Scots pine P. Slyvestris L. is a coniferous tree from the Pinaceae family. The Scots pine trees are among the most commercially important tree species in the world. While its principal use is as a Christmas tree, it yields softwood valued for their timber and wood pulp. Enormous quantities of pine sawdust are produced as by−products from timber and wood pulp industry.
Pine sawdust suggests a broad potential application to adsorbent production. The cell wall of pine sawdust consists of mainly celluloses, hemicelluloses and lignin. The cellulose content consists of numerous hydroxyl groups (OH−) which facilitates cationic dye removal instead of anionic dye removal. Hemi−cellulose contains not only hydroxyl groups but also carboxylic groups (H+), binding active sites for anionic dye. Lignin is built up with polyphenlic groups (i.e an aromatic ring with a three−carbon side chain.). Generally, soft wood (25−35%) contains more lignin than hard wood −18−25% ( Batzias and Sidiras,2007b) . Tannin, a soluble complex polyhydric phenol, is also present in pine sawdust. The detailed chemical composition of Scots pine were reported by Sidiras et al. (2011) and SjÓ§strÓ§m (1993), as presented in Table 2.12.
Table 2.12: Chemical composition of Scot pine sawdust
Component
Percentage,%
Cellulose measured as glucan (52.5% XRD degree crystallinity)
40.1
Hemicellulose
28.5
Mannan
16
Xylan
8.9
Arabian
3.6
Acid insoluble lignin
27.7
Ash
0.2
Extractives and acid soluble components (e.g. acid soluble components)
3.5
The intrinsic composition, inexpensiveness and availability of pine sawdust make it a well favoured low cost adsorbent. Different forms of pine sawdusts and cones have been previously used as adsorbent for the removal of dyes and heavy metals, as shown in Table 2.13 and 2.14 respectively.
Adsorbent
Pollutant
Performance
Untreated P. Slyvestris L sawdust
Lead and Vanadium
Contact Time− 160 min
Adsorption Kinetics− Pseudo second order
Adsorption Isotherm− Freundlich parameter presented unfavourable intensity for sorption of vanadium.
Maximum removal percentage:
Pb : 99 % at pH4
V : 95 % at pH4
(Kaczala et al.,2009)
Untreated pine sawdust
Metal complex dye:
Acid Blue 256
Acid Yellow 132
Contact Time− 120 min
Adsorption Kinetics− Pseudo second order and Elovich equation
Adsorption Isotherm− Langmuir
Maximum adsorption capacity:
Acid Blue 256: 280.3 mg dye per g of pine sawdust
Acid Yellow 256: 280.3 mg dye per g of pine sawdust
( ÈŒzacar and Sengil,2005)
Table 2.13: Studies on pine sawdusts as an adsorbent
Adsorbent
Pollutant
Performance
H2SO4 modified Aleppo pine sawdust
Phosphate ions
Contact Time− 40 min
Adsorption Kinetics− Pseudo second order
Adsorption Isotherm− Freundlich
The low value of activated energy of adsorption, 3.088 KJ mol−1, indicates that the phosphate ions are easily adsorbed on the sawdust.
(Benyoucef and Amrani, 2011 )
Untreated Pinus Sylestris L. fillings
Cr6+
Contact Time− 4 hours
Adsorption Kinetics− Pseudo second order
Adsorption Isotherm− Langmuir and D-R isotherm
The Cr6+ removal increased from 34.8 % to 69.41 % as the adsorbent dosage increased from 3.0 to 8.0 g/L, while the uptake of Cr6+ decreased from 6.09 mg/g to 4.78 mg/g as the biosorbent dosage increased from 3.0 to 8.0 g/L, when the intial Cr6+ concentration was 50 mg/L.
( Mudhoo and Seenauth, 2011)
Tartaric acid modified red pine sawdust
Cr6+
Contact Time− 120 min
Adsorption isotherm− Freundlich
Maximum adsorption observed at pH 3.0 and the adsorption ranges from 8.3 to 22.6 mg/g.
(Gode et al.,2008)
Adsorbent
Pollutant
Performance
Untreated Pinus halepensis sawdust
Basic Methylene Blue
Cr6+
Contact Time− 10−20 min
Adsorption Kinetics− Pseudo second order
Adsorption Isotherm− Freundlich
Favourable pH at 9. An adsorbent dose of 10g/L was optimum for almost complete cadmium removal within 30 min from a 5mg/L cadmium solution. For all contact times, an increase in cadmium concentration resulted in decrease in the percent cadmium removal (100-87%), and an increase in adsorption capacity (0.11-5.36 mg/g).
(Semerjian,2010)
Auto hydrolysis by the organic acids produced by the Scot pine sawdust itself
Basic dyes, methylene blue ,bismarck brown and acridine orange.
In the case of MB, the Freundlich's adsorption capacity KF increased from 5.60 to 15.7 (mg g−1)(L mg−1)1/n, the amount of dye adsorbed when saturation is attained (Langmuir constant qm) increased from 38.7 to 88.0 mg g−1, and the Bohart−Adams adsorption capacity coefficient N increased from 8046 to 14157 mg L−1, indicating the extent that the severity of the autohydrolysis treatment enhanced the adsorption behaviour of the material.
(Sidiras et al.,2011)
Table 2.14: Studies on pine cones as an adsorbent
Adsorbent
Pollutant
Performance
Raw pine and HCl acid treated pine cone powder
Anionic dye congo
Contact Time− 100 min
Adsorption Kinetics− Pseudo second order
Adsorption Isotherm− Freundlich
Maximum adsorption of 32.65 mg/g occurred at pH 3.55 at an initial dye concentration of 20 ppm by raw cone whereas with acid treated pine, the maximum adsorption of 40.19 mg/g (Dawood and Sen,2012).
Potassium hydroxide treated pine cone powder
Cu(II) and Pb(II)
Adsorption Kinetics− Pseudo second order
Adsorption Isotherm− Langmuir
A maximum uptake of 26.32 mg/g was obtained with Cu(II) and 32.26 mg/g with Pb(II) (Ofomaja et al.,2010).
Chemically activated pine cone by NaOH, KOH, CaOH2
Cu2+
Based treatment favoured the formation of new pores predominantly in the order :
Raw sawdust <CaOH2<KOH<NaOH
( Ofomaja and Naidoo,2011)
Cone biomass
of Pinus
sylvestris L.
Copper(II) and zinc(II)
Adsorption Kinetics− Pseudo second order
Adsorption Isotherm− Freundlich and Langmuir
The maximum biosorption efficiency of P. sylvestris was 67% and 30% for Cu (II) and Zn (II), respectively(Ucun et al. , 2009).
Studies show that treated pine sawdust has better adsorption capacity that untreated one. Most of the research done to date has focused on the use of chemically treated−pine sawdust for heavy metal uptake. Very few studies have been carried out on the effect of chemically -treated pine sawdust on dye removal, especially on reactive anionic dye.
Chemical treatment enhances the adsorption capacity of adsorbents by providing a higher number of active binding sites, improving its ion−exchange properties and leading to the formation of new functional groups that favour anions uptake. The most common modifying agents used can be classified as mineral organic acids (HCl, H2SO4), base solutions (NaOH, Na2CO3), organic salts (NaCl,MgCl2) and organic compounds (Formaldehyde, methanol). In this study, the effect of organic acid in the form of H2SO4 and base solution in the form of NaOH was investigated. The pine sawdust will be subjected to a pre−boiling treatment before any chemical pre−treatment.
Pre−Boiling Treatment
Pine sawdust contains water−soluble compounds like tannin which liberate a brown colour. Pre−boiling enables the removal of this brown colour which may interfere with the analysis of RB 221 dye. During the boiling process, the surface area of the scot pine is increased, thereby liberating more active binding sites (Mudhoo and Seenauth, 2011). The functional groups present in the scot pine will also open and polymerise, enhancing its adsorption ability.
Sulphuric Acid Pre−Treatment
Sulphuric acid is a more efficient organic acid compared to hydrochloric acid as it provides a higher amount of H+ ions. These H+ ions provide a net positive surface to the sawdust, thereby providing more chemically active sites for dye binding (Tumlos et al., 2011). Raw sawdust is typified by a highly oriented structure in the form of filaments filled with material, conferring an anisotropic character to the sawdust (Singh et al., 2011). This isotropy is eliminated by treatment with sulphuric acid. Sulphuric acid is selective in its effects (Singh et al., 2011). First, it cleans the filaments and enhances its porosity, thereby eliminating sawdust anisotropy and leaving empty channels. Then, it reacts with the sawdust components, thereby preserving a honeycomb structure (Alvarez et al., 2004; Camacho et al., 1996). The set of chemical reactions with the sawdust components is as follows:
Water−Soluble Glucose
Cellulose
Hemi−Cellulose
Xylose
Degradation Products
Lignin
Not Hydrolysed - Unaffected by mild H2SO4
Figure 2.6: Set of chemical reactions during H2SO4 modification in sawdust
Batzias and Sidiras (2007 b) investigated the effect of H2SO4 pre−treatment on beech sawdust. It was noted that the hemi−cellulose content, initially 27.3% w/w, decreased to 0.8 % after 4 h of H2SO4 treatment. The hydrolysis of hemi−cellulose results in the 'opening' of the structure of the lignocellulosic matrix (Sidiras and Koukios, 1989; Sidiras, 1998). However, the cellulose and lignin content remained practically unchanged. An increase in the BET surface area of the sawdust, from 2 to 6 m2/g, was also noted ( Batzias and Sidiras,2007b).
Sodium Hydroxide Pre−Treatment
Sodium hydroxide is more efficient than Sodium carbonate. This is due to higher number of Na+ ions in 1 g of NaOH compared to 1 g of Na2CO3. Sodium hydroxide is a good reagent for saponification, i.e the conversion of methyl esters , which are major component of cellulose, hemicelluloses and lignin to carboxylate, ligands and alcohol as shown in the equation below:
(Source: Wan Ngah and Hanafiah, 2008)
On a whole, NaOH improves mechanical and chemical properties of sawdust such as structural durability, reactivity and natural ion−exchange capacity (Chakraborty et al., 2011).Such alkaline treatment induced the swelling of the lingo−cellulosic materials which leads to an increase in internal surface area, average pore volume and pore diameter. It was noted by ŠÄ‡iban et al. (2006) that the surface area and average pore diameter of poplar tree sawdust increased about 1.5−2 times after NaOH modification. This treatment also leads to a decrease in the degree of polymerisation, a decrease in crystallinity, separation of structural linkages between lignin and carbohydrates and disruption of the lignin structure. NaOH treated sawdust also shows good settling property, making it easy to filter or separate the adsorbent from the solution (Wan Ngah and Hanafiah, 2008).
Table 2.15 shows a summary of different adsorption studies carried on pre-boiled ,H2SO4 and NaOH treated sawdust.
Table 2.15: Performance of Pre-boiled, H2SO4 , NaOH and treated sawdust.
Treatment
Wood
Pollutants
Absorbance,mg/g
