Journal of Industrial and Engineering Chemistry 21 (2015) 1354–1364

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Journal of Industrial and Engineering Chemistry

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Physical activation of carbon residue from biomass gasification: Novelsorbent for the removal of phosphates and nitrates from aqueoussolution

Sari Kilpimaa a, Hanna Runtti a, Teija Kangas a, Ulla Lassi a,b,*, Toivo Kuokkanen a

aUniversity of Oulu, Department of Chemistry, P.O. Box 3000, FIN-90014 Oulu, FinlandbKokkola University Consortium Chydenius, Unit of Applied Chemistry, Talonpojankatu 2 B, FIN-67100 Kokkola, Finland

A R T I C L E I N F O

Article history:

Received 24 March 2014Received in revised form 9 May 2014Accepted 2 June 2014Available online 10 June 2014

Keywords:SorbentPhysical activationActivated carbonPhosphateNitrate

* Corresponding author. Tel.: +358 40 02E-mail address: ulla.lassi@oulu.fi (U. Las

http://dx.doi.org/10.1016/j.jiec.2014.06.0061226-086X/ ã 2014 The Korean Society of

9 4090.si).

Industrial

A B S T R A C T

Carbon residue obtained as a by-product fromwood gasification process was employed as a precursor forpreparing adsorbent by physical activation. Adsorbent with BET surface area 590m2g�1 and pore volumeof 0.335 cm3g�1 was prepared and used as an adsorbent for phosphate and nitrate removal. The optimalpH and initial concentrations for phosphate and nitrate removal were determined. The kinetics showedthat the adsorption data followed pseudo-second-order kinetics. The isothermanalysis indicated that theadsorption data can be represented by the Langmuirmodel. Results showed that activated carbon residueis suitable adsorbent for phosphate removal.ã 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Introduction

Phosphate and nitrate containing wastewaters are a globalproblem. Phosphate inwater is one of themain factorswhich leadsto the eutrophication and deterioration of water bodies. Eutrophi-cation may occur when the concentration of phosphate is higherthan 0.02mgL�1. Wastewaters possess harmful levels of phos-phates and the removal of these before discharge is paramount.Biological, chemical and physical treatment methods have beendeveloped as a phosphate removal technology [1–3]. Adsorption isa common method to remove nutrients, especially phosphorus,from effluent by using porous materials such as zeolites [4].Chemical precipitation with, for example, iron salt is a simplephosphate removal technique but the disadvantages are itsrunning costs and the formation of sludge formation in thisprocess which are also problems that exist in the biologicaltreatment [1,2,5]. There are also various physical methodsdeveloped for phosphate removal such as electrodialysis, reverseosmosis and ion exchange [3,6,7]. Phosphate can also be removedby electro-coagulation (EC) [8]. Most of these methods are

and Engineering Chemistry. Publi

expensive as well as inefficient [5]. Adsorption onto porousmaterials such as zeolites is a suitable process for the removal ofphosphates from an aqueous solution [4]. Nitrate also stimulateseutrophication and it has been linked to outbreaks of infectiousdiseases. Excess nitrate in drinking water may cause methemo-globinaemia, also called a blue baby syndrome, in newborn infants[9]. According to the World Health Organisation [10], therecommended maximum concentration for nitrate in drinkingwater is 50mgL�1. Nitrate can be removed inmanyways such as byadsorption [11], biological denitrification [12], reverse osmosis[13], ion exchange [14] and catalytic denitrification [15].

Development of inexpensive water treatment chemicals hasbeen of great interest over the past decade because according tothe principle of sustainable development, the utilisation of wastematerials instead of their disposal is nowadays an important topicglobally. Activated carbon is typically used for purification ofwater,for example, wastewaters and drinking waters as well as airbecause it has a well-developed pore structure, high internalsurface area and good mechanical characteristics. It could also beused for the separation of gas mixtures. Consequently, commercialactivated carbon is fairly expensive and preparation of it is energyconsuming. Therefore other substitutive adsorbents must beinvestigated [16–18]. Activated carbon can be prepared from avariety of raw materials with high carbon content and therefore,

shed by Elsevier B.V. All rights reserved.

Table 1Preparation conditions for the physical activation of gasification carbon residue.

Sample Temperature (�C) Activation time (h) Activating agent

1 600 1 CO2 600 3 CO3 800 1 CO4 800 3 CO5 600 1 CO2

6 600 3 CO2

7 800 1 CO2

8 800 3 CO2

9 600 1 N2

10 600 3 N2

11 800 1 N2

12 800 3 N2

S. Kilpimaa et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 1354–1364 1355

various novel materials such as coconut husk [19], agriculturalresidues [20], macadamia nutshell [21] and coffee endocarp [22]are studied as adsorbents. However, the abundance of by-productsmakes them good sources of raw materials for activated carbonswhich are the reason why locally available materials still needmore research.

Gasification is one of the most effective energy conversionmethods for theutilisationofbiomass forobtainingenergy (heatandpower) [23]. Syngas produced via biomass gasification can be useddirectly as fuel for an internal combustion engine or as a chemicalfeedstock to produce liquid fuels [24]. In the gasification process acarbon residue as a by-product is formed, the amount is evaluated tobe0.1–15%of thegasifiedmaterial, andanyhow, ratherhighvolumesare produced. [23] For improving the cost-effectiveness of thegasificationprocess theutilisationof thisby-product is important. Inaddition, this high carbon content residue is in the EU regionclassifiedaswastematerial, andtherefore, allutilisationapplicationswould be preferred. According to the national Finnish strategy onwaste materials, in accordance with the corresponding Europeanstrategy, all kinds of waste must be utilised primarily as material(reuse and recycling), secondary as energy and if neither of thoseutilisation methods are possible, they can be disposed viaecologically beneficial methods [25].

In principle, the methods for preparing activated carbon can bedivided into two categories: physical and chemical activation. Thereare several advantages and disadvantages of physical activationcompared to chemical activation. Themain advantages for chemicalactivation are, among others, lower temperature of activation,shorter activation time and higher development of porosity. Themain disadvantage for the use of chemical activation process is thepollution generated by the activation chemical, while chemicalconsumptionandpricemustalsobeconsidered.Chemical activationneeds also a washing stage and therefore, physical activation couldbe environmental friendly and an effective method to produce anadsorbent from waste material [18].

Preparation of activated carbon consists of two steps: carbon-isation and activation. The porosity formed in the carbonisationstep is typically not enough for most applications which is thereason to develop porosity further. Physical activation can beperformed for example with CO2 or steam [18]. Carbon residuesformed in the wood gasification process have a lower specificsurface area compared with commercial activated carbon sinceonly the carbonisation step took place during the gasificationprocess. The recycling of gasification carbon residues for activatedcarbon production may be possible since the carbon residue onlyneeds an activation step to develop a higher specific surface areaand pore structure. Therefore physical activation would berequired to increase the specific surface area of carbon residueand to develop the porosity further.

The overall aim of this investigation is to modify an industrialcarbonaceous by-product (carbon residue from the wood gasifica-tion process) by physical activation to maximise the adsorptionpotential of material and test it as an adsorbent for phosphate andnitrate removal on a laboratory scale. The effect of a physicalactivation procedure was examined in order to obtain the optimaloperation conditions for the activation stage. Different physicalactivating agents, duration of activation and temperature werestudied. The target was to produce an adsorbent with a highsurface area, high pore volume and also keep the yield of theactivation process high. Physically activated carbon residue wastested as an adsorbent for phosphate and nitrate removal and theinfluence of adsorption time, initial phosphate or nitrate concen-tration and pH was also studied. Commercial activated carbon andcarbon residue without any pre-treatment process were used asreference samples. Kinetic calculations, isotherm analysis anderror analysis were also performed.

Experimental

Raw material

Carbon residue from the biogasification process used in thelaboratory experiments was acquired from a biogasification pilot-plant (Sievi, Finland) which involved a 150kW downdraft gasifieroperating at around 1000 �C.100% Finnishwood chipswere used asthe raw material for fuel at a rate of 50 kgh�1. There was noseparate carbon residue collector in the gasifier. The gas producedin the gasifier is washed by a water scrubber and the carbonresidue formed in the process is collected from thewater container.Prior to activation the carbon residuewas dried at 110 �C overnight,crushed and sieved to obtain particle size <150mm. A commercialactivated carbon (Norit GCN 1240) produced from coconut shellsby steam activation, used as a reference sample, was also dried,crushed and sieved before use.

Physical activation

Physical activation was carried out by using two differentactivation agents, CO and CO2. In addition, reference samples wereprepared by thermal treatment without an activating agent undera nitrogen atmosphere. A literature review has shown that carbonmonoxide is not a typical physical activating agent but preliminaryexperiments have shown that it might be a suitable activatingagent for gasification carbon residue. Temperature and duration ofactivation were the two main parameters investigated in additionto the yield of the activation process. Temperatures of 600 and800 �C were tested while the duration of activation was 1 or 3h.Activation conditions during physical activation are presented inTable 1. Activating agent flow rate and nitrogen flow rate was0.09 Lmin�1 and 0.18 Lmin�1 respectively in all experiments.

Physical activation was carried out in a vertical furnace, where1 g of gasification carbon residue was placed in a reactor tube(diameter 40mm) which was prepared specifically for a Carboliteoven. The oven was continuously heated at a rate 10 �Cmin�1

under a nitrogen atmosphere until the target activation tempera-ture was reached. At this temperature, nitrogen flow was stoppedand the activation agent was fed at the selected flow rate. Nitrogenwas used also during the cooling stages.

Characterisation of adsorbents

The specific surface areas of products were determined fromnitrogen adsorption–desorption isotherms at the temperature ofliquid nitrogen (�196 �C) by using a Micromeritics ASAP 2020. Thesame equipment was also used for the determination of pore sizeand pore volume. Microscopic analysis was performed by using afield emission scanning electron microscope (FESEM, Zeiss Ultra

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Plus Gemini) while carbon, hydrogen, nitrogen, sulphur andoxygen content were determined by elementary analysis using aFlash 2000 analyser. This method is based on combustion wherethe sample elements are converted into simple gases (CO2, H2O, N2

and SO2) in a pure oxygen atmosphere. Gases are separated anddetected by a TCD detector. XRD analysis was done by usingSiemens Diffraktometer D5000 equipment.

During the physical activation process some weight loss takesplace. The degree of burn-off is calculated as the weight loss of thecarbon residue after activation to the dried carbon mass before theactivation process (Eq. (1)).

burn� off�% ¼ weightðinitialÞ �weightðfinalÞweightðinitialÞ � 100% (1)

Adsorption experiments

Laboratory experiments were carried out in order to examinethe effects of different adsorption variables on the adsorptionefficiency of phosphates and nitrates on the three adsorbents(carbon residue, activated carbon residue and commercialactivated carbon). The studied variables were pH, initial concen-tration of phosphate or nitrate in solution and adsorption time. Theexperimental data of the adsorption properties were obtainedusing a batch type process. The effect of pH on phosphate or nitrateremoval was studied over a pH range of 4–8, while optimisationexperiments were performed by using a solution whose concen-tration was 80mgL�1. The effect of initial solution concentrationwas studied at optimum pH within a range of 10–140mgL�1 ofphosphate or nitrate. The model solution of phosphate wasprepared by adding solid KH2PO4 to distilled water while thenitrate test solution was prepared from solid NaNO3. Bothchemicals were p.a quality provided by Merck. Each type ofadsorbent was weighed so that the adsorbent dose was 5 g L�1 inall experiments.

The optimum pH and effect of initial concentrations weredetermined by using 25mL sample volume in 250mL polyethyleneflasks, in which a solution of phosphate or nitrate and adsorbentwas mixed together whose pH were adjusted by adding 0.1M HClor 0.1M NaOH (provided by FF-Chemicals). This was done afteradding the adsorbent to ensure the solution had the correct pHbecause the carbon residue from the gasification process is clearlyalkaline (pH 9). This is a consequence to the fact that carbonresidue has some buffer capacity which means that it is able toretain an almost constant pHwhen a small amount of acid is addedto the solution, and therefore, it possesses ions that have the abilityto neutralise the added hydrogen ions. These ions are similar tohydrogen carbonates, carbonates, and hydroxides [26]. Thesolution was shaken by a laboratory shaker using an orbitalmovement at a speed of 230 rpm at room temperature for 24h.

Effect of adsorption time studies were performed based on theanalysis results from the batch experiments for different pH valuesand initial concentrations of phosphate or nitrate solutions withusing the three adsorbents. The rate of adsorptionwas determinedat the following time intervals: 1min, 2min, 5min, 10min, 20min,30min, 1 h, 2 h, 4 h, 6 h, 8 h, 12h and 24h. The adsorbent dose in allexperiments was constant (5 g L�1) while pH and initial concen-tration were optimal during kinetic studies. Kinetic studies werecarried out by using a 1000mL sample volume in a two litreErlenmeyer flask while at the same time as shaking was takingplace by amagnetic stirrer with an agitation speed of 1000 rpm. Allsamples including the initial samples were filtered through a0.45mm filter so that the phosphate or nitrate concentrationscould be analysed by ion chromatography (Metrohm 761CompactIC). The amount of phosphate or nitrate adsorbed (qe in mgg�1)was determined as follows:

qe ¼ðC0 � CeÞ � V

m(2)

where C0 and Ce are the initial and equilibrium concentrations ofphosphate or nitrate in a solution (mgL�1), respectively, V is thevolume of solution (L) and m is the mass of the adsorbent (g)[27,28].

Results and discussion

Effect of physical activation atmosphere, time and temperature

It is well known that the most important property of activatedcarbons and other adsorbents is their adsorption capacity which isrelated to the specific surface area, pore volume and pore sizedistribution. Typically adsorption capacity is higher when specificsurface area is larger so there is a great importance to investigatethe parameters which affect to the activation process and specificsurface area [17].

The mechanism involved in the activation of a carbonaceousmaterial is not clear. It is known that during physical activationwith CO2 the following reaction occurs:

Cþ CO2 ¼ 2COðReaction 3ÞThe chemical mechanism of physical activation is not as simple

as presented in reaction 3, but the following mechanism ispostulated to be:

Cþ CO2$Cþ CðOÞðReaction 4ÞIn reaction 4 a surface oxygen complex C(O) is initially formed.

It became stable under the reaction conditions and acting as aretardant by blocking reaction sites as presented in reaction 5:

CðOÞ$C��OðReaction 5ÞHowever, it may decompose leaving the surface as CO

CðOÞ ! COðReaction 6Þinwhich the “free” surface carbon atom stays available for reaction[18].

According to Guo et al. [17] there are three stages in thedevelopment of pore structure during the physical activationprocess:

- opening the previously inaccessible pores,- creation of new pores by selective activation,- widening of the existing pores into the mesoporous range.Table 2 lists the burn-offs during the activation procedure as

well as the properties of physically activated samples, includingthe specific surface areas, pore sizes, total pore volumes,microporevolumes and macro-mesopore volumes. According to our results,the activating agent and activation temperature had an importantrole of developing porosity but the duration of activation only hada significant role when carbon dioxide was used as an activatingagent. Burn-offs increased clearly when temperature and durationof activation increased. In this case, duration of activation camesignificant at temperature of 800 �C. Duration of activation has notnotable effect on the total pore volume but increasing temperatureincreased also pore volume. The highest surface area obtained inour studies was 590m2g�1 and it was obtained by using carbondioxide as a physical activating agent, duration of activation of 3hand a temperature of 800 �C. Burn-off of samples was also quitehigh when other conditions were compared but the specificsurface area was notably high and therefore, this sample waschosen for further consideration.

For comparison, results published byGuo et al. [17], whichwereobtained by using carbon dioxide as the physical activating agentand coconut shell as a raw material, are presented in Table 3. Guoet al. [17] obtained a yield as a result which is opposite for burn-off

Table 2Properties of various physically activated carbon residues and reference samples. Preparation conditions of sample were presented in Table 1.

Sample Burn-off(%)

BET(m2g�1)

Pore size(nm)

Total pore volume(cm3g�1)

Micropore volume(cm3g�1)

Macro-mesopore volume(cm3g�1)

1 15.6 117 6.04 0.176 0.050 0.1262 16.6 126 5.83 0.183 0.064 0.1193 20.5 135 6.02 0.204 0.056 0.1484 25.6 133 6.16 0.205 0.052 0.1565 15.0 150 5.25 0.196 0.062 0.1346 16.3 152 5.20 0.197 0.075 0.1227 40.1 353 3.86 0.340 0.172 0.1688 55.2 590 3.44 0.335 0.175 0.1609 17.8 136 5.61 0.191 0.052 0.139

10 18.2 145 6.08 0.220 0.050 0.17011 15.7 156 5.52 0.215 0.069 0.14612 30.9 160 5.98 0.240 0.058 0.182

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presented in our investigation. The authors reported yields in thesame order of magnitude as in our studies, but for higher specificsurface areas and pore volumes. On the other hand, they used alonger activation time and a higher temperature which increasesthe cost of activation. Their raw material was also different incomparison to the raw material used in our studies.

FESEM images of the physically activated carbon residues arepresented in Fig. 1 which illustrates pores and crystals around theadsorbent’s surface. Specific surface area increases in any physicaltreatment and some porous structure is formed, but the mostporous surface was obtained by using carbon dioxide as a physicalactivating agent, a temperature of 800 �C and duration of activationof 3h (Fig. 1).

Physical and chemical characteristics of adsorbents

The physical and chemical characteristics presented in Table 4,are for adsorbent which have been taken for further considerationand which have been used in adsorption experiments. The studiedadsorbents were carbon residue, physically activated (3h in CO2 at800 �C) carbon residue and commercial activated carbon.

Physical activation decreases the carbon content of carbonresidue but significantly increases the specific surface areawhich isin the same order of magnitude as commercial activated carbon.Activation clearly increases the carbon residues’micropore volumeand therefore average pore size decreases. The majority of surfaceareas in the activated carbon residue come frommicropores whichis the reasonwhy average pore size seems to decrease. Comparisonof XRD pattern of studied adsorbents (Fig. 2) shows thatcommercial activated carbon is amorphous material and it containlarge amount of carbon. Carbon residue included also amorphous

Table 3Results obtained by physical activation with carbon dioxide of coconut shell presented

Temperature(�C)

Activation time(min)

CO2 flow rate(cm3min�1)

Yield(%)

750 240 600 72.00800 240 600 64.67850 240 600 50.00900 240 600 29.33950 240 600 11.33900 120 600 58.67900 180 600 45.33900 300 600 24.00900 360 600 17.33900 240 200 38.67900 240 400 34.00900 240 800 23.33900 240 1000 10.67

and crystalline material and the main peak is calcite. Crystallinityincreases during physical activation comparing with unactivatedcarbon residue and carbon content decreases but amount of calciteincreases. Carbon content results presented in Table 4 support alsothis.

Adsorption experiments

Effect of pHTo determine the optimum pH, the equilibrium adsorption of

phosphate and nitrate was studied over a pH range of 4–8 togetherwith an adsorption time of 24h and initial phosphate or nitrateconcentration of 80mgL�1. The surface of the adsorbent becamepositively charged in acidic conditions and negatively charged inalkaline conditions. Phosphate ions are negatively charged and itappears that there are repulsion forces between phosphate ionsand the negatively charged adsorbent surface in alkaline con-ditions. This occurs because at active sites OH� ions and othernegatively charged anions give the appearance that they are incompetition with each other. [27]. Industrial by-products such asthe carbon residue formed in the biogasification process neverinclude a single element and therefore many components canaffect the adsorption capacity. For example, carbon residueincludes a notable amount of calcium (42.3 g kg�1) determinedby ammonium acetate extraction [26]. It is well-known that highpH and high calcium concentration are advantageous for calciumphosphate precipitation [29]. Precipitation reactions often co-occur with the adsorption process and thus it is difficult todistinguish these individual processes [30]. The purpose of thisstudywas to investigate the adsorption, sowe chose the conditionsin such away that precipitation should not occur and therefore, pH

by Guo et al. [17].

BET(m2g�1)

Total pore volume(cm3g�1)

Micropore volume(cm3g�1)

613 0.437 0.307860 0.590 0.452

1305 0.689 0.5571391 0.838 0.7301323 0.878 0.671724 0.506 0.367

1075 0.744 0.5741653 1.089 0.8581331 0.819 0.6991013 0.671 0.5211192 0.768 0.6191700 1.135 0.8821360 0.928 0.697

[(Fig._1)TD$FIG]

Fig.1. FESEM-images fromphysically activated carbon residues (a) CO 1h 600 �C, (b) CO 3h 600 �C, (c) CO 1h 800 �C, (d) CO 3h 800 �C, (e) CO21h 600 �C, (f) CO2 3h 600 �C, (g)CO2 1h 800 �C, (h) CO2 3h 800 �C, (i) N2 1h 600 �C, (j) N2 3h 600 �C, (k) N2 1h 800 �C and (l) N2 3h 800 �C.

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8 was eliminated [31]. This meant having mildly acidic conditionswhich are also realistic conditions found in wastewaters contain-ing phosphate. pH increases during the adsorption process whichcan cause precipitation and therefore precipitation cannot betotally neglected. From an utilisation point of view it isinsignificant how phosphate removal occurs, i.e. if the mechanismis by adsorption or precipitation. The results suggest that theoptimal pH for phosphate removal over activated carbon residue,carbon residue and activated carbonwas 6 which were optimal pHvalues when alkaline conditions were eliminated.

Depending on the pH of the system, phosphorus can exist as

H3PO4, H2PO4�, HPO2�

4 or PO3�4 . Speciation diagram presented in

Fig. 3 has been drawn by usingMINEQL+ softwarewhich is a tool tocalculate chemical equilibrium in a system.

In the case of nitrate removal, the optimal pH valuewas 4 for alladsorbents. Nitrate ions are also negative and as in the case ofphosphate, repulsion forces also exist between these ions andadsorbent in alkaline conditions. The poor removal efficiency of

Table 4Physical and chemical composition of adsorbents.

Parameter Unit CRa ACRa ACa

C % 69.5 52.1 91.9H % 1.2 0.4 0.5N % 0.9 <0.5 <0.5S % <2 <2 <2O % 8.1 8.0 2.6Specific surface area m2 g�1 52.4 590 786Pore size (average) nm 8.16 3.44 2.21Micropore volume (total) cm3 g�1 0.0236 0.175 0.394Macro+meso volume (total) cm3 g�1 0.0832 0.160 0.041

a CR: carbon residue, ACR: activated carbon residue and AC: activated carbon.

anions in alkaline conditions was most likely due to the increasingcompetition for sites betweenOH� and nitrate ions [27]. pHwas an

essential variable in which formed phosphate (PO3�4 ) existed, but

nitrate (NO�3 ) occurs naturally in nitrogen cycles and resided in a

stable form in oxygenated nitrogen systems. Nitrate can bereduced to nitrite (NO�

2 ) by a microbial action under reducingconditions, however, nitrite is unstable and can be oxidised back tonitrate. Chemical and biological processes can also reduce nitriteinto various compounds. Typically carbon based adsorbents showspoor adsorption towards anions while published results on thesorption of nitrates by activated carbon are limited [32]. Afkhamiet al. [33] discovered that the acid treatment of carbon-basedadsorbent increases its adsorbent capacity towards nitrate andnitrite. They assumed that the acid treatment produces positivesites on the carbon surface by protonating the OH-groups locatedthere. This increases the electrostatic adsorption of anions whichmay also explain why the optimal pH for nitrate adsorption isfound in acidic conditions.

Effect of initial phosphate or nitrate concentrationThe effect of initial phosphate or nitrate concentration on the

removal efficiency of anions between the concentration levels of10–140mgL�1 are presented in Fig. 4. Optimum initial phosphateconcentration for phosphate removal was 80mg L�1, 20mgL�1 and10mg L�1 for carbon residue, activated carbon residue andactivated carbon respectively. Phosphate removal efficiency overthe adsorbents decreased when the initial concentration wasincreased from 20mgL�1 in the case of activated carbon residue.The same effect was observed for activated carbon but there thehighest removal efficiencywas observed at a concentration level of10mg L�1. This can be explained by the fact that optimal adsorptionsites are occupied first at low concentrations while on the other

[(Fig._2)TD$FIG]

Fig. 2. XRD pattern of (a) physically activated carbon residue, (b) carbon residue and (c) activated carbon.

S. Kilpimaa et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 1354–1364 1359

hand, some driving forces exist when the concentration increases.This explains why an optimum concentration of 80mgL�1 isobserved for carbon residue. In the case of nitrate, the optimuminitial concentrations were 10, 10 and 20mgL�1 for activatedcarbon residue, carbon residue and activated carbon respectively,since the optimum adsorption sites are occupied first.

Adsorption isothermsAdsorption isotherms are generally used for the design of an

adsorption system since they represent the amount of speciesadsorbed versus the amount of species left in the solution phase atequilibrium. Therefore isotherm analysis was performed forphysically activated carbon residue and activated carbon. TheLangmuir [34] and Freundlich [35] equations are by far the mostcommonly used equations for describing the adsorption isotherm.The Langmuir isotherm assumes that adsorption occurs at specifichomogeneous sites within the adsorbent without any interactionsbetween the adsorbed substances. The Freundlich model isempirical in nature and assumes that the uptake of ions occurson a heterogeneous surface. The linear form of the Langmuir andFreundlich isotherm models are given by Eqs. (7) and (8),respectively, as follows:

1qe

¼ 1qm

þ 1qmbCe

(7)

[(Fig._3)TD$FIG]

Fig. 3. Different phosphate species presented as total concentration as a function ofpH.

logqe ¼ logKf þ1nlogCe (8)

where qe (mgg�1) is the amount of species adsorbed per unit massof adsorbent, Ce (mg L�1) is the solution concentration atequilibrium while qm (mgg�1) and b (Lmg�1) are the Langmuirconstants related to the capacity and energy of adsorption,respectively. Kf (L g�1) and n (dimensionless) are Freundlichconstants related to the adsorption capacity and intensity ofadsorption, respectively. Furthermore, the constants of Langmuirand Freundlichwere obtained from the linear plots of 1/qe versus 1/Ce, and log qe versus logCe, respectively.

In addition to Langmuir and Freundlich isotherm models,Tempkin and Dubinin–Radushkevich (D–R) isotherm models havecommonly been applied in linear form. The heat of the adsorptionand the adsorbent-adsorbate interaction were evaluated by usingTempkin isothermmodel [36,37]. The D–R model was also appliedto estimate e.g. the characteristics of adsorption [38]. That modeldoes not assume a homogeneous surface or constant adsorptionpotential. Linear forms of isotherms are presented in Eqs. (9) and10 and [39,40]:

qe ¼ BlnAþ BlnCe (9)

lnqe ¼ lnqm � be2 (10)

where B= (RT)/b is the Tempkin constant related to heat of theadsorption (Jmol�1) andA is theTempkin isothermconstant (L g�1).In the D–R isotherm, qe is the amount of ion adsorbed to adsorbentper unit dosage (mol g�1),b (mol2 (J2)�1) is a constant related to theadsorption energy, qm (mol g�1) is the theoretical saturationcapacity, e is Polanyi potential,which is described inEq. (11) [40,41].

e ¼ RTln 1þ 1Ce

� �(11)

The mean free energy of adsorption E (kJmol�1) describes freeenergy change when one mole of ion is transferred from thesolution to the surface of the sorbent and it can be calculated asfollows [40]:

E ¼ 1ffiffiffiffiffiffiffiffiffiffi�2b

p (12)

The values of different isotherm constants and correlationcoefficients are presented in Tables 5 and 6.

[(Fig._4)TD$FIG]

Fig. 4. The effect of initial phosphate or nitrate concentration on the removal (%) efficiency of phosphate (a) and nitrate (b) between the concentration levels of 10–140mgL �1

with an adsorbent dose of 5 g L�1. Experiments were performed at room temperature together with the optimum pH values for phosphate (pH 6) and nitrate (pH 4).

Table 5Isotherm constant parameters and correlation coefficients calculated for theadsorption of phosphate ions onto activated carbon residue and activated carbon atroom temperature.

Isotherm ACRa ACa

Langmuirqm (mgg�1) 30.211 8.6957b (Lmg�1) 0.0116 0.0275R2 0.9976 0.9924

FreundlichKf (L g�1) 0.3157 0.5746n 1.0908 1.8720R2 0.9680 0.9612

TempkinB (Jmol�1) 4.9734 1.1717A (L g�1) 0.1965 0.2295R2 0.9601 0.9605

Dubinin and Radushkevichb (mol2 J2) �1.3�10�5 �6.5�10�6

qm (mol g�1) 9.7865 4.8298E (kJmol�1) 196.12 277.35R2 0.8785 0.7527

a ACR: activated carbon residue and AC: activated carbon.

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According to the results in Tables 5 and 6, the low value ofcorrelation coefficients of Freundlich, Tempkin and D–R modelsshow not usefulness of these models to fit experimental data.Results show that phosphate and nitrate adsorption onto activatedcarbon residue and activated carbon can be represented by theLangmuir model. The fact that the Langmuir model fits theexperimental data very well may be due to the homogeneousdistribution of active sites on physically activated carbon residueand commercial activated carbon [40]. In case of phosphateadsorption, qm is notable higher for activated carbon residuecomparing with commercial activated carbon and therefore, it canbe said that more phosphate can be adsorbed to activated carbonresidue surface than for activated carbon surface. In case of nitrateremoval, Langmuir adsorption capacity is slightly lower foractivated carbon residue comparing with activated carbon.

Li et al. [42] have used fly ash as an adsorbent for phosphateremoval and they found Langmuir adsorption capacity 58–78mg/gdepending on treatment technique. Kumar et al. [43] were removedphosphate over coir-pith activated carbon and they reportedLangmuir adsorption capacity 7.3–7.7mg/g depending on tempera-ture.Mezenner andBensmaili [44] studiedphosphate adsorptiononiron hydroxide-eggshellwaste and they found qmvalue 10.6mg/g atroom temperature. Afkhami et al. [33] compared the nitrateadsorption capacity of 4M H2SO4-treated carbon cloth to distilledwater-treated carbon cloth and they found Langmuir adsorptioncapacity of 126 and 24mg/g. Bhatnagar et al. [45] reportedadsorption capacity 10.2mg/g for ZnCl2 treated coconut granularactivated carbon. Mishra and Patel [46] reported adsorptioncapacity 1.10mg/g for wheat straw charcoal, 1.30mg/g for mustardstraw charcoal and 1.22mg/g for commercial activated carbon.

Namasivayam and Sangeetha [47,48] treated coconut coir pithwithZnCl2 and they found Langmuir adsorption capacity of 10.3mg/g.Katal et al. [11] studied aminated rice husk as an adsorbent fornitrate removal and they found Langmuir adsorption capacity56mg/g. Adsorption capacity for nitrate vary a lot, organiccompounds added to aluminosilicates had lowadsorption capacity(<10mg/g)andadsorptioncapacityupto363mg/gwasobtainedbysurface protonationmethods [49]. Our results are in the sameorder

Table 6Isotherm constant parameters and correlation coefficients calculated for theadsorption of nitrate ions onto activated carbon residue and activated carbon atroom temperature.

Isotherm ACRa ACa

Langmuirqm (mgg�1) 11.198 14.599b (Lmg�1) 0.0172 0.0447R2 0.9922 0.9407

FreundlichKf (L g�1) 3.117 12.360n 1.4126 4.8614R2 0.9854 0.8988

TempkinB (Jmol�1) 2.4746 2.5705A (L g�1) 0.1902 0.6926R2 0.8939 0.9271

Dubinin and Radushkevichb (mol2 J2) �1.1�10�5 �3.2�10�7

qm (mol g�1) 5.2662 8.5797E (kJmol�1) 213.201 395.285R2 0.7463 0.9052

a ACR: activated carbon residue and AC: activated carbon.

S. Kilpimaa et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 1354–1364 1361

of magnitude when comparing with adsorbents which areproduced on waste materials.

Effect of adsorption timeThe adsorption of phosphate and nitrate on three different

adsorbents was investigated as a function of adsorption time(1min–24h) using the optimum pH and optimum initialconcentration which are presented in Table 7. Phosphate removalpercentages as a function of time are presented in Fig. 5a, whichclearly shows that phosphate removal efficiencies increase rapidlywith time especially when activated carbon residue was used asthe adsorbent. Results show that the removal efficiency is notablyhigher for activated carbon residue compared with commercialactivated carbon. On the other hand, removal efficiency is higherfor activated carbon residue compared with unactivated carbonresidue and therefore, activation can notably enhance theadsorption capacity. In the case of phosphate removal, pHincreases during reaction and therefore, it can be assumed thatafter 24h reaction time some removal efficiency is based onprecipitation. Nitrate removal percentages as a function of time arepresented in Fig. 5b. Results show that activated carbon residue iswell adsorbent, whose removal efficiency is slightly higher than40% which is significantly better compared with the adsorptioncapacity of unactivated carbon residue. Commercial activatedcarbon shows good adsorption capacity towards nitrate ions. Fornitrate, the removal rate is very fast in case of carbon residue andactivated carbon residue. The best removal efficiency is achieved atthe start of experiments but after that the removal efficiency startsto decrease gradually. This can be explained by its pH value: in thebeginning pH was optimum but then it starts to increase andtherefore, it is not optimum anymore.

Table 7Optimum pH and concentration values for different adsorbents.

Adsorbent Phosphate Nitrate

pH Concentration(mg L�1)

pH Concentration(mg L�1)

ACRa 6 20 4 10CRa 6 80 4 10ACa 6 10 4 20

a ACR: activated carbon residue, CR: carbon residue and AC: activated carbon.

Rates of adsorptionThe kinetics of phosphate and nitrate adsorption on different

adsorbent surfaces was studied by using pseudo-first-order andpseudo-second-order kinetic models to identify the dynamics ofthe adsorption process [50,51]. The Lagergren’s equation [51] iswidely used for liquid adsorption studies and the linear form ofpseudo-first-order rate expression is given as follows [28]:

logðqe � qtÞ ¼ logqe �kf

2:303t (13)

where qe and qt are the amounts of phosphates or nitratesadsorbed (mgg�1) at equilibrium (1440min in this study) and attime t. The pseudo-first-order rate constant is marked as kf[min�1].

Adsorption kinetics was also calculated by the pseudo-second-order process which can be expressed in a linear form [28]:

tqt

¼ 1ksqe2

þ 1qet (14)

where ks [gmg�1min�1] is the rate constant of pseudo-second-order kinetics.

The rate constants, linear correlation coefficients (R2) and bothexperimental values ofqe(exp) and theoreticalqe(cal) uptakes for bothadsorbates are given in Table 8. Pseudo-second-ordermodel gave agoodfit for different adsorbents and for both adsorbates. The valuesof qe(exp) and qe(cal) indicate that the theoretical values agree wellwith the experimental uptake values in the case of pseudo-second-orderkineticmodel forbothphosphateandnitrateadsorptionforalladsorbents studied. The number of experimental points were 14 inall cases expect for phosphate adsorption onto a carbon residuesurface. In that case, only 5 points could be taken into accountduring calculations because adsorption only started after 360minand therefore the calculations before that point cannot be made.However, for nitrate adsorptionover a carbon residue surface, the ksvaluewas negative and is not included in Table 8. The negative rateconstant can be explained by the fact that the maximal nitrateremoval efficiency was achieved at the beginning of experimentsbut then it starts todecreasewith timeaspresented in Fig. 5. Resultsshow poor adsorption capacity towards nitrate in case of carbonresidue which also explains the negative rate constant.

Weber and Morris intraparticle diffusion modelKinetic data was analysed using the intraparticle diffusion

model on the theory proposed by Weber and Morris [52]. Authorsconsider the phosphate adsorption onto activated carbon residuebecause it was more suitable adsorbent comparing with nitrateadsorption. The amount of phosphate adsorbed (qt) at a particularpoint in time (t) was plotted against the square root of time (t1/2),according to the following equation:

qt ¼ kipt1=2 þ C (15)

where kip is the intraparticle diffusion rate constant and C is theintercept related to the thickness of the boundary layer. If theadsorption mechanism only follows the intraparticle diffusionprocess, a plot of qt versus t1/2 should be a straight line from theorigin. If the data exhibits multi-linear plots, then the process isgoverned by two or more steps. As can be seen in Fig. 6, the data ofphosphate adsorption on activated carbon residue exhibits multi-linear plots (three plots) and plots do not pass through the origin.The same effect is for phosphate adsorption over activated carbon.In case of nitrate adsorption, commercial activated carbon includestwo plots. Weber–Morris model cannot be done for nitrateremoval over activated carbon residue because adsorptionefficiency decreases a little as a function of time. The first stage

[(Fig._5)TD$FIG]

Fig. 5. The removal efficiency of phosphate (a) and nitrate (b) as a function of time for different adsorbents. Experiments were performed at room temperature using theoptimum pH values and initial phosphate/nitrate concentration values presented in Table 7. Adsorbent dose was 5 g L�1.

1362 S. Kilpimaa et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 1354–1364

in multi-linear plots (Fig. 6) can be attributed to the instantaneousor external surface adsorption, for example the diffusion of ionsthrough the solution to the external surface of the adsorbent. Thesecond stage can be attributed to the slow diffusion of ions fromthe surface sites into inner pores, where intra-particle diffusioncan be assumed as a rate-controlling step. In the third stage intra-particle diffusion starts to slow down due to low ion concen-trations left in the solution [27,28,53–55].

Type of adsorption and adsorption mechanism is quite hard toevaluate. Commercial activated carbon includes typically variableamounts of heteroatoms such as oxygen, hydrogen, nitrogen andsulphur which have originally in raw material or they have addedto the carbon surface during production step. Those can haveinfluence the charge, hydrophobicity and electronic density of theadsorbents surface [56]. According to Dias et al. [56], typically

Table 8Pseudo-second-order model parameters, calculated qe(cal) and experimental qe(exp) valu

Adsorbent Phosphate

qe(exp)(mg g�1)

qe(cal)(mg g�1)

ks(gmg�1min�1)

R2

ACRa 3.84 3.88 0.0075 0.99CRa 0.97 0.96 0.0966 0.99

ACa 0.74 0.75 0.0526 0.99

qe: amount of phosphate or nitrate adsorbed on the adsorbent (mgg�1) at equilibriucoefficient.

a ACR: activated carbon residue, CR: carbon residue and AC: activated carbon.

theremay be for example followingmechanisms bywhich ions areremoved from an aqueous solution:

process is based on electrostatic adsorbate–adsorbent interac-tion which is dependent on the existence of adsorbent’s surfacefunctionality;

enhanced adsorption potential which typically occurs in thenarrowest microporosity, may be strong enough to adsorb ions.

Adsorbents tested in this study have no functional groups basedon our IR-measurements. Therefore, it can be assumed thatadsorption is based to the porosity of material.

Error analysisThe traditional methods of determining the isotherm param-

eters by linear regression equations appear to give good fits toexperimental data. However, the use of R2 is limited to solve linear

es for different adsorbents in nitrate and phosphate removal experiments.

Nitrate

qe(exp)(mg g�1)

qe(cal)(mgg�1)

ks(gmg�1min�1)

R2

74 0.76 0.75 5.619 0.999593 0.28 0.28

–0.9976

70 3.64 3.62 0.0537 0.9996

m; ks: rate constant for pseudo-second-order kinetics and R2: linear correlation

[(Fig._6)TD$FIG]

Fig. 6. Weber and Morris intraparticle diffusion model plot of phosphateadsorption on activated carbon residue.

S. Kilpimaa et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 1354–1364 1363

forms of isotherms equation which measure the differencebetween experimental data and theoretical data in linear plotsonly, but not the errors in isotherm curves. Beside, the parametersof non-linear isotherm equations cannot be solved from the linearequation. [57] To evaluate the fit of isotherm equations to theexperimental data, different error functions were used. Theresidual root mean square error (RMSE), the chi-square test (x2)and the sum of squares errors (SSE) were used. Those errors can bedefined as follows [58–60]:

RMSE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

n� 2

Xni¼1

ðqeðexpÞ � qeðcalcÞÞ2vuut (16)

x2 ¼Xni¼1

ðqeðexpÞ � qeðcalcÞÞ2qeðexpÞ

(17)

SSE ¼Xni¼1

ðqeðcalcÞ � qeðexpÞÞ2 (18)

where qe(exp) and qe(calc) show the experimental and calculatedvalues and n is the number of observations in the experimentalisotherm. If RMSE value is small, curve fitting is well. If data fromthe model are similar to the experimental data, x2 will be a smallnumber; If they are different, it will be large number. Also SSE isused to evaluating the goodness of fit. The error analysis was donefor best fitmodels of activated carbon residue and activated carbonwhich were best adsorbents in phosphate and nitrate removal.Results of error analysis were presented in Table 9.

In cases of phosphate removal over activated carbon residueand carbon residue and nitrate removal over activated carbonresidue, the Langmuir model fit was excellent (>0.99). Erroranalysis shows also that RMSE and x2 are low. In case on nitrate

Table 9Error analysis for best fit models during isotherm and kinetic calculations ofactivated carbon residue and activated carbon.

Phosphate Nitrate

Langmuir ACRa ACa ACRa ACa

RMSE 0.09733 0.07146 0.3177 1.0890x2 0.4650 0.1764 0.5701 1.9466SSE 1.1679 0.8575 3.8122 13.068

Pseudo-second-orderRMSE 0.0836 0.0057 0.0329 0.6019x2 0.4397 0.4972 0.1760 4.5247SSE 0.9749 0.1368 0.1495 14.4466

a ACR: activated carbon residue and AC: activated carbon.

removal, Langmuirmodelfitwas not perfect but it was still the bestone when comparing with other isotherm models. Although theLangmuirmodeldidnot showaprecisefit, its adsorptionparameterscanstillprovidevaluable information.Errorsare lowalso forpseudo-second-order model which was expected because fit was excellent(>0.99) and calculated and experimental qe values presented inTable 8 were quite same by using that kinetic model.

Conclusions

The experimental results in this study indicated that thephysically activated carbon residue may be used as a low-costadsorbent for the removal of phosphates and nitrates, for example,in wastewater treatment. Based on our results, activated carbonresidue had a notably higher adsorption capacity compared withcommercial activated carbon in the case of phosphate removal.Adsorption capacity is also substantially better for activated carbonresidue compared with carbon residue and therefore, theadsorption capacity can clearly be enhanced by activation. Theuse of a gasification carbon residue is not only an effective andinexpensive adsorbent, but it also has the potential to significantlyreduce waste streams.

The influences of experimental parameters such as initialconcentration, contact time and pH on the removal efficiencywere investigated. The removal efficiency for phosphate wasfound to be at its maximum at pH 6 while pH 4 was the optimumpH value for the removal of nitrate from an aqueous solution forall adsorbents. The optimum initial phosphate concentration was20, 80 and 10mgL�1 for activated carbon residue, carbon residueand activated carbon, respectively and 10, 10 and 20mg L�1,respectively in the case of nitrate removal. The used adsorbentdose was 5 g L�1 and adsorption batch experiments wereperformed at room temperature. Kinetic modeling was done byusing pseudo-first and pseudo-second-order kinetic calculations.The kinetics showed that the adsorption data followed in goodagreement with pseudo-second-order kinetics in phosphate andnitrate adsorption. Langmuir, Freundlich, Tempkin and D–Risotherm models were tested. Isotherm analysis indicated thatthe adsorption data can be represented by the Langmuir model inphosphate and nitrate adsorption onto activated carbon residueand activated carbon.

Acknowledgements

The authors gratefully acknowledge the HighBio2 project(Biomass to fuels and chemicals/EU Interreg Nord, 304-8455-10)and partnering companies for their financial and technical support.The authors would like to thank Mr. Jaakko Pulkkinen for hisassistance in phosphate and nitrate analysis, and M.Sc HenrikRomar and Ph.D Pekka Tynjälä for BET, pore size and pore volumemeasurements. Our gratitude also goes to the staff at TraceElement Laboratory at the University of Oulu for their assistancewith elementary analysis.

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