Advanced Powder Technology 25 (2014) 695–703

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Advanced Powder Technology

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Original Research Paper

A novel method of combination of Kraft lignin with synthetic mineralsupport

0921-8831/$ - see front matter � 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rightshttp://dx.doi.org/10.1016/j.apt.2013.10.016

⇑ Corresponding author. Tel.: +48 61 665 37 47; fax: +48 61 665 36 49.E-mail address: filip.ciesielczyk@put.poznan.pl (F. Ciesielczyk).

Filip Ciesielczyk ⇑, Łukasz Klapiszewski, Karolina Szwarc-Rzepka, Teofil JesionowskiPoznan University of Technology, Institute of Chemical Technology and Engineering, M. Sklodowskiej-Curie 2, PL-60965 Poznan, Poland

a r t i c l e i n f o

Article history:Received 27 June 2013Received in revised form 27 September 2013Accepted 25 October 2013Available online 9 November 2013

Keywords:Inorganic mineral supportKraft ligninBiosorbentsThermal and electro-kinetic stabilityPorous structure

a b s t r a c t

The main goal of this research was to produce and to give a full physicochemical description of a newgroup of products obtained by combining a commercially available Kraft lignin with the synthetic inor-ganic support MgO�SiO2. Hybrid systems of this type may have a wide range of applications, particularlyconsidering the variety of functional groups present in the structure of lignin, as well as the large surfacearea presented by the inorganic oxide system. These features allow the products to be classed as effectiveadsorption materials, with a broad range of users connected with protection of the environment. Thelignin was combined with the surface of the synthetic mineral support by way of initial activation ofthe lignin, followed by its reaction with the precipitated oxide system MgO�SiO2. The materials (biosor-bents) thus obtained were subjected to thorough physicochemical analysis, including evaluation of theirdispersive–morphological character, thermal and electro-kinetic stability, and porous structure parame-ters. Additionally, to confirm the effectiveness of the combining operation, the FT-IR spectra were ana-lyzed and the elemental composition of this new group of hybrid biosorbents was determined.� 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

Over recent years a great deal of research has been carried outusing materials of biological origin to prepare hybrid products withspecific physicochemical and utility properties. The materials usedinclude in particular lignin, chitin and chitosan [1–6]. The combin-ing of materials of this type, offering unique properties, with othermaterials such as inorganic mineral substances makes it possibleto obtain systems with very interesting parameters and a widerange of applications. Such systems can be successfully used as rel-atively cheap polymer fillers or as selective biosorbents of certaininorganic/organic pollutants [7–10]. The need to protect the envi-ronment, along with economic factors, has made it a very impor-tant objective to develop innovative biosorbents, which may inthe near future come to replace the sorption materials that are cur-rently in general use.

The unique properties of lignin, including the presence of func-tional groups such as hydroxyl, ether and carbonyl, enable it to becombined relatively easily with mineral supports. Such supportsinclude co-precipitated inorganic oxides, which are characterizedby large specific surface area, a high degree of homogeneity, ther-mal stability, and defined electro-kinetic properties [11–14]. Thepresence of hydroxyl groups on the surface of such systems is a

determining factor in their reactivity and facilitates their combina-tion with numerous compounds, including lignin [15–18].

Many scientific investigations have been carried out to evaluateorganic/inorganic systems, and it is believed likely that work onsuch materials will bring useful results, since these composites of-fer very good properties and significant functionality [19–21].

The paper [22] presents a process for obtaining a silica–ligninxerogel. The researchers first obtained a silica material (using thesol–gel method) and then modified the silica using lignin. It wasfound that an increase in the quantity of lignin used causes signif-icant changes in the parameters of the porous structure. The intro-duction of lignin into the mineral matrix causes an increase inspecific surface area, as well as a decrease in pore volume. Similarresults were obtained when a hybrid biosorbent was preparedusing rice husks as a precursor for both lignin and silica [23].

The synthesis of a hybrid biocomposite based on lignin andhydrated silica is described in [15]. The system was prepared byway of initial precipitation of silica in a polar medium, its surfacemodification using aminosilane, followed by combination withactivated Kraft lignin. A silica/lignin hybrid was obtained with verylarge specific surface area and optimum dispersive–morphologicalparameters. It was also shown that the properties of the hybridmaterial are determined chiefly by the quantity (by weight) of lig-nin used in the process and by the initial functionalization of themineral support with alkoxysilane. In [24] a comparison was madeof lignin-derived hybrid biocomposites obtained using silicas

reserved.

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produced in polar and non-polar media and by the Stöber process.The results indicate clearly the effectiveness of the proposedmethod for combining lignin with a mineral support. The bestphysicochemical parameters, including primarily the defined elec-tro-kinetic stability, were obtained for the biocomposites based onStöber silica. The effectiveness of the process was confirmed byXPS, FT-IR and elemental analysis.

In view of the dynamic development of industry and continuingneed for new multifunctional materials, an attempt was made toproduce a new group of hybrid systems based on lignin and aninorganic mineral support, with defined physicochemical proper-ties, including above all specified porous structure parameters.The materials obtained in this manner may constitute a new groupof functional biosorbents.

2. Experimental

2.1. Materials

The synthetic mineral oxide system was obtained in a process ofprecipitation from aqueous solutions of sodium silicate and mag-nesium sulfate, as described in previous work [25–26]. As a resultof this process, an oxide system characterized by micrometer-sizedparticles (dominant particle diameter 7.8 lm), amorphous struc-ture and a relatively large BET surface area (514 m2/g) wasobtained. In the combination process, Kraft lignin (Sigma–Aldrich)was used in quantities of 3, 5, 10, 20, 30, 40 and 50 parts by weight(samples TS 1.1–7.1) relative to the MgO�SiO2.

2.2. Process of combination of Kraft lignin with mineral support

The preliminarily obtained mineral support – MgO�SiO2 – wasused in the process of combination with Kraft lignin. A technolog-ical diagram of this process is shown in Fig. 1. First, two solutionswere made. Solution 1 was made of lignin dissolved in a mixture ofdioxane:water (9:1, v/v). Solution 2 (oxidizing) was made ofsodium periodate (Sigma–Aldrich) dissolved in water. Then solution2 was dosed into solution 1 at a rate of 1.1 cm3/min, in the dark. Tothe mixture of the two solutions the mineral support was added,

Fig. 1. Technological diagram of the produc

and the whole of the content was stirred for 1 h. Finally the solventwas removed in a vacuum evaporator, and the product was sub-jected to convection drying at 105 �C for 24 h.

2.3. Evaluation of physicochemical properties of the hybrid material

The dispersive characteristics of the MgO�SiO2/lignin hybridmaterial were determined with a Mastersizer 2000 apparatus(Malvern Instruments Ltd.), using the laser diffraction methodand measuring particles of sizes from 0.2 to 2000 lm. Themorphology and microstructure of the materials obtained wereanalyzed using a Zeiss EVO40 scanning electron microscope. Theobservations enabled evaluation of the degree of dispersion, thestructure of particles and their tendency towards aggregation oragglomeration. With regard to the color of the Kraft lignin andits presence in the mineral support structure after combination,colorimetric analysis was performed using a colorimeter (Specbos4000, JETI Technische Instrumente GmbH). Daylight (D65) wasused as a standard light source. The instrument evaluated the colorin terms of the CIE L�a�b� color space system. In this color space, L�

represents the brightness, and a� and b� are appropriate color coor-dinates. Using a Zetasizer Nano ZS (based on the non-invasive backscattering light method) it was also possible to measure theelectrophoretic mobility using laser Doppler velocimetry (LDV),and indirectly the zeta potential (the Zetasizer Nano ZS softwareprovides the ability to convert electrophoretic mobility values tozeta potential based on the Henry equation). The electro-kineticpotential was measured over the whole pH range in the presenceof 0.001 M NaCl electrolyte, which made it possible to determinethe electro-kinetic curves. The hybrid material was also subjectedto thermal stability analysis with the use of STA 449F3 apparatus(Netzsch GmbH). The tests were carried out in a nitrogen atmo-sphere, with the temperature varying within a range of 30–1000 �C. The specific surface area ABET (BET method) was calculatedbased on data measured by low-temperature adsorption of nitro-gen. The isotherms of nitrogen adsorption/desorption were mea-sured at 77 K using an ASAP 2020 apparatus (MicromeriticsInstrument Co.). With regard to the high accuracy of the instru-ment used (±0.0001 m2/g) the surface area values were rounded

tion of a lignin-based hybrid material.

Table 1Dispersive parameters of hybrid biosorbent.

Sample Abbreviation Dispersive parametera (lm)

D[4.3] d(0.1) d(0.5) d(0.9)

MgO�SiO2 TS 0 7.8 3.7 7.0 13.1MgO�SiO2 + 3 wt./wt. of lignin TS 1.1 12.4 3.1 10.0 25.6MgO�SiO2 + 5 wt./wt. of lignin TS 2.1 15.9 3.5 10.2 29.3MgO�SiO2 + 10 wt./wt. of lignin TS 3.1 12.9 3.0 10.3 26.9MgO�SiO2 + 20 wt./wt. of lignin TS 4.1 15.9 3.0 13.0 33.5MgO�SiO2 + 30 wt./wt. of lignin TS 5.1 21.7 4.0 14.4 41.3MgO�SiO2 + 40 wt./wt. of lignin TS 6.1 19.1 2.9 15.5 41.0MgO�SiO2 + 50 wt./wt. of lignin TS 7.1 41.4 4.7 33.3 90.6

a D[4.3] – mean particle diameter, d(0.1) – 10% of particles have diameter lower than d(0.1) value, d(0.5) – 50% of particles have diameter lower than d(0.5) value, d(0.9) –90% of particles have diameter lower than d(0.9) value.

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up to whole numbers, and the mean pore size (Sp) and total porevolume (Vp) calculated using the BJH algorithm were rounded toone and two decimal places respectively. The presence of desiredfunctional groups was confirmed by Fourier transform infraredspectra (FT-IR), recorded on an EQUINOX 55 spectrophotometer(Bruker). The elemental composition of the products was estab-lished with the use of a Vario El Cube instrument (Elementar Anal-ysensysteme GmbH), which gave the percentage contents ofcarbon and sulfur following high-temperature combustion of thesamples.

3. Results and discussion

3.1. Characteristics of the hybrid material

The tests carried out on the obtained hybrid biosorbent madepossible a precise characterization of particle sizes and surfacemorphology, parameters which are of particular importanceconsidering the possible applications of the material. The resultsof these analyses are presented in the form of table and of SEMphotographs of the products.

Fig. 2. SEM images of mineral oxide support (a) and MgO�SiO2/lignin hybrid m

Table 1 contains the dispersive parameters of the hybrid biosor-bents (MgO�SiO2 oxide system/lignin) obtained using the proposedmethod, with different proportions of lignin relative to MgO�SiO2.The initial sample of the mineral support (sample TS 0) has a 10%content of particles with diameters not exceeding 3.7 lm, 50% ofparticles not exceeding 7.0 lm, and 90% of particles smaller than13.1 lm. The SEM image of this sample indicates that it is highlyhomogeneous product (with a narrow range of particle diameters)and has an average particle diameter of D[4.3] = 7.8 lm. The hybridmaterials obtained have very different dispersive characteristicscompared with sample TS 0. Sample TS 1.1, obtained using thesmallest amount of lignin (3 wt./wt.), has the best dispersive prop-erties of all of the hybrid materials. It contains 10% of particles withdiameter smaller than 3.1 lm, 50% smaller than 10.0 lm, and 90%smaller than 25.6 lm. In turn, sample TS 7.1, prepared using thegreatest amount of lignin, displays a significant spread of particlesize and a large value of D[4.3] = 41.4 lm, which shows conclu-sively that the quantity of lignin used has a significant effect onthe agglomeration of the particles of the resulting hybrid materials.This is confirmed by the microscopic SEM images shown in Fig. 2,which confirm the irregular shape of the particles of the resulting

aterials prepared using (b) 5, (c) 20, and (d) 40 parts by weight of lignin.

Table 2Colorimetric parameters of hybrid biosorbent.

Sample Abbreviation Colorimetric parameter

Brightness (L�) Color (from greento red) (a�)

Color (from blueto yellow) (b�)

Chroma (C�) Hue (h�) Total colorchange (dE�)

MgO�SiO2 TS 0 93.85 �0.41 �0.21 0.46 207.13 3.13MgO�SiO2 + 3 wt./wt. of lignin TS 1.1 65.35 4.27 14.98 15.58 74.10 31.27MgO�SiO2 + 5 wt./wt. of lignin TS 2.1 54.66 4.67 13.73 14.50 71.20 40.94MgO�SiO2 + 10 wt./wt. of lignin TS 3.1 51.29 6.31 17.05 18.18 69.69 45.30MgO�SiO2 + 20 wt./wt. of lignin TS 4.1 44.08 7.01 18.08 19.39 68.79 52.58MgO�SiO2 + 30 wt./wt. of lignin TS 5.1 35.99 5.68 11.32 12.66 63.38 58.74MgO�SiO2 + 40 wt./wt. of lignin TS 6.1 34.17 7.56 18.39 19.89 67.65 57.27MgO�SiO2 + 50 wt./wt. of lignin TS 7.1 33.51 7.52 19.34 20.75 68.75 62.13

Fig. 3. Electro-kinetic curves of MgO�SiO2/lignin hybrid materials.

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systems and their significant tendency to agglomerate, particularlyin the case of the materials obtained using larger amounts of lignin.The photographs also show how the particles of the mineralsupport are covered with activated lignin. The analysis confirmedthat modification of the oxide system using lignin leads to asignificant tendency towards agglomeration of the particles of thehybrid biosorbent. The situation is similar in the case of a largenumber of oxide systems modified with selected organiccompounds, particularly when they are used in larger quantities[27–28].

The next stage of the study consisted of colorimetric analysis ofthe hybrid materials obtained. The tests carried out enabled inter-pretation of the color of the products. The results are given inTable 2.

Analysis of the experimental data shows that the quantity oflignin used to obtain the biosorbent has a significant effect onthe brightness parameter L�. It was noted that as the quantity oflignin increases, the brightness of the biosorbent consistently de-creases. This is shown by the change in the value of L� from93.85 for a sample of pure mineral support MgO�SiO2, to 65.35for sample TS 1.1, prepared using 3 parts by weight of lignin. Thesmallest value L� = 33.51 was recorded for sample TS 7.1, madeusing 50 parts by weight of lignin. Moreover, analysis of thecoefficients a� and b�, which define the proportional contributionof particular primary colors, showed that their values increaseproportionally to the quantity of lignin used in the preparation ofthe hybrid materials. These parameters attain their highest valuesfor the samples obtained using the largest quantities of lignin(sample TS 7.1 and others). Positive values of a� and b� indicaterespectively the contribution of red and yellow in the obtainedbiosorbents. The tests also indicate an increase in the parameterdE, which defines the total color change. The highest valuedE = 62.13 was recorded for sample TS 7.1. The changes in the var-ious color parameters provide conclusive proof of the presence oflignin in the resulting biosorbent structure.

In view of the possible uses of materials of this type in variousfields, tests of their electro-kinetic and thermal stability are ofgreat importance. Fig. 3 shows the electro-kinetic curves for sam-ples of the produced hybrid materials.

All of the systems (including sample TS 0) have negative zetapotential values over a pH range of 2.5–11, and as the pH increasesthe electro-kinetic potential consistently decreases. The largestchange in zeta potential (from 2.9 to �22.7 mV) with increasingpH of the system was recorded for sample TS 0, which was the onlysample to attain an isoelectric point at a pH of around 2.3. More-over the precipitated mineral support MgO�SiO2 has a highly stablepotential over a pH range of 6–10.9. Among the obtained biosor-bents, the greatest change in electro-kinetic potential (from �8.1to �24.3 mV) as well as the greatest electro-kinetic stability wererecorded for sample TS 7.1, which was prepared using the largestamount of lignin (50 wt./wt.). Sample TS 2.1 is the biosorbent with

the highest initial potential (�2.9 mV); as the pH increases thevalue of the zeta potential decreases consistently, reaching�15.8 mV. All of the samples have their highest zeta potentialvalues in an acidic environment. The analysis showed that combin-ing of lignin with the inorganic oxide system MgO�SiO2 causeschanges in the charge on the biosorbents’ surface relative to sam-ple TS 0. Moreover, the increase in the electro-kinetic stabilitytogether with increasing amount of lignin used for biosorbentpreparation, was observed. Similar relationships have been foundfor hybrid systems prepared using lignin and precipitated silica[15,24,28].

The thermal stability of the unmodified mineral sample (TS 0),pure Kraft lignin and selected hybrid biosorbents was determinedby means of thermal analysis. Thermogravimetric curves (TG), aswell as their first derivatives (DTG), were obtained (Fig. 4).

The thermogravimetric curve for the MgO�SiO2 oxide system(sample TS 0), shown in Fig. 4a and b, indicates a single-stage masschange. The loss of mass of the oxide support was found to begineven at the starting temperature of 30 �C (Fig. 4a), and the ob-served mass loss in the temperature range 30–200 �C is relativelylarge, at around 5%. This is also indicated by the clear peak appear-ing on the derivative curve (Fig. 4b). Above a temperature of 700 �Cthe sample stabilizes, its mass remaining virtually unchanged. Theunmodified MgO�SiO2 oxide support was found to lose only 8% ofits total mass.

For lignin the mass loss over the temperature range studied wassignificant and reached 65% (Fig. 4a). This result confirms the datapublished in other work, also for other types of lignin [29–30]. The

Fig. 4. Thermograms: (a) and (b) of native precursors, (c) and (d) of MgO�SiO2/lignin hybrid materials.

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thermogram shows a three-stage degradation process. The firststage, with a small mass decrease of around 11% up to a tempera-ture of 200 �C, involves chiefly the local elimination of water boundon the lignin surface. The second stage, with a mass loss of approx-imately 40% in the temperature range 200–580 �C, involves thecomplex thermal decomposition of the compound and theformation of new bonds as a result of cross-linking reactions. Thesignificant change in mass is also revealed by the sharp peak onthe DTG curve (Fig. 4b), with a maximum at approximately310 �C. Thermal processing above 600 �C (up to 1000 �C) at thethird stage causes a gradual loss of mass by a further 15%, causedby molecular fragmentation resulting from unclear and uncon-trolled reactions.

In the case of certain hybrid biocomposites (Fig. 4c and d) athree-stage decomposition process was also noted, similarly as inthe case of the pure biomodifier. The first change in mass involvesthe loss of water, while further thermal decompositions areresponsible for the formation of new bonds as a result of cross-linking reactions. The obtained MgO�SiO2/lignin composites havefairly high thermal stability. The only clear difference was notedin the case of biocomposite sample TS 7.1 (Fig. 4c), whose mass de-creases by 50% relative to the initial mass of the sample, thus dem-onstrating poorer thermal properties. This may indicate that thequantity of lignin (50 parts by weight) relative to the quantity ofmineral support is too large. However, there was found to be aclear improvement in the thermal stability of the hybrid materialsas compared with pure lignin, as is shown graphically in Fig. 4a.The favorable test results indicate that the biocomposites mightbe successfully used, among other things, as a new generation ofbiodegradable and relatively cheap polymer fillers.

In order to determine possible applications of a hybrid materialbased on lignin and the oxide system MgO�SiO2 as a biosorbent, the

parameters of its porous structure were evaluated. Fig. 5a showsthe nitrogen adsorption/desorption isotherms for the systemsobtained.

The character of the isotherms indicates a mesoporous struc-ture for both the pure mineral support MgO�SiO2 and for the hybridbiosorbents produced. It was observed that the quantity of nitro-gen adsorbed for all samples increases slightly up to a relativepressure of 0.8. After this value of p/p0 is exceeded, the quantity in-creases rapidly. This applies to all of the samples, although thegreatest increase in the quantity of adsorbed nitrogen was re-corded for the pure oxide system MgO�SiO2, which attained a valueof 465 cm3/g (at p/p0 = 1). It was also found that as the quantity oflignin used to prepare the biosorbent increased, the quantity of ad-sorbed nitrogen decreased. The greatest quantity of adsorbednitrogen (406 cm3/g) was recorded for sample TS 1.1, which wasobtained using the smallest quantity of lignin (3 wt./wt.). Basedon the adsorption and desorption isotherms, a determination wasmade of parameters such as BET surface area, pore volume andpore diameter. The studied samples were found to have relativelylarge surface area. The maximum value (514 m2/g) was recordedfor pure MgO�SiO2, and as the quantity of lignin in the structureof the biosorbent increased, the surface area of the hybrid systemsdecreased, reaching as little as 125 m2/g for sample TS 7.1. Thesame pattern was followed in the case of pore volume (Fig. 5b),the maximum value (0.72 cm3/g) being that of the MgO�SiO2 sys-tem. Pore diameter tended to increase with the quantity of ligninin the hybrid biosorbent; the largest pore diameter value(8.2 nm) was recorded for sample TS 5.1, which was made using30 wt./wt. of lignin.

It was thus concluded that the surface area of the hybrid biosor-bent is relatively large, and has a significant effect on its surfaceactivity and possible applications. It was observed that the porous

Fig. 5. N2 adsorption/desorption isotherms (a) and pore size distribution (b) of hybrid biosorbents TS 1.1, TS 3.1, TS 5.1 and TS 7.1 obtained using 3, 10, 30 and 50 parts byweight of lignin respectively.

Fig. 6. FT-IR spectra of (a) native precursors and (b) MgO�SiO2/lignin hybrid materials.

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structure parameters were determined to a significant degree bythe quantity of lignin used in obtaining the hybrid. It should alsobe noted that the biomodifier itself, whose advantage is the pres-ence of varied functional groups, has markedly inferior porousstructure parameters. The value for the surface area of lignin is just0.8 m2/g, the pore volume is 0.001 cm3/g and the pore diameter2.9 nm.

In addition, from observation of the relationships shown inFig. 5b, it was concluded that the lignin positioned itself effectively

in the pores of the mineral support, as is shown by the decreasingpore volumes with increasing contribution of biomaterial to theMgO�SiO2/lignin hybrid. The conclusion is also confirmed by thepore diameter values, which did not show significant changes. Thisproves the effectiveness of the proposed method of combining lig-nin with a mineral support. In spite of the reduction in the specificsurface area as the quantity of lignin in the hybrid materials in-creases, it should be noted that these systems may serve as usefulbiosorbents. This is because of the introduction of functional

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groups which are capable of interacting, for example, with toxicheavy metals.

3.2. Evaluation of efficiency of combination of lignin with mineralsupport

To confirm the effectiveness of the combination of lignin withthe mineral support, FT-IR spectral and elemental analyses wereperformed on the resulting hybrid materials.

The FT-IR spectra of the products were taken to check for thepresence of characteristic functional groups in their structure.Fig. 6a shows the FT-IR spectra of the MgO�SiO2 mineral supportand Kraft lignin (the precursors of the hybrid biosorbent).

The spectrum of MgO�SiO2 reveals the presence of the charac-teristic bonds of „SiAOASi„, represented by the bands at1096 cm�1 and 805 cm�1, which are characteristic for theirstretching vibrations. Moreover there is a band that correspondsto the stretching vibrations of the „SiAOH bond at 960 cm�1.The other bands, that are the one corresponding to stretchingvibrations of AOAH (3600–3200 cm�1) and one at �1630 cm�1,are attributed to water physically bound in the mineral supportstructure. The FT-IR spectrum of Kraft lignin shows bands attrib-uted to the stretching vibrations of AOAH (3600–3200 cm�1),„CAH (2960–2835 cm�1), ketone group @C@O (1710–1550 cm�1), and bands at 1600 cm�1, 1510 cm�1 and 1420 cm�1

attributed to stretching vibrations of the „CAC„ bonds in the aro-matic skeleton. There is another group of bands at 1375 cm�1,1265 cm�1, 1220 cm�1 and 1045 cm�1 corresponding to thestretching vibrations of „CAOA and ether bonds „CAOAC„.

Fig. 7. Predicted mechanism of the combination

The spectrum also shows a group of bands below 1000 cm�1,attributed to the in-plane and out-of-plane vibrations of aromatic„CAH bonds. All these data are in agreement with the literature[24].

The FT-IR spectrum of the MgO�SiO2/lignin hybrid biosorbents(Fig. 6b) confirms the effectiveness of the proposed method ofsynthesis. It is associated with an increase in the intensity of thebands, especially those at 1600 cm�1, 1510 cm�1 and below1000 cm�1, attributed to the corresponding lignin functionalgroups, compared with the spectrum of the unmodified mineralsupport. Moreover, a decrease in the intensity of the band at3600–3200 cm�1 can be considered evidence of chemical interac-tions between lignin and the mineral support. The decrease inthe intensity of this band is related to the quantity of lignin com-bined with the mineral support, the highest value being observedfor sample TS 7.1, prepared using 50 wt./wt. of lignin. In accor-dance with the results presented and discussed here, a predictedmechanism of the process was proposed (Fig. 7).

A further stage of tests having the aim of confirming the effec-tiveness of the proposed method of combination consisted of ele-mental analysis of the produced hybrid biocomposites. Thepercentage content of carbon and sulfur in the structure of thematerials was determined (Fig. 8). It was shown that the carboncontent increases proportionally to the quantity of lignin used toprepare the hybrids. In the case of sulfur, which may come fromthe reagents used in the precipitation of the mineral support, butmay also result from the presence of the element in the structureof lignin, much smaller quantities were recorded, and conse-quently there was only a small increase in its percentage content

of lignin with MgO�SiO2 mineral support.

Fig. 8. Elemental contents of carbon and sulfur in the structure of MgO�SiO2/ligninhybrid material vs. amount of lignin used in the preparation process.

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in the product. On the other hand, the increase in the sulfur con-tent observed with increasing quantities of lignin used to preparethe hybrid biosorbents proves that the main source of this elementis in fact the lignin. This provides further conclusive confirmationof the effectiveness of the proposed method for obtaining func-tional materials based on lignin and the mineral support MgO�SiO2,similarly as in the case when silica was used as the support [24].

The results obtained indicate conclusively the possibility ofusing lignin in the preparation of hybrid systems with specificphysicochemical properties. Although the tests were carried outusing commercially available lignin, they are of huge significancein terms of applications, since they may also lead to the recyclingof significant quantities of this waste biomodifier in specific tech-nological processes.

4. Conclusions

The proposed new method for combining lignin with a min-eral oxide support has enabled the obtaining of valuable hybridsystems with specific properties. It was confirmed that the phys-icochemical properties of the materials produced are stronglydependent on the quantity of lignin used in their preparation.By controlling the contribution of lignin to the mineral matrix,hybrid systems were obtained with diversified particle size andirregular particle shape. Also the combination of lignin with theinorganic support MgO�SiO2 caused a change in its color, whichmay be of significance for potential applications, particularlythose in which color properties play an important role. Also ofgreat importance in this regard are the results of the tests ofelectro-kinetic and thermal properties, which confirmed the highstability of the inorganic/organic hybrid systems, particularly inspecific conditions of thermal processing or pH. Changes in theproperties of the mineral support following its combination withlignin, and the effectiveness of the process, were confirmed byFT-IR tests and the results of elemental analysis. In view of thelarge surface area of the mineral support and the variety of func-tional groups in the structure of lignin, systems of this type mayserve as hybrid biosorbents, as is confirmed by the analysis oftheir porous structure. It has again been confirmed that by select-ing an appropriate value for the contribution of lignin to the inor-ganic matrix, it is possible to efficiently design hybrid materialswith defined physicochemical and utility properties. In view ofthe unique properties of the biosorbents obtained, they will con-tinue to be studied in a further stage of research, which will dealwith the adsorption of selected heavy metals and toxic organiccompounds.

Acknowledgement

The study was financed within the Polish National Centre of Sci-ence funds according to Decision No. DEC-2011/03/D/ST5/05802.

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