Accepted Manuscript

Preparation of carbon adsorbents from lignosulfonate by phosphoric acid acti-

vation for the adsorption of metal ions

M. Myglovets, O.I. Poddubnaya, O. Sevastyanova, M.E. Lindström, B.

Gawdzik, M. Sobiesiak, M.M. Tsyba, V.I. Sapsay, D.O. Klymchuk, A.M. Puziy

PII: S0008-6223(14)00884-7

DOI: http://dx.doi.org/10.1016/j.carbon.2014.09.032

Reference: CARBON 9329

To appear in: Carbon

Received Date: 10 June 2014

Accepted Date: 13 September 2014

Please cite this article as: Myglovets, M., Poddubnaya, O.I., Sevastyanova, O., Lindström, M.E., Gawdzik, B.,

Sobiesiak, M., Tsyba, M.M., Sapsay, V.I., Klymchuk, D.O., Puziy, A.M., Preparation of carbon adsorbents from

lignosulfonate by phosphoric acid activation for the adsorption of metal ions, Carbon (2014), doi: http://dx.doi.org/

10.1016/j.carbon.2014.09.032

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1

PREPARATION OF CARBON ADSORBENTS FROM LIGNOSULFONATE BY 1

PHOSPHORIC ACID ACTIVATION FOR THE ADSORPTION OF METAL IONS 2

3

M. Myglovets1,2, O.I. Poddubnaya1, O. Sevastyanova3, M.E. Lindström3, 4 B. Gawdzik4, M. Sobiesiak4, M.M. Tsyba1, V.I. Sapsay5, D.O. Klymchuk5, 5 A.M. Puziy1*

7 6

1 Institute for Sorption and Problems of Endoecology, NASU, Kyiv, Ukraine 8 2Taras Shevchenko National University of Kyiv, Kyiv, Ukraine 9

3The Royal Institute of Technology, KTH, Department of Fiber and Polymer 10 Technology, Stockholm, Sweden 11

4 Maria Curie-��������� University, Lublin, Poland 12 5 M.G. Kholodny Institute of Botany, National Academy of Sciences of Ukraine, 2 13

Tereshchenkivska St., 01601 Kyiv, Ukraine 14 15 16

Abstract 17

Activated carbons were prepared from sodium lignosulfonate by phosphoric acid 18

activation at carbonization temperatures of 400-1000 °C. The resulting materials were 19

characterized with regard to their surface area, pore volume, pore size distribution, 20

distribution of surface groups and ability to adsorb copper ions. Activated carbons were 21

characterized by nitrogen adsorption, scanning electron microscopy, Fourier transform 22

infrared spectroscopy and thermal gravimetric analyses. The results indicate that with 23

increasing carbonization temperature, the surface area decreased from 770 m2/g at 400 °C 24

to 180 m2/g at 700 °C and increased at higher temperatures to 1370 m2/g at 1000 °C. The 25

phosphorus content peaked at 11% for carbon obtained by carbonization at 800 °C. 26

Potentiometric titration revealed the acidic character of all the phosphoric acid-activated 27

* Corresponding author e-mail: alexander.puziy@ispe.kiev.ua fax: +38-044-4529325

2

carbons, which were found to have total concentrations of surface groups of up to 28

3.3 mmol/g. The carbons showed a high adsorption capacity for copper ions even at pH 29

values as low as 2. 30

1. Introduction 31

Driven by the absolute necessity of a fossil fuel-independent future with a 32

bioeconomy based on natural feedstock, an interest in the development of technologies for 33

upgrading biowaste has emerged. The conversion of lignin to marketable value-added 34

products is an essential part of the integrated biorefinery concept [1]. Lignin is one of the 35

main components of lignocellulosic materials and is the second most abundant 36

macromolecule in nature [2]. Native lignin is an amorphous and physically and chemically 37

heterogeneous polyphenol material [3]. Technical lignins are generated as by-products in 38

pulp and paper manufacturing and in biomass pre-treatment processes, such as in kraft, 39

soda, sulfite, organosolv, and hydrolysis methods, and are readily available in large 40

quantities. The structure and properties of such lignin molecules vary depending on the 41

wood species from which they are derived and, to an even greater extent, on the industrial 42

process employed to isolate them [4]. Lignin plays an important role in the chemical 43

recovery process and is commonly used as a low-grade fuel for pulping operations. However, 44

the potential of lignin as a raw material to use in value-added applications is underexploited 45

- only 1-2% of the various known technical lignins are used in non-fuel high-value 46

applications. 47

As a polyaromatic macromolecule with a carbon content greater than 60%, lignin 48

may provide the high carbon yield required for the commercial manufacturing of activated 49

carbon and can partly substitute for non-renewable coal-based resources. Lignin can be 50

3

converted into activated carbon by physical or chemical activation, where the latter is more 51

preferable because it requires lower activation temperatures and gives higher product 52

yields. From both economic and environmental perspectives, phosphoric acid activation is 53

the preferred processing method because the activation temperature required is relatively 54

low compared with physical activation and because the phosphoric acid can be recovered 55

post-activation [5]. Over the past few decades, the H3PO4 activation of lignin has been 56

extensively studied owing to its low cost and high availability [1,6]. In most studies, kraft 57

lignin was used [6–13]. Preparation of carbon adsorbents by phosphoric acid activation using 58

other types of lignin such as Alcell lignin [14,15] and brewer’s spent lignin [16] has also been 59

reported. It has been shown that lignin is a suitable precursor for the preparation of carbon 60

adsorbents for the adsorption of phenol [9,12], trichlorophenol and chromium (VI) [9], other 61

phenolic compounds and metal ions [16]. Carbons obtained from lignin by the phosphoric 62

acid activation method have an acidic character [10] and show catalytic activity in acid-63

catalyzed reactions like the dehydration of 2-propanol [14]. Phosphorus-containing carbons 64

produced from Alcell lignin have also shown enhanced oxidation resistance [15]. 65

Lignosulfonates still have not been adequately explored as precursors for the 66

preparation of carbon adsorbents. Lignosulfonates (LS) are obtained as by-product of sulfite 67

pulping, in which the delignification of wood is performed through the use of HSO3- and SO3

2- 68

ions [17]. As result, lignosulfonates are water-soluble anionic polyelectrolytes that contain a 69

large number of charged groups. The degree of sulfonation of lignosulfonate molecules is 70

0.4-0.5 per phenylpropane unit [17]; they have relatively high molecular weights with a 71

broad distribution and a quite high ash content [4]. Lignosulfonates are produced in 72

relatively large quantities, totaling approximately 2 million tons per year as dry solids [18]. 73

The production of lignosulfonates has been commercialized by many companies; the major 74

4

producer is Borregaard LignoTech, with a capacity of approximately 500,000 metric tons per 75

year [4]. 76

In this study, the impact of the pyrolysis temperature and impregnation ratio on the 77

porous structure and surface chemistry of carbon adsorbents produced from sodium 78

lignosulfonate were investigated. In addition, the adsorption of copper into these structures 79

was studied. 80

2. Experimental 81

2.1. Activated carbon preparation 82

High-molecular-weight softwood sodium lignosulfonate was used as the precursor 83

for the preparation of activated carbons. Lignosulfonate has a degree of polymerization of 84

1650, an ash content of 16%, and a degree of sulfonation of approximately 0.5 per 85

phenylpropane unit. The lignosulfonate was obtained from Borregaard LignoTech 86

(Sarpsborg, Norway). The lignosulfonate was impregnated with 60% phosphoric acid at an 87

impregnation ratio of 1:1, dried in air at 110 °C for 1 h and then carbonized in a flow of argon 88

(0.5 L/min) at temperatures in the range of 400–1000 °C for 30 min. After carbonization, the 89

carbons were extensively washed with hot water in a Soxhlet extractor until a neutral pH of 90

wash waters. On average, the washing procedure lasted approximately 15 hours with 91

automatically changing wash waters that refreshed every 15-20 min. After washing, the 92

carbons were dried at 110 °C for 24 hours. The carbons were abbreviated as PXXX, where 93

XXX refers to the carbonization temperature. For comparison purposes, a carbon C800 94

samples was obtained by carbonization of the same lignosulfonate precursor at 800 °C 95

without the addition of phosphoric acid prior to heating. 96

5

2.2. Characterization methods 97

2.2.1. Porous structure 98

The porous structures of the carbons produced were characterized by nitrogen 99

adsorption–desorption isotherms measured at �196 °C in the 0.001–1 relative pressure 100

range using an Autosorb-6 gas adsorption analyzer (Quantachrome, USA). Prior to 101

measurement, all the samples were degassed overnight at 200 °C. Pore size distributions 102

were calculated by Autosorb-1 software (Quantachrome, USA) using the QSDFT method and 103

the slit/cylindrical pore model [19]. The surface area (SBET) was calculated by the standard 104

BET method using nitrogen adsorption data in the relative pressure range from 0.01 to 0.10 105

because deviations from linearity of the BET plot were observed at relative pressures below 106

0.01 and above 0.10. The total pore volume (Vtot) was calculated by converting the amount 107

of nitrogen adsorbed at a maximum relative pressure to the volume of liquid adsorbate. The 108

micropore (Vmi) and mesopore (Vme) volumes were calculated from the cumulative pore size 109

distribution as the volume of pores with sizes less than 2 nm and between 2 and 50 nm, 110

respectively. 111

2.2.2. Thermogravimetric analysis 112

Thermogravimetric analysis (TGA) was carried out using a Netzsch STA 449 F1 Jupiter 113

(Netzsch) thermal analyzer over a temperature range of 50 to 1000 °C with a heating rate of 114

10 °C/min. Approximately 5 mg of a sample was placed in an Al2O3 crucible and studied 115

under a helium atmosphere. The flow rate of helium was 40 mL/min. 116

2.2.3. SEM imaging 117

In order to perform SEM analysis of the carbon samples, the dried specimens were 118

sputter-coated with a gold film with a thickness of 5 ± 2 nm. Specimens were then examined 119

6

in a JSM 6060 LA scanning electron microscope (JEOL, Japan) at an accelerating voltage of 30 120

kV. 121

2.2.4. Surface chemistry 122

The carbons were characterized by energy-dispersive X-ray spectroscopy (EDX) using 123

an EX-54175 JMU accessory coupled to the previously mentioned scanning electron 124

microscope (JEOL, Japan). A ZAF standard-less correction [20] was applied to the raw data 125

using the CalcZAF software (Probe Software, USA)†

Attenuated total reflectance (ATR) spectra were obtained using a Bruker FTIR 127

spectrophotometer TENSOR 27. 128

. 126

The identity and quantity of the surface groups on the carbons were investigated by 129

potentiometric titration at 25 °C in a thermostatic vessel under a flow of argon [21,22]. First, 130

0.1 g of carbon was placed in 20 mL of 0.1 M NaCl solution, equilibrated for at least 8 hours 131

and then titrated with 0.1 M NaOH (for basic titration) or 0.1 M HCl (for acidic titration) 132

using a 672 Titroprocessor combined with a 655 Dosimat (Metrohm, Switzerland). The 133

proton concentration was monitored using an LL pH glass electrode (Metrohm, Switzerland). 134

The electrode electromotive force was calibrated to the proton concentration by a blank 135

titration. The amount of protons adsorbed at each titration point was calculated using the 136

following equation: 137

� = ������ ([��] [���] [��] + [���] ) (1)

where V0 and Vt are the volume of background electrolyte and the volume of added titrant, 138

respectively, and m is the mass of adsorbent. The subscripts i and e refer to the initial and 139

† http://www.probesoftware.com/Technical.htm

7

equilibrium ion concentrations. Solution equilibria and a correction factor for possible 140

carbonate and silicate contamination were calculated using EST software [23]. 141

Proton affinity distributions (F(pK)) were calculated by solving the adsorption integral 142

equation using the CONTIN method [21,24]: 143

� ��(��) = � ����(��, ��)�(��)������������ (2)

where Qexp is the experimentally measured proton binding, Qloc is a kernel function 144

describing the local proton binding to surface center with a given pK, F(pK) is the sought 145

after distribution function, also known as the proton affinity distribution (PAD), and pKmin 146

and pKmax are the integration limits. The distribution function, F(pK), describes the site 147

concentration as a function of the proton binding constant and is characteristic of the 148

adsorbent material used. To reproduce both the positive and negative parts of the proton 149

binding isotherm, a combination of two Langmuir equations was used as a local isotherm: 150

���� =��� �������

� + ������� , �� � �� �������

� + ������� , �� ! �� "#$

(3)

where PZC is the point of zero charge. 151

2.2.5. Cu(II) adsorption 152

To investigate Cu(II) adsorption to the carbon samples, 0.1 g of carbon was shaken 153

with 20 mL of 0.001 M Cu(NO3)2 solution containing 0.1 M NaCl as the background 154

electrolyte. The pH of the solution was adjusted by the addition of 0.1 M HCl or 0.1 M NaOH. 155

After equilibration for 24 h, the pH was measured with a pH glass electrode, and the copper 156

concentration was determined by titration with standard EDTA solution. The results are 157

8

presented below as the relative amount of copper adsorbed on the carbon, calculated by the 158

relation 1-C/C0, where C is the equilibrium concentration and C0 is the initial concentration. 159

3. Results and Discussion 160

3.1. Carbonization 161

Carbonization of lignosulfonate at 400-800 °C in the presence of H3PO4 resulted in 162

the formation of a carbonaceous residue in yields of 49-55% (Table 1). At higher 163

temperatures, the yield progressively decreased due to thermal degradation of the 164

phosphocarbonaceous species and formation of volatile phosphorous-containing 165

compounds (phosphorus(V) oxide and elemental phosphorus) as a result of reduction of the 166

phosphates present to elemental phosphorus [25]. Investigations into the oxidation 167

protection conferred by phosphorous functionalities also showed that the phosphorus 168

compounds tend to evaporate from the carbon surface at temperatures higher than 800 °C 169

[26,27]. Formation of elemental phosphorus was also observed during the phosphoric acid 170

activation of Nomex polymer fibers [28] and during pyrolysis of phosphorus-containing 171

phenol resins [29,30]. Volatile phosphorus compounds may be formed by the following 172

reactions: 173

4 H3PO4 + 10 C = P4 + 10 CO + 6 H2O (4)

4 H3PO4 + 10 C = P4O10 + 6 H2O (5)

P4O10 + 10 C = P4 + 10 CO (6)

Thermodynamic analysis showed that the Gibbs free energy for each of the reactions 174

that could occur during phosphoric acid activation is negative at temperatures above 750 °C 175

(Fig. 1), indicating that their occurrence is favorable (spontaneous) [31]. It should be noted 176

9

that the reduction of phosphates is the industrial process utilized for the production of 177

elemental phosphorus [32]. A decrease in yield at temperatures higher than 800 °C was also 178

observed for carbons prepared from other carbonaceous precursors, including various 179

polymers [33–35] and fruit stones [36,37]. The addition of phosphoric acid to lignosulfonate 180

prior to carbonization increased the yield of carbon at 800 °C by 7.4%. 181

The carbon yields achieved in the present study (50-55% at 400-800 °C) are quite 182

comparable to those of carbons obtained by phosphoric acid activation of other types of 183

lignin. The yield of carbons derived from kraft lignin was reported to be in the range 50-88% 184

at 400 °C [6,10,11]. A yield of 44% was obtained at 500 °C for carbon produced from Alcell 185

lignin [15]. The high carbon yield achieved in this study suggests that there is great potential 186

in using lignosulfonate for the production of activated carbons. 187

188

189

Fig. 1. Temperature dependence of the Gibbs free energy of the reactions that can occur 190 during phosphoric acid activation. 191

192

-1000

-500

0

500

1000

1500

2000

0 200 400 600 800 1000

�G°T

, kJ/

mol

Temperature, °C

4 H3PO4 + 10 C = P4 + 10 CO + 6 H2O 4 H3PO4 = P4O10 + 6 H2O P4O10 + 10 C = P4 + 10 CO

10

3.2. SEM 193

SEM imaging shows that lignosulfonate forms pebble-like granules with a smooth 194

surface (Fig. 2). Carbonization of lignosulfonate in the absence of phosphoric acid (carbon 195

C800) led to a loose swollen structure, whereas carbons obtained by phosphoric acid 196

activation had a foam-like structure with a smooth surface. Because a foam-like structure 197

was observed for the carbons obtained at all temperatures, it is evident that this structure 198

formed during the drying step before carbonization. Formation of foam was observed 199

visually – the volume of the lignosulfonate and phosphoric acid mixture increased several 200

times during drying at 110 °C. It should be noted that the foam-like structure and smooth 201

surface of the carbons derived from lignosulfonate are markedly different than the features 202

of carbons produced from kraft lignin, which form as agglomerated particles with rough 203

surfaces[6]. The difference may be ascribed to the solubility of lignosulfonate in water and in 204

phosphoric acid. When the mixture of lignosulfonate and phosphoric acid was dried at 205

110 °C, the formation of foam was observed. The foam-like structure was preserved during 206

carbonization at high temperatures as well. 207

Lignosulfonate C800

11

P400 P500

P600 P700

P800 P900

12

P1000

Fig. 2. SEM images of lignosulfonate and the carbons obtained from it. 208

209

3.3. Chemical composition 210

Energy dispersive X-ray analysis indicated the presence of carbon, oxygen and 211

phosphorus in all of the carbons (Table 1). With increasing carbonization temperatures up to 212

700 °C, the carbon content decreased at the cost of increasing oxygen and phosphorus 213

contents. A further increase in the carbonization temperature resulted in an increasing 214

content of carbon. The oxygen content decreased with a carbonization temperature rise 215

from 400 to 500 °C due to the dehydration action of phosphoric acid [38]. The increasing 216

oxygen content within the temperature range of 500-700 °C, on the other hand, was due to 217

the progressive formation of phosphates/polyphosphates bound to the carbon via C-O-P 218

linkages [25,38–40]. At higher temperatures, the oxygen content decreased due to the 219

evaporation of the phosphorus compounds as a result of the rupture of the C-O-P linkages. 220

In a similar trend, with increasing carbonization temperature up to 800 °C, the phosphorus 221

content of the carbons increased to a maximum and decreased at higher temperatures 222

(Table 1). Again, the increasing phosphorus content was due to the progressive formation of 223

polyphosphate ethers, while the decreasing phosphorus content was due to both the 224

13

evaporation of phosphorus compounds because of the thermal instability of the C-O-P 225

linkage and phosphate reduction by carbon [25,28,31]. The same trends were observed for 226

other phosphoric-acid-activated carbons obtained from polymer [33–35] and lignocellulosic 227

[36,37] precursors. The maximum amount of phosphorus introduced to the carbon formed 228

from lignosulfonate, at 10.6% (Table 1), is quite comparable to the 8.5-12.2% achieved for 229

carbons obtained at 800 °C from polymer [33–35] and lignocellulosic [36,37] precursors. This 230

fact indicates the high reactivity of lignosulfonate towards phosphoric acid. It should be 231

noted that the lower phosphorus content (1.5%-5.7%) reported for lignin-based carbons was 232

almost certainly due to the low carbonization temperature employed in those studies, 233

approximately 500 °C [10,14,15]. 234

The calculated O/P atomic ratios for these experiments follow the trends observed 235

for carbons obtained from polymer [33] and lignocellulosic precursors [36]: a drastic 236

decrease in the ratio occurs with increasing carbonization temperatures up to 600 °C, 237

followed by a much slower decrease in the ratio down to a value of 3 at 800 °C. The O/P 238

atomic ratio achieved at 800 °C is close to that present in polyphosphates with the general 239

formula Hn+2PnO3n+1, where n = �������is evidence of the formation of polyphosphates with 240

a high degree of polymerization at high temperatures during chemical activation. Recently, 241

direct evidence of polyphosphates in phosphorus-containing carbons was obtained by mass 242

spectrometry [39]. 243

EDX analysis did not reveal the presence of sulfur in any of the carbons, which could 244

be present in the carbon structure because the precursor material contains sulfonic groups 245

[17]. Chemical analysis revealed there to be 6.2% of sulfur in lignosulfonate (Table 1). The 246

sulfur content decreased 2.5 times upon carbonization of lignosulfonate at 800 °C without 247

14

the addition of phosphoric acid. Phosphoric acid activation of lignosulfonate caused a 248

decrease of one order of magnitude in the sulfur content. Higher amounts of sulfur in the 249

acid-free carbon indicate that phosphoric acid promotes the removal of sulfur from the 250

lignosulfonate structure. With increasing carbonization temperatures, the content of sulfur 251

was found to decrease in the phosphoric-acid-activated carbons. 252

A small amount of sodium originating from the precursor was also observed in the 253

carbons that were obtained at temperatures higher than 800 °C. The presence of sodium 254

could be explained by the formation of phosphate glass [41], a compound that is chemically 255

stable and could survive extensive washing after carbonization. This is supported by the 256

formation of phosphorus pentoxide in this temperature range [25], which could form 257

phosphate glass with the necessary sodium coming from the lignosulfonate precursor. 258

259

260

261

262

263

Table 1. Chemical composition of phosphoric-acid-activated carbons made from 264 lignosulfonate. 265

Carbon Yield %

C %

O %

Na %

P %

P/O C* %

H* %

N* %

S* %

Lignosulfonate 49.5 4.52 0.17 6.19 P400 55.0 80.6 17.0 0.03 2.1 15.3 79.0 2.96 0.31 0.67 P500 49.2 81.3 16.7 0.03 3.0 10.7 78.9 2.71 0.30 0.63 P600 51.4 78.6 17.4 0.04 7.0 4.0 77.8 2.28 0.26 0.45 P700 53.4 70.4 18.99 0.05 10.6 3.7 71.0 1.52 0.24 0.38 P800 48.6 73.8 15.3 0.25 17 2.8 71.7 0.78 0.27 0.34 P900 36.0 73.2 14.4 2.81 8.7 3.2 70.3 0.15 0.58 0.34

15

P1000 26.2 82.6 11.2 1.79 4.4 5.8 77.5 0.25 0.66 0.18 ���� 41.2 56.7 0.26 0.59 2.52

* obtained by chemical analysis 266

267

3.4. Thermogravimetric analysis 268

Thermogravimetric analysis shows that the mass loss was greater for the carbons 269

obtained without the addition of phosphoric acid (Fig. 3a). Carbon C800 retained 49% of its 270

original mass at the highest temperature tested of 985 °C, whereas carbon P800 obtained at 271

the same temperature in the presence of phosphoric acid retained 65% of its carbonaceous 272

mass. The temperature at which these carbons reached a mass loss of 20% differed by more 273

than 200 °C, as it was 636 °C for carbon C800 and 847 °C for P800. This indicates that 274

phosphoric acid promotes the transformation of lignosulfonate to thermally stable 275

carbonaceous materials. Differential thermogravimetric (DTG) curves (Fig. 3b) show that 276

thermal degradation began at approximately 300 °C for carbon C800 and at 500 °C for 277

carbon P800. 278

All phosphoric-acid-activated carbons showed appreciable mass loss in the 800 -279

900 °C temperature range, peaking in the 850-880 °C range (Fig. 3b). The most intense mass 280

loss was observed for carbons with the highest amount of phosphorus, which corroborates 281

the earlier attribution of the mass loss to the evaporation of phosphorus compounds by 282

thermal decomposition of the C-O-P linkage and phosphate/polyphosphate reduction by 283

carbon [28,35,42]. Indeed, a perfectly linear relationship was observed between the mass 284

loss in the 800 - 900 °C temperature range and the phosphorus content of the carbons 285

obtained at temperatures of 600 °C and higher. Carbons obtained at the lowest 286

carbonization temperatures (400 and 500 °C) as well as the acid-free carbon C800 deviate 287

16

from this trend because the mass loss for these carbons is determined by oxygen-containing 288

functionalities not bound to phosphorus. The thermal degradation of phosphocarbonaceous 289

compounds at temperatures above 830-850 °C has also been reported for other phosphorus-290

containing carbons [43–45]. 291

292

293

294

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800 900 1000

Mas

s, %

Temperature, °C

P400 P800 P1000 C800

(a)

-0.16

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0 100 200 300 400 500 600 700 800 900 1000

Mas

s/t,

%/°

C

Temperature, °C

P400 P800 P1000 C800

(b)

17

Fig. 3. TG (a) and DTG (b) curves of phosphoric acid-activated carbons produced from 295 lignosulfonate. 296

297

298

Fig. 4. Dependence of mass loss in the 800-900 °C temperature range on the phosphorus 299 content of phosphoric acid-activated carbons produced from lignosulfonate. 300

301

3.5. FTIR 302

The FTIR spectra of the sodium lignosulfonate precursor, the carbons obtained 303

without acid pretreatment, and the seven phosphoric acid-activated carbons produced here 304

were recorded over the 400-4000 cm-1 region (Fig. 5). 305

The FTIR spectra for lignosulfonate showed features typical of lignins, including a 306

high-intensity absorption band between 3000 and 3600 cm-1 that peaked at 3455 cm-1 and 307

was attributed to the –OH vibrational frequencies of hydroxyls bound to lignin as well as 308

those on -SO3H groups; bands in the 2800-3000 cm-1 region from the C-H stretches of methyl 309

and methylene groups; signals between 1400 and 1700 cm-1 attributed to aromatic skeletal 310

P400

P500 P600

P700

P800

P900

P1000

C800

R² = 0.9931

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8 9 10 11 12

Mas

s los

s, %

P content, %

18

vibrations (1423, 1464, 1512 and 1605 cm-1); and a high-intensity peak at 1037 cm-1 311

attributed to aromatic C-H in-plane deformations. Signals in the 1100-1300 cm-1 region 312

typically arise from aromatic C-H in-plane deformations but also from C-O deformations in 313

secondary alcohols and aliphatic ethers, as well as C=O stretching of esters. The signal 314

observed at 1267 cm-1 on the lignosulfonate spectrum is characteristic of the quaiacyl ring 315

found in softwood lignin [3]. The distinct band containing characteristic peaks at 523 cm-1 316

and 652 cm-1 that appears in the 450-660 cm-1 region of the lignosulfonate spectrum was 317

assigned to the S-O stretching vibration of sulfonic groups present on the aliphatic side chain 318

of the lignosulfonate [46]. 319

As the result of the carbonization at 800 °C, some changes in the FTIR spectra, mostly 320

related to the intensities of the signals, could be observed. The high-intensity signal in the 321

3000-3600 cm-1 region observed for untreated lignosulfonate was transformed into a 322

broader and weaker signal in the 2600-3600 cm-1 region with a maximum at 3420 cm-1 for 323

carbon C800, illustrating the decrease in the number of OH-groups present after 324

carbonization. Bands from aromatic skeletal vibrations (1679 cm-1), C-H deformations in –325

CH3 and –CH2- (1450 cm-1), aromatic C-H in-plane deformations (1114 cm-1) and aromatic 326

C-H out-of-plane deformations (in the region 800-1000 cm-1) could clearly be observed for 327

carbon C800. Furthermore, two distinct peaks at 551 and 617 cm-1 observed for the carbon 328

C800 could be assigned to –SO2- bending vibrations [47]. 329

The FTIR spectrum of all of the phosphoric-acid-activated carbons showed much less 330

intense absorption bands than C800 carbon (Fig. 5a). This clearly shows the activating 331

function of phosphoric acid that facilitates a restructuring of the organic precursor to a 332

carbon structure. 333

19

The FTIR spectra of carbons obtained by phosphoric acid activation of lignosulfonate 334

at different temperatures (Fig. 5a) show multiple functionalities that are similar to those 335

observed in the case of carbons prepared from fruit stones [36,37] and polymers [33,34]. 336

The broad absorption band at 3600–3200 cm-1 with a maximum at approximately 3420 cm-1 337

is characteristic of the stretching vibration of hydrogen-bonded hydroxyl groups of 338

carboxyls, phenols or alcohols. All the spectra show an absorption band at 1570-1630 cm-1 339

due to combined stretching vibrations of conjugated C=O groups and aromatic rings [48]. 340

The unconjugated C=O stretching vibrations appear at 1698-1713 cm-1. The absorption band 341

at 1759-1767 cm-1 was assigned to carboxylic acid vibrational frequencies. 342

All phosphoric-acid-activated carbons have FTIR spectra with absorption bands at 343

1160-1180, 1060 cm-1 and 950-990. These bands are characteristic of phosphorus and the 344

phosphocarbonaceous compounds present in the phosphoric acid-activated carbons [49–345

52]. The peak at 1151-1180 cm-1 can be assigned to the stretching vibration of hydrogen-346

bonded P=O groups [51,52] of phosphates or polyphosphates, to the O–C stretching 347

vibration in the P–O–C(aromatic) linkage [see also web service‡

‡

], and to P=OOH [web 348

service ]. The peak at 1060-1066 cm-1 could be due to a combination of the P+–O- bond in 349

acid phosphate esters [51] and the symmetrical vibration of the polyphosphate chain P–O–P 350

[53,54]. The peak at 1000–990 cm-1 may be due to aliphatic P–O–C stretching [51,52,55], 351

aromatic P–O–C asymmetric stretching [52], P–O stretching in >P=OOH [52], P–OH bending 352

[51], P–O–P asymmetric stretching in polyphosphates [52,55], and symmetrical stretching of 353

PO present in phosphate–carbon complexes [54]. It should be noted that the carbonization 354

of lignosulfonate treated with phosphoric acid eliminated signals previously attributed to 355

‡ http://www.science-and-fun.de/tools/

20

sulfonic groups in the spectrum of lignosulfonate and sulfonyl groups in the spectrum of 356

acid-free carbon C800. 357

358

21

359

360

0 500 1000 1500 2000 2500 3000 3500 4000

Tran

smitt

ance

, %

Wavenumber, cm-1

P400

P500

P600 P700 P800

P900 P1000

C800

LS

(a)

P400

P500 P600

P700

P800

P900 P1000

C800

PC2

(16%

)

PC1 (71%)

(b)

22

Fig. 5. FTIR spectra of phosphoric-acid-activated carbons from lignosulfonate precursor 361 (a) and a PCA score plot, obtained from the first two principal components (PC1 362 vs. PC2) of the FTIR spectra. 363

364

Principal component analysis (PCA) was used to discriminate between carbons 365

obtained by different methods [56]. The PCA score plot obtained from the first two principal 366

components (PC1 vs. PC2) is shown in Fig. 5b. PC1 describes 71% of the total variance in the 367

data, while PC2 describes 16%; the two PCs together express 87% of the original 368

information. A clear discrimination between the two preparation methods is obtained. All of 369

the phosphoric-acid-activated carbons are located along a line, while the acid-free carbon 370

C800 occurs far from this trend on the plot. PCA analysis gives additional evidence of the 371

dissimilarity between phosphoric acid-activated carbons and thermally treated carbon 372

produced without the addition of an activating agent. 373

3.6. Potentiometric titration 374

The surface groups of the carbons were further investigated by the potentiometric 375

titration method [21]. Proton-binding isotherms (Fig. 6a) show positive (proton adsorption) 376

and negative (proton desorption or dissociation) portions with a point of zero charge (PZC) 377

where the degrees of adsorption and desorption of protons are equal. These isotherms 378

revealed great differences between the carbons obtained with and without the addition of 379

phosphoric acid. The isotherm for the acid-free carbon C800 (Fig. 6a) was mostly in the 380

positive region, corresponding to proton adsorption and to the formation of a positive 381

charge at a pH lower than the PZC, which occurred at 8.4. The phosphoric acid-activated 382

carbons showed the opposite trend – proton-binding isotherms were located mostly in the 383

negative region, indicating that the surface groups dissociate protons. The PZC of the 384

23

phosphoric acid-activated carbons was in the acidic region at pH 2 to 3. This means that the 385

phosphoric acid-activated carbons are negatively charged at pHs higher than the PZC and 386

could thus adsorb cations like ion-exchange resins in this range. 387

388

389

Fig. 6. Proton-binding isotherms (a) and proton affinity distributions (b) for phosphoric 390 acid-activated carbons derived from lignosulfonate. 391

392

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1 2 3 4 5 6 7 8 9 10 11 12

Q, m

mol

/g

pH

P400

P800

C800

(a)

0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 5 6 7 8 9 10 11 12 13

F(pK

)

pK

P400

P800

C800

(b)

24

39

3

Tabl

e 2.

Cha

ract

eris

tics o

f the

surf

ace

grou

ps o

f pho

spho

ric a

cid-

activ

ated

car

bons

form

ed fr

om li

gnos

ulfo

nate

. 39

4

Carb

on

PZC

Qa

mm

ol/g

Q

1 m

mol

/g

pK1

Q2

mm

ol/g

pK

2 Q

3 m

mol

/g

pK3

Q4

mm

ol/g

pK

4 Q

5 m

mol

/g

pK5

Q6

mm

ol/g

pK

6 Q

7 m

mol

/g

pK7

P400

3.

30

1.40

0.

14

3.50

0.

11

5.38

0.

13

6.85

0.

05

7.76

0.

25

9.01

0.

72

10.3

2 P5

00

2.98

1.

37

0.16

3.

19

0.11

5.

53

0.14

6.

83

0.07

7.

82

0.33

9.

45

0.56

10

.77

P600

2.

60

1.64

0.

28

2.72

0.

20

4.76

0.

28

6.49

0.

13

7.92

0.

43

9.66

0.

33

11.0

2 P7

00

2.13

1.

77

0.37

2.

36

0.25

4.

23

0.22

5.

63

0.27

6.

97

0.19

8.

95

0.47

9.

83

0.40

11

.52

P800

1.

97

3.27

0.

66

2.12

0.

30

4.25

0.

50

6.37

0.

18

8.13

0.

79

9.84

0.

84

11.3

4 P9

00

2.90

1.

89

0.23

3.

10

0.19

4.

25

0.16

5.

54

0.39

6.

68

0.09

8.

05

0.26

9.

42

0.56

10

.54

P100

0 6.

81

1.54

0.

20

6.90

0.

22

8.89

1.

11

10.2

8 C8

00

8.36

1.

75

0.26

9.

01

1.49

10

.07

39

5

25

Proton affinity distributions show surface groups that can undergo dissociation 396

(Fig. 6b). There are up to seven types of such surface groups present on phosphoric acid-397

activated carbons from lignosulfonate (Table 2, Fig. 6b). The commonly accepted procedure 398

for assigning a chemical structure to the surface groups is based on a comparison of the 399

dissociation constants of surface groups with those of simple compounds. This classification 400

is, however, conditional and does not give explicit information regarding the nature of the 401

surface groups because the proposed compounds for comparison have pKs in broad and 402

overlapping regions. For example, the pKs of carboxylic groups typically fall between 0.7 and 403

7.5, the pKs of enol groups are between 5.8 and 10.7, and the pKs for phenols lie between 404

7.6 and 10.3 [57]. Lactones are expected to hydrolyze resulting in the formation of carboxylic 405

acid at pHs higher than 8 [58]. Thus, surface groups of types 1-4 may be carboxylic, surface 406

groups of types 3-7 may be enolic, surface groups of types 5-7 may be phenolic, and surface 407

groups of types 6-7 may be lactones. In addition, carbons obtained by phosphoric acid 408

activation contain a significant amount of phosphorus (Table 1) in the form of phosphate 409

and polyphosphate groups bound to carbon [25,39,59]. Taking into account the dissociation 410

constants of phosphate groups [57], surface groups of type 1 and type 4 may also be due to 411

the first and second dissociation constants of phosphate groups. 412

There is a tendency for each type of surface group to increase in amount with an 413

increasing carbonization temperature up to 800 °C, with a subsequent decrease at higher 414

temperatures. The same trend is observed for the dissociation constants of the most acidic 415

surface groups (Table 2). Similar temperature dependence of the amount of acidic surface 416

groups was observed for carbons produced from fruit stones [36,37] and polymers [33–35]. 417

The change in the amount of surface groups that occurs with increasing carbonization 418

temperature occurs in parallel with the variation of phosphorus content. This provides 419

26

sufficient evidence to associate the acid properties of the carbon with phosphorus and 420

oxygen-containing surface groups. Phosphorus-containing groups have the structure of 421

phosphates/polyphosphates bound to the carbon structure by a C-O-P linkage [25,31,39,59]. 422

Aside from phosphorus-containing groups, however, there are acidic oxygen-containing 423

surface groups that originate both from the precursor and the oxidative action of phosphoric 424

acid. 425

In contrast to the phosphoric acid-activated carbons, only phenolic groups were 426

observed on the acid-free carbon ���, demonstrating that carbonization in the presence of 427

phosphoric acid dramatically changes the surface chemistry of the carbon. 428

3.7. Copper binding 429

The presence of large amounts of surface groups (Table 2) is expected to be 430

beneficial for the binding of metal ions. In the present study, the sorption properties of the 431

lignosulfonate-based activated carbons were evaluated by studying uptake of copper ions 432

from aqueous solutions over a wide pH range. Fig. 7 shows the relative amount of copper 433

adsorbed by the lignosulfonate-derived carbons at different solution pHs. With increasing 434

pH, the adsorption of copper by all of the carbons increased because during surface complex 435

formation, copper cations and H+ ions compete for adsorption sites [60]. This trend has been 436

previously observed for many carbon adsorbents [61–63]. It is interesting to note that all of 437

the carbons exhibited appreciable adsorption of copper at pH values lower than the PZC, i.e., 438

at a pH where copper adsorption should be hindered by a repulsive force between the 439

copper cation and the positively charged carbon surface. This phenomenon has been 440

reported for the adsorption of Cd(II), Hg(II) and Cr(III) and has been explained as being due 441

to an ion-exchange reaction between ������������������ ��� ����-electrons of the 442

27

graphene layer (–��–H3O+) and the metal ion [64,65]. This mechanism is corroborated by the 443

fact that despite the lower amount of surface groups (Table 2), carbon C900 showed a 444

greater degree of copper adsorption than carbon C800. Another piece of experimental 445

evidence in support of the formation of �-complexes with copper is the relatively high level 446

of copper adsorption by acid-free carbon that contain surface groups that are in a 447

protonated state at the investigated pH range; their corresponding pKs are 9 and 10 448

(Table 2). However, for carbon C800, due to its relatively high sulfur content (Table 1), 449

copper uptake may also be ascribed to the formation of surface complexes with sulfur-450

containing groups. 451

With an increasing carbonization temperature up to 900 °C, the adsorption of copper 452

by the phosphoric acid-activated carbons increased while uptake decreased when carbon 453

was obtained from 1000 °C treatment. Additionally, carbons obtained at 800-1000 °C show 454

complete copper binding at pH values higher than 5, where copper hydroxides are formed. 455

This means that surface complexes of copper are more stable than copper hydroxides. 456

Carbons obtained at 800 °C and especially at 900 °C showed a very high adsorption of copper 457

at very low pHs, suggesting that lignosulfonate-based phosphoric acid-activated carbons 458

have considerable promise for the purification of aqueous solutions by heavy metal ion 459

removal. 460

461

28

462

Fig. 7. Copper binding by phosphoric acid-activated carbons formed from lignosulfonate. 463

464

3.8. Porous structure 465

Nitrogen adsorption isotherms of the lignosulfonate-based carbons (Fig. 8a) belong 466

to a mixed type of IUPAC classification. The initial part of the isotherms is of type I with 467

significant uptake by some carbons at low relative pressures, corresponding to adsorption 468

within their micropores. At intermediate and high relative pressures, the isotherms are of 469

type IV, with a hysteresis loop of type H4 associated with monolayer–multilayer adsorption 470

followed by capillary condensation in narrow slit-like pores. Nitrogen uptake by the carbons 471

decreased with increasing carbonization temperature up to 700-800 °C but increased when 472

carbonization was performed at higher temperatures. 473

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7 8 9

Cu b

indi

ng

pH

P400 P600 P800 P900 P1000 C800

29

474

475

Fig. 8. Nitrogen adsorption-desorption isotherms (a) and pore size distribution (b) for 476 phosphoric acid-activated carbons made from lignosulfonate. 477

478

479

480

481

482

483

0

5

10

15

20

25

30

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

N2

adso

rptio

n, m

mol

/g

P/Po

P400 P700 P900 P1000 C800

(a)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.1 1 10 100

dV/d

log(

w)

Pore size, nm

P400 P700 P900 P1000 C800

(b)

30

Table 3. Porous structural parameters of phosphoric acid-activated carbons made from 484 lignosulfonate. 485

Carbon ABET m2/g

Vtot cm3/g

Vmi cm3/g

Vmi/Vtot %

Vme cm3/g

Vme/Vtot %

P400 766 0.45 0.25 55% 0.20 45% P500 587 0.46 0.17 38% 0.28 62% P600 319 0.30 0.08 28% 0.22 72% P700 179 0.26 0.04 16% 0.22 84% P800 230 0.29 0.06 20% 0.23 80% P900 904 0.65 0.31 48% 0.34 52% P1000 1373 0.97 0.41 42% 0.56 58% C800 142 0.14 0.05 34% 0.09 66%

486

All of the carbons obtained by phosphoric acid activation have a more developed 487

porous structure than acid-free carbon carbonized at 800 °C (Table 3). Increasing the 488

carbonization temperature up to 700 °C resulted in a gradual decrease in the BET surface 489

area and micropore volume. The same tendency was observed previously for phosphoric 490

acid-activated carbons produced from fruit stones [36,37], where the decrease was 491

attributed to contraction of the material [36–38,66–68]. During the chemical activation of 492

lignocellulosic precursors, the phosphoric acid forms phosphate and polyphosphate bridges 493

that connect the biopolymer fragment, avoiding the contraction of the material due to the 494

temperature. At temperatures above 450 °C, however, contraction occurs due to the 495

catalytic activity of phosphoric acid, which allows the growth and alignment of polyaromatic 496

clusters, producing a more densely packed structure and some loss of porosity. Another 497

reason for decreasing porosity with increasing carbonization temperature up to 700 °C could 498

be the progressive formation of polyphosphates that block the pore space. This is supported 499

by the increase in phosphorus content with temperature in this range (Table 1). Increasing 500

the carbonization temperature above 700 °C caused a gradual increase in the porous 501

structure due to thermal degradation of phosphocarbonaceous species and the formation of 502

31

volatile phosphorus-based compounds (phosphorus(V) oxide and elemental phosphorus) 503

due to reduction of these phosphates to their elemental form [25]. This volatilization can 504

produce new channels (pores) and contribute to the observed increase in porosity above 505

700 °C. This is supported by the decreasing phosphorus content for carbons obtained at 506

temperatures higher than 800 °C accompanied by a decreasing concentration of surface 507

groups (Table 1, Table 2). 508

Pore size analysis revealed pores with sizes in the 0.9-1.3 nm range and of 2.3 nm 509

(Fig. 8b). The volume of these pores exhibits the same temperature dependence as do the 510

BET surface area and micropore volume. With increasing carbonization temperature, there is 511

a progressive increase in the number of pores in the range of 6-20 nm in the phosphoric 512

acid-activated carbons, while the acid-free carbon showed no increase in porosity in this 513

region (Fig. 8b). 514

4. Conclusions 515

Phosphoric acid activation of lignosulfonate prior to carbonization led to the 516

formation of porous phosphorus-containing carbons in high yields up to 55%. The 517

phosphorus content peaked at 11% for carbon obtained at a carbonization temperature of 518

800 °C. Activated carbons derived from lignosulfonate contain significant amounts of acidic 519

surface sites that can be as concentrated as 3 mmol/g. These lignosulfonate-derived 520

activated carbons also possess a considerable capacity for adsorbing metal ions and are thus 521

promising tools for use in new water treatment methods. The largest BET surface areas were 522

obtained when carbonization was carried out at the lowest (400 °C; 780 m2/g) and at the 523

highest (1000 °C; 1370 m2/g) temperatures investigated in this study. 524

32

Acknowledgements 525

The authors thank Borregaard LignoTech, Norway, for providing lignin samples; the 526

Swedish Institute, Baltic Sea Unit, National Academy of Sciences of Ukraine (projects 527

0110U001330 and 0110U004545) for the financial support of this joint work; and the Knut 528

and Alice Wallenberg Foundation in association with the Wallenberg Wood Science Center 529

(WWSC) for the financial support of the work of Dr. Olena Sevastyanova. Dr. Galina Dobele, 530

from Latvian State Institute of Wood Chemistry, is acknowledged for providing chemical 531

composition measurements. 532

533

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