration curve of protonated raspberry seeds

4. Conclusion

In this study, the surface chemical functional groups of new biosorbent (post-extraction residues of

strawberry and raspberry seed) were identified. This by-product of supercritical extraction of berries

seeds has a potential to be applied as a component of fertilizers. This approach fits the sustainable waste

management that encourages the recycling and recovery of waste that is produced. The role of the surface

chemistry of the biomass in biosorption process was investigatedusing FTIR and potentiometric titrations.

It was shown that both typesof biomass had good biosorption capacity, especially for Cu(II)

ions. The content of copper in the enriched biomass was 2 times higher than zinc and manganese in the

case of strawberry seeds, 5 times higher for manganese and more than two times higher for zinc in the

case of raspberry seeds. Potentiometric titration and FTIR technique showed that carboxyl, hydroxyl and

amino groups were dominating on the surface of the biosorbent. While the biosorption was carried out in pH

5, carboxyl groups participated mainly in the biosorption process. ICP-OES analysis of the biomass

confirmed that the main mechanism of the biosorption was ion exchange of micronutrients with K(I), Ca(II),

Mg(II) ions.

Acknowledgements

The work was supported by Polish National Science Centre, project no. 2012/05/E/ST8/03055 entitled:

Biosorption of metal ions to the biomass of seeds of berries.

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