Biomass Based Technologies
Lecture7
Biomass Based Adsorbents
Among the oldest of methods for treatment of wastewater is the use of adsorbents derived from
biological matter, or biomass. Because of its low cost, widespread availability, biomass has often been
tried for effluent treatment.
The term biomass includes a wide range of materials,dead plant and animal matter agricultural, forest,
fermentation and shellfish by ‐ products. This excludes charcoals, activated carbons, clays, soils,
diatpmaceous earth, activated sludges, compost, living plant communities,which are used in biological
treatment systems.
WASTEWATER DECOLOURIZATION
Most current practices for wastewater decolourization fall into two classes:
(i)
(ii)
those that destroy or modify the offending colour molecules, and
those that remove the coloured species.
The specific practices to achieve this are:
biological or chemical methods to destroy dyes. This can leave harmful organic residues and sludges
removal by precipitation, ion‐exchange and sorption. This results in the need for solid waste disposal
recycling of process waters directly or after some treatment to remove and reclaim salts and processing
agents.
CHITIN AND CHITOSAN
Chitin and chitosan are very common biomass materials used for pollutant removal from textile
effluents.
Chitin is a polysaccharide very similar in structure to cellulose, being composed of poly
2‐acetamido ‐2‐deoxy ‐ D‐glucose.
Chitin is a white flaky material which does not melt and which also is insoluble in water, dilute
Acids,cold alkalis, and organic solvents.
Chitosan is a derivative of chitin, obtained by deacetylation.
Sorption of Dyes
Sorption of dye on chitin and chitosan occurs due to dye/sorbent interaction and depends on
sorbent surface area, particle size, temperature, pH, and contact time. Sorbents have ionic interactions and a highly porous structure with extremely high specific surface
area of chitin and chitosan, which is ideal for sorption. The nature, size and shape of sorbent particles determines the manner of use (i.e. batch reactor I
clarifier vs continuous/ filter) which is essentially based on contact times, sorption rates, setting
times etc.
The effectiveness and limitations of various biomasses in dye adsorption are through the data
shown in Table as
Chitosan has an extremely high affinity
for many dyes, including disperse, direct,
reactive, acid, vat, sulfur and naphthol.
Rate of diffusion of dyes in chitosan is
similar to cellulose .
Chitosan also has versatility due to its
ability to sorb metals and surfactants, as
well as to be derivatized to attract basic
dyes and other moieties
Because sorption of dyes by chitosan is
exothermic, the resulting increase in
temperature leads to an increase in dye
sorption rate, but diminishes total
sorption capacity.
Wastewater pH may be an important factor in thesorption of certain dyes onto chitosan because at low pH, chitosan‘s free amines are protonated, causing them to attract anionic dyes.
Continued…
¾ Contact time or, inversely, flux (wastewater flow per unit cross sectional area) affects rate of sorption due to contact time and boundary layer effects. ¾ At high flux, diversion of liquid into channels around the particles and turbulent flow occurs. Low flux tends to give more complete contaminant removal while high flux treats higher volumes with lower contaminant removal. In a batch reactor high agitation is preferred to uniformly mix the free particles of the sorbent with the solution, because it reduces boundary layer width while increasing contact. ¾ Furthermore, use of certain additives in dyeing such as salt and surfactant can accelerate or retard dye sorption processes. Loading significantly increases the sorption rate because the driving forces for sorption decrease, leading to an ultimate saturation value beyond which further sorption is not feasible.
A Study of Dye Binding Properties
¾ Dye binding properties may be studied by following simple experiment. Weigh 0.5 or 2.0 g chitin or chitosan in centrifuge tubes, add 20 g of aqueous dye solution (5 to 40 mg dye/liter) and then shake the closed centrifuge tubes for 30 min at 200 rpm at a horizontal position.
¾ The supernatant decanted and the water uptake of chitin and chitosan is collected and measured for absorbance at 505 nm using decolorized water as blank. The weight of the supernatant is used for calculation of the total amount of dye bound or released. ¾ Note that chitosan formed gels at pH values below 5.5 and no dye binding measurements could be obtained.
¾ A marked difference between water uptake of chitin and chitosan is found with chitosan having more water uptake than chitin. ¾ Differences in the amount of covalently bound protein residues might also effect water uptake.
Continued….
¾ An effect of chitin/ chitosan: dye solution ratio on water uptake was observed to be higher at 0.5 g: 20 ml ratio than at 2.0 g : 20 M:L ratio. Similar trends were found with dye‐binding capacity.
¾ This difference could be caused by differences in rate of water uptake (wettability) at different chitin to aqueous dye solution ratio. Dye concentration affects dye‐binding capacity of chitin and chitosan.
¾ The dye‐binding capacity of both chitin and chitosan correlated significantly with dye concentration.
¾ The effect of pH on dye‐binding capacity of chitin and chitosan is also quite significant. ¾ Dye‐binding capacity of chitin and chitosan decreases above pH 7.0. Within a pH range of 2.0‐ 7.0 dye‐binding capacity of chitin is stable while chitosan forms gel below pH 5.5
Fixed-bed Systems
Fixed bed systems, sustain high pressure drop losses if fine adsorbent particles were used, and they
have advantage because adsorption depends on the concentration of solute in the solution being
treated. The adsorbent is continuously in contact with fresh solution; hence the concentration in the
solution in contact with a given layer of adsorbent in the column is constant.
The main fixed bed variables include bed height, dye flow rate, and chitin particle size, and these have
been correlated with design parameters, i.e. the bed depth, service time, etc.
CHITOSAN FIBRES
Chitsoan can be spun into fibres, which have much improved absorption kinetics. Chemical cross linking
of chitosan fibres enables the fibres to be used at low pH, which improves their dye‐binding capacity,
without solubilizing the chitosan. Moderately crosslinked chitosan fibres have an Acid Orange II (a monovalent anion) binding capacity of
about 4.5 mol/kg at pH 3‐4. The binding capacity decreases with increasing pH and temperature, but is little affected by salt
concentrations. The crosslinked fibres can be regenerated by treatment with NaOH.
OTHER BIOMASS SYSTEMS
Other than chitin/ chitosan, biomass for colour removal include microbial biomass, unmodified
lignocellulose biomass and chemically‐modified cellulose and lignocellulose. A brief description of the
these biomass systems are as
(a) Microbial biomass
Many industrially useful fungi have chitin and chitosan in their cell walls. Hence, the fungal biomass by ‐
products of industrial fermentation processes serves as an alternative to crustacea as a source of chitin‐
based dye adsorbents. Bacterial biomass has been shown to adsorb textile dyes. Hu examined the
adsorption of eleven reactive dyes to Aeromonas biomass. Dye binds to the bacterial cell wall fraction
which is not composed of chitin or chitosan.
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(b) Chemically‐modified Cellulose and Lignocellulose
Chemical modification of chitin improves its dye adsorption. Derivatization of cellulose or lignocellulose biomass by grafting improves their dye binding properties. This material, composed of 10‐30% cellulose, has a high adsorption capacity for acidic dyes. The adsorption capacity for Direct Blue 86 dye of the PAE‐cellulose (25% cellulose) material is 1.0 mol/kg,
which is very low rate of dye adsorption, requiring three days at 30°C to reach equilibrium.
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The adsorption capacity of PAE‐cellulose is pH dependent (similar to chitosan). The adsorbed
dye can be removed from the cellulose derivative with NaOH. Quaternary ammonium groups can be introduced into cellulose and lignocellulosic materials. The
quaternary ammonium group introduces a positive charge into substrate, making the materials
acidic dye adsorbent.