Valuable Characteristics Responsible For Efficiency

Published Date: 02 Nov 2017

Table 2.10: Kinetic models

Kinetic Model

Purpose

Pseudo 1st order

Used for reversible reaction with an equilibrium being established between a solid and liquid phase (Radnia et al., 2011).

It was found to not fit well over the entire contact time range, but was generally applicable over the initial periods of the sorption process.

Diffusion mechanism cannot be identified.

Pseudo 2nd order

Good correlation of data by the pseudo 2nd order equation over the whole contact time (Sag and Aytay, 2002).

The rate limiting factor may be chemisorption (Low et al., 2000).

Diffusion mechanism cannot be identified.

Intraparticle Diffusion

Intra−particle diffusion as being the rate controlling step (Venkata et al., 2008).

Film diffusion is negligible due to stirring (Radnia et al, 2011).

Adsorbent: Scots Pine Sawdust

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 Sjӧstrӧm (1993), as presented in Table 2.11

Table 2.11: 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 have been previously used as adsorbent for the removal of dyes and heavy metals, as shown in Table 2.12.

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 2.12: Studies on pine 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 )

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)

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

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)

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)

Sulphuric acid modified Scot pine sawdust

Basic methylene blue

Cr6+

(Politis and Sidiras,2012)

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)

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)

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. 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 Šćiban 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 2.12: Performance of H2SO4 , NaOH and Pre-boiled treated sawdust.

Reagents

Wood

Pollutants

Absorbance,mg/g