WO2015066816A1

WO2015066816A1 - Generation of high quality birch-based activated carbons for adsorption of heavy metals and pah from water

Info

Publication number: WO2015066816A1
Application number: PCT/CA2014/051077
Authority: WO
Inventors: Barrie William FIOLEK; Stephanie Lee MACQUARRIE; Khaled Walid OMARI
Original assignee: B.W. Bioenergy Inc.
Priority date: 2013-11-07
Filing date: 2014-11-07
Publication date: 2015-05-14

Abstract

Birch-based biomass was physically and chemically activated generating highly porous carbons that showed excellent ability to adsorb heavy metals (Fe, Hg, Pb, Zn, Al and Cu) and PAH (fluorene and phenanthrene) from water. The method of producing activating highly porous Birch-based carbons includes providing bark free dried birch hardwood chips, subjecting the chips to a pyrolysis process at a temperature in a range from about 250 °C to about 1000 °C to give a fully carbonized product, and subjecting the fully carbonized product to a quenching process, in an absence of oxygen, to rapidly reduce the temperature of the fully carbonized product. The concentrations of these heavy metals and PAH were significantly reduced and can be greater than ca. 99.9%. Certain pesticides were adsorbed using the activated carbon. Importantly, this product improved water quality. Heavy metals and PAH/pesticides were determined using inductively coupled plasma-mass spectrometry (ICP-MS) and gas chromatography-mass spectrometry (GC-MS), respectively.

Description

GENERATION OF HIGH QUALITY BIRCH-BASED ACTIVATED CARBONS FOR ADSORPTION OF HEAVY METALS AND PAH FROM WATER.

FIELD OF THE INVENTION

The present invention relates to a method for physically and chemically activating highly porous Birch-based carbons with an excellent ability to adsorb heavy metals and other pollutants.

BACKGROUND OF THE INVENTION

Current commercially available feedstocks of activated carbon include coconut shells [1 -6], coal [7-1 1 ] and lignite [12-14], all of which suffer limiting drawbacks. Most critically, lignite and coal are non-renewable and costly resources and coconut shells are not readily available in North America [15]. Coconut shells, a primary raw material used to produce activated carbon nearly doubled in price in 2004, while the cost of coal increased 3.5 times the same year. Coupled with the increasing costs associated with transportation, these raw materials are not sustainable, stable resources for generating activated carbon.

Activated carbons are used in a multitude of applications including gas purification [16-18], decaffeination [19], water purification [20-23], synthesis [24,25], sewage treatment [26,27], air filters in gas masks and respirators [28], and metal extraction [29-31 ]. The most important industrial application of activated carbons involves their use as adsorbents to remove pollutants like heavy metals and organic compounds from water systems [15]. Activated carbons have different surface area, porous structure and chemical properties [10,14,15,17,32]. The removal of species recognized as toxic pollutants from water systems, like aromatics and heavy metals, is of critical concern since these pollutants adversely affect human, animal life, domestic and industrial activities [33-36].

Polycyclic aromatic hydrocarbons (PAH) are ubiquitous contaminants and well known for their potential health hazards as many have known carcinogenic, and mutagenic effects [37-39]. The major source of PAH is the incomplete combustion of organic material such as coal, oil and wood, and their presence in water systems affects the use and reuse of water [40,41 ].

This has led to an upsurge of interest in developing and implementing methods for their removal from water and soil using adsorption [41 -45].

Extensive research has been carried out demonstrating the adsorption of aromatic compounds from water systems using carbon adsorbents [42,46,47]. However, most of these systems involve the use of activated carbons generated from non-renewable, expensive sources as described above.

The removal of heavy metals (e.g. Cu, Pb, Fe, Cd, Mn, etc.) from industrial wastewater is of primary importance since they represent the main source of environmental pollution into water systems [48-50]. Such metals may be discharged into the wastes from various industries, including coal- fired power plants, dying, textile, fertilizer, and other chemical industries. With the continuous increase in industrial technology, a great effort must be devoted towards minimizing hazardous pollutants in water systems, limiting their toxic effects on animals, plants and humans [51 -53]. Effects of exposure to toxic levels of many of the above mentioned heavy metals include reduced growth and development, cancer, organ damage, nervous system damage, and in extreme cases, death [54,55]. Exposure to some metals, such as mercury and lead, may also cause development of autoimmunity leading to rheumatoid arthritis, and diseases of the kidneys, circulatory system, and nervous system [56,57]. The removal of heavy metals via adsorption on activated carbons is one of the most convenient methods used; however, many of the activated carbons commercially available are expensive and come from non-renewable resources [29-31 ].

Pollution control and management is a high priority globally and a challenging task for researchers and stakeholders worldwide. The effective removal of the above two kinds of pollutants is crucial to environmental protection and for the sustainable development of local industry. Additionally, the pore size and surface area of activated carbon play a necessary role in its adsorption efficiency.

It would be very advantageous to provide a readily available, renewable, economical activated carbon based material capable of adsorbing known pollutants whether heavy metals or other chemicals.

SUMMARY OF THE INVENTION

Birch-based biomass was physically and chemically activated generating highly porous carbons that showed excellent ability to adsorb heavy metals (Fe (III)), (Hg (II)), and (Pb (II)) and PAH (fluorene and phenanthrene) from water. The concentrations of these heavy metals and PAH were significantly reduced. The percentage removal can be greater than ca. 99.9%. Importantly, this product improved water quality. Heavy metals and PAH were determined using inductively coupled plasma-mass spectrometry (ICP-MS) and gas chromatography-mass spectrometry (GC-MS),

respectively.

The Birch based carbons were developed from bark free properly sized and dried hardwood chips. Then, the chips are subjected to a pyrolysis process at a defined temperature. Once the birch-based carbons are considered fully carbonized they are subjected to a quenching process to reduce temperature rapidly in the absence of oxygen. This process increase yield and pore volume.

Thus, an embodiment of the present method of producing activating highly porous Birch-based carbons includes providing bark free dried birch hardwood chips, subjecting the chips to a pyrolysis process at a temperature in a range from about 250 °C to about 1000 °C to give a fully carbonized product, and subjecting the fully carbonized product to a quenching process, in an absence of oxygen, to rapidly reduce the temperature of the fully carbonized product.

The temperature range over which the pyrolysis may be performed may be over a narrower range, in a range from about 350 °C to about 750 °C.

The quenching process may include submerging the fully carbonized product in any one or combination of water, ice, water-methanol, water- acetone, dry ice, and dry ice-acetone. However, a preferred quenching medium is water, which is more economical than using the other chemicals.

Further features of the invention will be described and become apparent in the course of the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows the BET surface area of chemically activated Birch- ground wood source of carbon.

Figure 2 shows adsorption of heavy metals ions (A) Fe, (B) Hg, and (C) Pb using different chemically activated carbon compared with the Birch- ground wood.

Figure 3 shows adsorption of heavy metals ions (Fe, Hg, and Pb) using different physically activated carbon (saw dust, ground wood, and hard wood).

Figure 4 shows the BET surface area of quenched ground wood and hard wood chips.

Figure 5 shows nitrogen adsorption isotherms at 77 K of quenched activated carbon.

Figure 6 shows the adsorption of heavy metals ions (A) Fe, (B) Hg, and (C) Pb using different quenched carbons.

Figure 7 shows iodine number for activated carbons (untreated (pyrolysis) and quenched carbons).

Figure 8 shows the heavy metals removal comparison among Birch hard wood (pyrolysis, Q/H₂0), wood, peat, and coal-based activated carbons.

Figure 9 shows the adsorption of PAH (fluorene and phenanthrene) using the best Birch-based activated carbons.

Figure 10 shows the adsorption of pesticides using the best Birch- based activated carbon. DETAILED DESCRIPTION OF THE INVENTION

Generally speaking, the systems described herein are methods for physically and chemically activating carbons from cost-efficient feedstocks, specifically the Birch trees that showed excellent ability to adsorb heavy metals and other pollutants. As required, embodiments of the present invention are disclosed herein. However, the disclosed embodiments are merely exemplary, and it should be understood that the invention may be embodied in many various and alternative forms. The Figures are not to scale and some features may be exaggerated or minimized to show details of particular elements while related elements may have been eliminated to prevent obscuring novel aspects. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. For purposes of teaching and not limitation, the illustrated embodiments are directed to methods for physically and chemically activated generating highly porous carbons from cost-efficient feedstocks.

As used herein, the term "about", when used in conjunction with ranges of dimensions, temperatures or other physical properties or characteristics is meant to cover slight variations that may exist in the upper and lower limits of the ranges of dimensions so as to not exclude embodiments where on average most of the dimensions are satisfied but where statistically dimensions may exist outside this region. The present disclosure discloses activation of carbon produced from Birch trees employing several chemical methods at reasonable temperatures in short time periods discloses the quenching effect via reducing the temperature of carbon after pyrolysis. The activated carbon samples have been employed as adsorbents for the removal of heavy metal contaminants from water.

Materials

Birch-based carbon samples were developed by B. W. BioEnegry Inc. using a biomass carbonization process which is under development. The samples were ground using an agate mortar with pestle. The ground carbon was sieved using 60 and 1 00 mesh sizes, which are 250 and 149 \xm, respectively. The sample size between 60 and 1 00 mesh size was used in the most of experiments. Ammonium persulphate ((NH₄)₂S208, min 98%), hydrogen peroxide (H₂0₂, min 30%), phosphoric acid (H₃P0₄, min 85%), Pb(N0₃)₂, Fe(N0₃)₃-9H₂0, and Hg(N0₃)₂-H₂0, min 99.99% of metal nitrate compounds, were purchased from Sigma-Aldrich. Nitric acid (HN0₃, 68-70%), potassium hydroxide (KOH, min 85%), and potassium chloride (KCI, min 99.5%) were purchased from Caledon Laboratories Inc. and BDH Inc., respectively. Boric acid (H₃B0₃, 1 00.1 %) was purchased from Fisher

Scientific. DigiFILTER 0.45 micron and 50 ml DigiTUB (PerkinElmer®) were purchased from PerkinElmer. Wood, peat, and coal-based activated charcoal Norit® were purchased from Sigma-Aldrich. The 1 7 pesticides standard was donated by Dr. Allen Britten at the department of chemistry-Cape Breton University. Deionized water was obtained from a Barnstead NANOpure ultrapure water system (IOWA, USA).

Instruments

Micromeritics® Instrument Corporation ASAP™ 2020 automatic volumetric adsorption analyzer was used to determine the Multi point BET surface area measurements of the activated carbon samples. Nitrogen adsorption/desorption isotherms were performed at 77 K (-1 96 °C) using a standard static volumetric technique. Other determined parameters were micropore area, external surface area, and pore diameter.

Inductively coupled plasma-mass spectrometry (ICP-MS) was used to quantify the heavy metals concentrations. The analysis was performed using NexlON 300D ICP-MS (PerkinElmer Shelton, CT)). Sample uptake rate, integration time, and replicate per sample were 250 μΙ_Ληίη, 500 ms, and 3, respectively. Calibration curves of Hg, Pd, and Fe showed 0.9986, 0.9999, and 0.9985 correlation coefficients (r²), respectively. Percentage recovery of Hg, Pb, and Fe were 94-99, 1 03-105, and 96-102, respectively. The percentage accuracy of Hg, Pb, and Fe were 3.5, 4.0, and 1 .0, respectively. The percentage of relative standard deviation (% RSD) for each tested heavy metal was less than 4.0. The limits of detection (LoD) of Fe, Hg, and Pb were 100, 100, and 10 part per trillion (ppt), respectively.

Gas chromatography-mass spectrometer (GC-MS) from Agilent Technologies, USA was used to quantify the PAH (fluorene and

phenanthrene) concentrations. It was equipped with 6890 N Network GC System (El source) coupled to 5973 inert Mass Selective Detector. The SIM of fluorene and phenanthrene were 166 and 178 amu, respectively, which were used for quantitation purposes. Samples were injected automatically using 7683 B series injector. The separation column was NSP-PAH, J&K 1518012. The conditions of the oven were 80 °C (hold, 2 min), 20 °C/min - 240 °C (hold, 0 min), total run was 10 min. The retention times of fluorene and phenanthrene were 7.76 and 9.09 min, respectively. The r² of the calibration curves of fluorene and phenanthrene were 0.9983 and 0.9993, respectively. Percentage recovery of fluorene and phenanthrene were 95-124 and 102- 1 17, respectively. The % RSD was less than 5.0 for both compounds. The LoD of fluorene and phenanthrene were 0.4 and 0.2 ppb, respectively. For pesticides, the conditions of the oven were 140 °C, 30 °C/min-250 °C, 15 °C/min-280 °C, 75 °C/min-320 °C (0 min). Flow rate was 1 .5 mL/min. The 17 pesticides, retention times, and their SIM (selected ion monitoring) are shown in Table 1.

Table 1 - The 17 pesticides, SIM for quantitation, and their retention times.

SIM

(min)

5.54 Methoxychlor 227

4.99 Endosulfan sulfate 272

4.86 Endrin aldehyde 67

4.80 m,p'-DDD 235

4.62 Endosulfan II 207

4.56 Mitotane 235

4.44 Endrin 263

4.18 Dieldrin 79

4.12 P,P'-DDE 246

3.96 Endosulfan 241

3.70 Heptachlor epoxide 353

3.33 1,4:5, 8-Dimethanonaphthalene 66

3.09 Heptachlor 100

3.19, 2.91, 2.88, 2.59 Lindane isomers 181

Carbon Chemical Activation Method

Porous carbonized Birch-based carbon obtained from B.W. BioEnergy was mixed with various chemicals in water and refluxed for 6 hours (h) for activation at 1 00 °C. The hot sample was filtered and washed several times with hot water to remove impurities and excess chemicals. The filtrate was tested to check for chemicals. For example, in case of using KCI, the filtrate was tested using AgN0₃.The activated carbons were then dried at 1 10 °C for 24 h in an oven. The porous carbons generated from this procedure are denoted AC-X, see Figures 1 and 2, where X represents the chemical used for activation.

Adsorption tests

In order to determine the optimum volume of adsorbate solution for adsorption test, a preliminary set of experiments were conducted at ambient temperature with 0.1 g of birch carbon treated in 5, 50 and 100 mL adsorbate solution in the concentrations range of 50-250 mg/L by stirring at 150 rpm for 24 h. The adsorbate solution volume of 5 mL was chosen since under the experimental conditions at least 80 % removal of all the metals was achieved in comparison to every other volume used. Based on these preliminary results, 5 mL volume was used for detail study of the heavy metals and varying the metals concentration from 50 to 200 mg/L. Consistent results were obtained from repeated experiments with insignificant error margin. The mixtures were filtered using DigiFILTER 0.45 micron for 50 ml DigiTUB (PerkinElmer®) to be ready for analysis using ICP-MS.

Additionally, PAH adsorption tests were performed using 100 mg of activated carbon. A 5.0 mL of 10,000 ppb fluorene and phenanthrene (in 1 :1 water: methanol) was added to the activated carbons and stirred at 150 rpm over 24 hours at room temperature. The mixtures were filtered using

DigiFILTER 0.45 micron for 50 ml DigiTUB (PerkinElmer®). An aliquot of the filtrate was extracted using dichloromethane (3 x 2 mL). After drying the extraction, the residue was reconstituted in 1000 \xL dichloromethane. 1 \xL was injected into the GC-MS. For the pesticides, the adsorption tests were performed using 10 mg of activated carbon. A 1 .0 mL of 100 ppm of 17 pesticides, in methanol, was added to the activated carbons and stirred at 150 rpm over 24 hours at room temperature. The mixtures were filtered using DigiFILTER 0.45 micron for 50 ml DigiTUB (PerkinElmer®). 1 μΙ_ was injected into the GC-MS. Results and discussion

Chemically Activated Carbon

Effects of Chemical Activation on the Adsorbent Properties of Birch- based Carbon

Birch-based carbon was activated chemically using nitric acid (HN0₃), phosphoric acid (H₃P0₄), hydrogen peroxide (H₂0₂), boric acid (H₃B0₃) potassium hydroxide (KOH), water (H₂O), potassium chloride (KCI), and ammonium persulfate ((NH₄)₂S₂O₈). Figure 1 shows the BET surface area of the activated carbons, which were activated using these eight (8) chemical agents. More particularly, Figure 1 shows that in most cases the chemical activation had little impact on the BET surface areas and pore size of Birch- based carbon. Notably, activation with either KOH [58] or (NH₄)₂S₂O₈ leads to a decrease in the surface area (from 209 m²/g to79 m²/g and 1 15 m²/g, respectively). The reduction in the surface area could be due to changing on the surface chemistry of the activated carbons. In an attempt to oxidize the surface, H₂O₂ was used in the activation process and the largest increase in surface area was observed (243 m²/g).

Heavy Metals Adsorption Using Birch-Based Chemically Activated Carbons Figure 2 shows the adsorption of heavy metal ions Fe³⁺, Hg²⁺, and

Pb²⁺ using the chemically activated Birch-based carbons. The Birch-based carbon activated with boric acid had the highest adsorption removing 99.89% iron (Fe³⁺), 99.96% mercury (Hg²⁺) and 99.99% lead (Pb²⁺), see equation (1) which gives the heavy metals percentage adsorbed calculation.

% Adsorbed = ^—^- x 100 (1)

i

Where "i" is the initial concentration of the adsorbate in a solution before adsorption and "f" is the final concentration of the adsorbate in the solution after adsorption using an activated carbon substance.

Importantly, the highest percentage of iron, mercury and lead ion adsorbed was from the untreated Birch-based ground wood carbon activated, see the first point from the left in Figure 2, only through physical pyrolysis (99.87% Fe, 99.99% Hg, and 100% Pb removal) followed by the boric acid activated carbon (99.89 %Fe, 99.96% Hg, and 99.99% Pb removal). This shows that activation of the Birch-based carbons using chemicals is unnecessary for effective adsorption of the metal ions. This promising result indicates benign Birch-based carbon activated by physical pyrolysis may be used as an effective adsorbent for metal removal from waste water streams and coal mine run-off.

Activated Carbon Screening

Physical Pyrolysis of Different Carbon Sources

Three different physically activated carbons were generated from Birch trees in the absence of air at ca. 500 °C. The samples were Birch-based sawdust, ground wood, and hard wood chips. The BET surface areas increased from saw dust to ground wood to hard wood chips, which were 1 69, 203, and 254 m²/g, respectively. Thus the best surface area was obtained using the hard wood chips activated carbon. The full details of each activated carbon in terms of surface area (SA) and pore size are shown in Table 2. Generally, when the micropore volume increased, the BET surface area also increased (Table 2).

Table 2 - BET surface area and porosity size of Birch-based carbon (saw dust, ground wood, and hard wood chips).

BET Micrp^b External Micrp Ads Ave P SA^a Area SA V^c W^d m²/g cm³/g A, nm

Saw Dust 1 69 129 40 0.0683 22, 2.2

Ground Wood 203 149 54 0.0789 23, 2.3 Hard Wood

254 1 98 56 0.1 05 22, 2.2

Chips ^aSA: surface area ^bmicrop: micropore °V: volume

dAds Ave P W: adsorption average pore width

The BET surface area {SBET) , total pore volume ( V_T), micropores volume ( V_M) and average pore diameter (D_p) obtained from the nitrogen adsorption-desorption isotherms at 77 K for the activated carbons prepared with and without quenching in cold water and ice are shown in Table 3. SBET increased from 254 m²/g, for non-quenched carbon to 415 m²/g for carbon quenched in ice/ water. The total pore volume of the carbon increased after quenching, i.e., from 0.1 34 cm³/g to 0.327 cm³/g, as shown in Table 3. The average iodine number (iodine absorbed (mg) by 1 g of carbon) for the quenched carbon was determined in order to obtain information on the surface area of the birch carbon that is composed of pores that are less than 2 nm. We speculate that rapid quenching of the birch carbon in ice/water mixture leads to increase in surface area and pore volume due to the sudden large drop in temperature upon immersion in ice/water mixture (from 500 ^QC to ambient temperature) in less than 5 min. Heat transfer between the carbonized shells and the quenching medium can lead to greater cracks in the interstitial spaces within the shells which eventually lead to more opening of the pores in the birch carbon. We intend in the future to conduct a detailed study to validate this hypothesis.

Table 3 - BET surface area and porosity of the birch activated carbon.

AC treatment SBET (m /g) V_t (m^a/g) v_m Dp (A)

method (cm³/g)

AC-no quenching 254 0.134 0.105 22

AC-Quenching 415 0.327 0.066 32

Heavy Metals Adsorption using Different Birch-based Carbon Sources After the Birch-based carbon samples were subjected to the pyrolysis processes, no treatment was performed on any of the three different types of Birch-based carbon (i.e., saw dust, ground wood, and hard wood). Each was studied for their ability to adsorb heavy metals (Fe, Hg, Pb), Figure 3. The best heavy metals adsorption was observed using hard wood chips, in which Fe, Hg, and Pb were adsorbed from 5000 ppb solution of heavy metals and the % adsorbed (removed) heavy metals were 99.98, 99.95, and ca. 100, respectively. All the carbon substances were able to remove the Pb totally (ca. 1 00% adsorbed). No significant difference was observed in reducing the concentration of Hg among carbon substances, which was adsorbed on an average of 99.95%. The ground wood was not able to reduce Fe via adsorbing greater than 99.87 ppb. Quenched Activated Carbon

Effects of Quenching on the Adsorbent Properties of Birch-based Carbon

In this study, quenching is a rapid cooling of a substance in neat water or chemical aqueous solutions, in which, more pores could be opened in an attempt to increase the BET surface area and enhance adsorption properties of activated carbons. At this end, the idea was to quench an activated carbon, right away, after pyrolysis via cooling it down in different aqueous solutions. Therefore, carbon substance will break down and open more pores to increase the BET surface area. Ground wood (sometimes referred to as pulp) and birch hard wood samples were quenched in water and sodium hypochlorite (NaCIO) aqueous solution. Furthermore, hard wood was quenched in HCI and CaCI₂ aqueous solutions and in ice. The quenching was continuously done after pyrolysis and the temperature was dropped from ca. 500 °C to 20 °C in about 5 min. The samples were washed thoroughly with water and dried at 1 10 °C overnight. The BET SA of hard wood quenched in water and ice were increased to reach 296 and 294 m²/g, respectively as shown in Figure 4. The seven (7) data points shown in Figure 4 are for ground wood and hard wood samples, with the chemicals following the letter Q, designating the quenching medium or chemicals, e.g., Q/H₂0 means the sample was quenched with water.

Water quenched samples showed larger surface areas compared to samples quenched with chemical aqueous solutions. Noteworthy, the quenching with water increased the yield of the product due to a decrease in the ash rate during the cooling time. For example, in both carbon sources (i.e., ground wood and hard wood chips), the surface area of activated carbon quenched in water is larger compared with those quenched with NaCIO(_aq) as shown in Figure 4. The largest surface area was obtained for the hard wood sample which was quenched in water (296 m²/g). The second largest surface area was 294, which was for hard wood quenched in ice. The largest micropore volume was for water and ice quenched carbon, which were 0.1 19 and 0.135 cm³/g, respectively as shown in Table 4. The nitrogen adsorption isotherms of quenched activated carbons are shown in Figure 5. The nitrogen-phase adsorption was type 1 that reached a complete monolayer saturated single monolayer of the adsorbate on the adsorbent surface. The quantity adsorbed for hard wood quenched in water and ice were the highest. Both of them had the same adsorbed amount at P/Po = ca. 0.3.

Table 4 - BET surface area and porosity size of quenched Birch-based

carbon.

BET SA Micrp^c Area External SA Micrp V^d Ads Ave P W^e m²/g cm³/g A, nm

Ground wood

Q^a/NaC10 236 189 47 0.100 22, 2.2

Ground wood

Q/H₂0 258 210 48 0.111 21, 2.1

Hard wood

Q/NaCIO 243 200 43 0.106 22, 2.2

Hard Wood

Q/H₂0 415 378 37 0.327 32, 3.2

Hard Wood

Q/HC1 220 160 60 0.0852 23, 2.3

Hard Wood

Q/CaCl₂ 213 194 19 0.103 19, 1.9

Hard Wood

Q/Ice 294 255 39 0.135 20, 2.0 ^aQ: quenched ^bSA: surface area °microp: micropore ^dV

eAds Ave P W: adsorption average pore width

Heavy Metals Adsorption Using Birch-Based Quenched Activated

Carbons

The percentage adsorption of metals in the concentration range (50- 200 ppm) of each metal for the birch carbon is shown in Table 5. For all metals, the percentage removal was greater than 80 % under the same contact time for 5 mL adsorbate solution volume. In this study, there was no addition of acid or base to alter the pH of the aqueous solution. The results for the percentage removal of heavy metals with respect to adsorbent initial concentration are also shown in Table 5. The amount of adsorption increase per unit mass of adsorbent with increase in initial concentration, but the percentage of metal removal showed a decrease with the increase in initial concentration of the metals. This can probably be explained by the fact that high initial concentration provides the necessary driving force to overcome the resistances to mass transfer of metal ions between the aqueous and solid phase carbon. On the other hand, the percentage removal of the metals decreases slightly with increase in initial concentration because the adsorbent dosage (100 mg) or number of adsorptive sites is fixed.

Table 5 - Equilibrium concentrations obtained by ICPMS analysis and removal efficiency of metals and calculated Langmuir adsorption isotherm parameters.

Co (mg/L) Ce (mg/L) q_e (mg/g) Percentage Qmax K_L r¹ removal (%) (mg/g) (L/mg)

Lead (Pb)

200 0.2594 9.9870 99.8703

150 0.1896 7.4905 99.8736

100 0.1006 4.9950 99.8994 20.576 3.5735 0.9916

50 0.0375 2.4981 99.9250

Zinc (Zn)

200 6.6392 9.6680 96.6804

150 1.900 7.4050 98.7333

100 0.5401 4.9730 99.4599 10.428 1.7405 0.9974

50 0.1293 2.4935 99.7414

Copper (Cu)

200 37.5340 8.1233 81.2330

150 15.5570 6.7222 89.6287

100 2.4715 4.8764 97.5285 8.532 0.3970 0.9957

50 0.8724 2.4564 98.2552

Aluminium

(Al)

200 1.6464 9.9177 99.1768

150 0.7106 7.4645 99.5263

100 0.2846 4.9858 99.7154 12.853 2.0367 0.9980

50 0.1239 2.4938 99.7522

Mercury (Hg)

200 0.9382 9.9531 99.5309

150 0.5612 7.4719 99.6259 100 0.2809 4.9856 99.7191 16.863 1.4937 0.9953

50 0.1136 2.4943 99.7728

Iron (Fe)

200 1.4832 9.9258 99.2584

150 0.4018 7.4799 99.7321

100 0.0544 4.9973 99.9456 10.373 11.7561 09953

50 0.0189 2.4990 99.9622

The equilibrium adsorption data was fitted with the Langmuir adsorption isotherm due to higher correlation coefficients (l values) obtained. The Langmuir adsorption isotherm equation was used to determine adsorption parameters. The linearized Langmuir equation is as follows:

(1 )

Where C_e is adsorbate equilibrium concentration (mg/L), q_m is maximum adsorption capacity (mg/g), K_L (L/g) is Langmuir constant related to energy of adsorption (affinity of binding sites), qe is the adsorption capacity of metal ions per unit weight of adsorbent at equilibrium (mg/g). This can be determined by using the following equation:

_ (C₀ - C_e )V

^Qe w

(2) Where C₀ (mg/L) is the initial concentration of adsorbate, V is the volume of adsorbate solution, and W is the weight of adsorbent.

The Langmuir adsorption parameters obtained are shown in Table 5 as well. The maximum adsorption capacity of the carbon for the metals is in the following order: Pb > Hg > Al > Zn > Fe > Cu. The substantially large value of K|_ (L/g) obtained in all cases for the birch carbon in this study suggests a very high affinity between adsorbate and the carbon.

Comparison with other studies in heavy metals removal

Noteworthy, some studies indicated that the pH (ca. pH < 6.5) of the aqueous solution (i.e., adjusted using an acid) played a crucial rule in increasing the adsorption of heavy metals using activated carbon [59-61 ]. In this study, there was no need to change the pH of the aqueous solution to increase the adsorption of the heavy metals. The pH of Birch-based carbon of ground wood (pyrolysis) (the first data point at the far left of Figures 1 and 2), and hard wood quenched in water, 2% activated carbon in water at room temperature, are 8.3 and 8.5, respectively. This study showed a better heavy metals removal from aqueous contaminated water using Birch-based carbon compared with other studies, Table 6. In comparison between a surface area and lead adsorption, an activated carbon that was produced from polyacrylonitrile fiber [59] of a surface area 71 1 m²/g removed 50% lead. In this study, ground wood and hard wood quenched in water had 254 and 296 m²/g surface areas, respectively and adsorbed ca. 99.99% lead.

Table 6 - A comparison of heavy metals removal between this study and other studies from the literature.

Adsorbed heavy metal (%)

Reference Activated carbon source

Fe Hg Pb

59 Coal NR' NR 63.6

60 Polyacrylonitrile fiber NR NR 50.0 62 Soy bean oil cake NR NR 47.6

63 Palm oil empty fruit bunch NR Ca. 100 Ca. 100

64 Olive stones NR 72.0 NR

65 Pitch Ca. 92.0 NR NR

Birch-Saw Dust (pyrolysis) 99.97 99.95 Ca. 100

Birch-Ground Wood (pyrolysis) 99.87 99.99 Ca. 100

This study Birch-Hard Wood Chips (pyrolysis) 99.98 99.95 Ca. 100

Birch-Hard Wood (pyrolysis, Q/H₂0) 99.91 99.93 99.99

'NR: not reported

Iodine number of the best activated carbon materials in this study,

Table 2, was tested according to ASTM international standard [66], Figure 7.

The average iodine number was 1 163 mg/g that is relatively high compared with the iodine number of high surface area carbon substances.

Comparison with commercially available activated carbons, heavy metals removal

Birch-Hard Wood (pyrolysis, Q/H₂0) based carbon was tested along with wood, peat, and coal-based activated carbons (Norit®), which are commercially available. Birch-based activated carbon showed competitive results, see Figure 8. Birch-based carbon adsorbed slightly higher levels of iron and lead compared to other activated carbons, which removed 99.98 and ca. 100% of the previously mentioned heavy metals, respectively. The other activated carbons removed > ca. 99.6 and 99.4% of Fe and Pb, respectively. For Hg, wood, peat, and Birch showed almost the same % removals that were 99.97, 99.96, and 99.95%, respectively. Coal-based activated carbon (nonrenewable depleting resource) showed a relatively higher removal of Hg ca. 100%. PAH (fluorene and phenanthrene) adsorption on the activated carbon

The adsorption tests were performed using Birch-Saw Dust (pyrolysis), Birch-Ground Wood (pyrolysis), Birch-Hard Wood Chips (pyrolysis), and Birch-Hard Wood (pyrolysis, Q/H₂0). The above mentioned activated carbon materials significantly removed the PAH in a high percentage, as can be seen in Figure 9. Flourene and phenanthrene were removed ca. 100 and >

99.91 %, respectively. Birch-Hard Wood (pyrolysis, Q/H₂0) was the best. It removed fourene and phenanthrene ca. 100% from the solution.

Chlorine adsorption on the activated carbon

Chlorine adsorption test were conducted using different amounts of birch carbon (5, 10, 15, 20 and 25 mg) in 50 ml, 10 ppm chlorine

concentration solution. The carbon samples were dispersed in the chlorine solution for 1 and 4 hours respectively. Chlorine was completely removed (adsorbed) by the carbon samples irrespective of the amount of carbon used or the time for which the adsorption experiment was monitored.

Pesticides adsorption on the activated carbon

The adsorption tests of pesticides were performed using Birch-Hard Wood (pyrolysis, Q/H20). The results, Figure 10, showed that Birch-based carbon adsorbed eleven (1 1 ) active ingredients of the pesticides (insecticides) out of the seventeen (1 7). The adsorbed pesticides were Endosulfan sulfate, 1 ,2,4-Methanocyclopenta [cd]pentalene-5-carboxaldehyde, m,p-DDD, Endosulfan II, Mitotane, ρ,ρ'-DDE, Heptachlor epoxide, 1 ,4:5,8- Dimethanonaphthalene, and 4 isomers of Lindane. Although the amount of the activated Birch-based carbon that was performed in the test was small (10 mg), the Birch-based activated carbon showed a high affinity toward adsorbing pesticides.

Thus, the present method of producing activating highly porous Birch- based carbons includes providing bark free dried birch hardwood chips, subjecting the chips to a pyrolysis process at a temperature in a range from about 250 °C to about 1000 °C to give a fully carbonized product, and subjecting the fully carbonized product to a quenching process, in an absence of oxygen, to rapidly reduce the temperature of the fully carbonized product.

The quenching process may include submerging the fully carbonized product in any one or combination of water, ice, water-methanol, water- acetone, dry ice, and dry ice-acetone. However, a preferred quenching medium is water, which is more economical than using the other chemicals. Conclusions

This study demonstrates that activated carbon can feasibly be produced from Birch trees. Pyrolysis and water quenched Birch-based hard wood chips carbon not only yielded a ca. 300 m²/g BET surface area and 0.1 19 cm³/g micropore volume substance but also reduced the ash converted carbon due to the fast cooling in water. The Birch-based activated carbon substances have high affinity for adsorbing heavy metals (Fe, Hg, and Pb) from an aqueous solution with no need to adjust the pH of a solution. The activated carbon also showed a significant adsorption toward PAH (fluorene and phenanthrene). There is no direct relationship between the surface area and heavy metals removal. This activated carbon is readily available from locally obtainable renewable biomass resource, which is a benign, inexpensive, and reliable renewable feedstock. Remarkably, the activated carbon was produced without adding any noxious chemical. Consequently, the process disclosed herein is very benign and satisfies the triple bottom line (TBL), which is profit, people, and planet.

As used herein, the terms "comprises" and "comprising" are to be construed as being inclusive and open rather than exclusive. Specifically, when used in this specification including the claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or components are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

It will be appreciated that the above description related to the invention by way of example only. Many variations on the invention will be obvious to those skilled in the art and such obvious variations are within the scope of the invention as described herein whether or not expressly described.

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Claims

THEREFORE WHAT IS CLAIMED IS:

1 . A method for producing and activating porous Birch-based carbons, comprising:

a) providing bark free dried birch-based biomass; and

b) subjecting the birch-based biomass to a pyrolysis process at a temperature in a range from about 250 °C to about 1000 °C to give a fully carbonized product.

2. The method according to claim 1 , further comprising the step c) of subjecting the fully carbonized product to a quenching process, in an absence of oxygen, to rapidly reduce the temperature of the fully carbonized product.

3. The method according to claim 2, wherein the quenching process includes submerging the fully carbonized product in any one or combination of water, ice, water-methanol, water-acetone, dry ice, and dry ice-acetone.

4. The method according to claim 2, wherein the quenching process includes submerging the fully carbonized product in water.

5. The method according to claim 4, wherein the quenched fully carbonized product is characterized by exhibiting BET surface area and micropore volume of about 400 m²/g and 0.1 19 cm³/g, respectively.

6. The method according to any one of claims 1 to 5, wherein the temperature during the pyrolysis process is in a range from about 350 °C to about 750 °C.

7. The method according to any one of the claims 1 to 6, wherein birch- based biomass is one of saw dust, ground wood and hard wood chips.

8. The method according to any one of the claims 1 to 6, wherein birch- based biomass is hard wood chips.

9. The method according to any one of the claims 1 to 8, further including chemical treatment of the birch-based biomass.

10. A activated carbon product produced by the method of any one of claims 1 to 9.

1 1 . The activated carbon product of claim 9 for use as an adsorbent for pollutants.

12. The activated carbon product of claim 1 1 wherein the pollutants are heavy metals.

13. The activated carbon product of claim 12 wherein the heavy metals are any one or combination of Fe³⁺, Hg²⁺, Pb²⁺, Zn, Al and Cu²⁺

14. The activated carbon product of claim 1 1 wherein the pollutants are pesticides.

15. The activated carbon product of claim 1 1 wherein the pollutants are polycyclic aromatic hydrocarbons (PAH).

16. A method for producing and activating porous Birch-based carbons, comprising:

a) providing bark free dried birch-based biomass; and

b) subjecting the birch-based biomass to a chemical treatment to produce an activated carbon product.