Lignosulfonates (LS) are water-soluble anionic polyelectrolyte : they are byproducts from the production of wood pulp using sulfite process.[ LS and SL both appear as free-flowing powders; the former is light brown while the latter is dark brown.]
Lignosulfonates have very broad ranges of molecular mass (they are very polydisperse). A range of from 1,000 to 140,000 Da has been reported for softwood lignosulfonates with lower values reported for hardwoods. Sulfonated Kraft lignin tends to have smaller molecules at 2,000–3,000 Da.[
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Preparation
Lignosulfonates
Lignosulfonates are recovered from the spent pulping liquids (red or brown liquor) from sulfite pulping. Ultrafiltration can also be used to separate lignosulfonates from the spent pulping liquid.
The Electrophile produced during ether cleavage react with bisulfite ions (HSO3−) to give sulfonates.
- R-O-R' + H+ → R+ + R'OH
- R+ + HSO3− → R-SO3H
The primary site for ether cleavage is the α-carbon (carbon atom attached to the aromatic ring) of the propyl (linear three carbon) side chain. The following structures do not specify the structure since lignin and its derivatives are complex mixtures: the purpose is to give a general idea of the structure of lignosulfonates. The groups R1 and R2 can be a wide variety of groups found in the structure of lignin. Sulfonation occurs on the side chains, not on the aromatic ring like in p-toluenesulfonic acid.
Sulfonated Kraft lignin
Kraft process from black liquor, which is produced in much higher amounts, may be processed into sulfonated lignin. The lignin is first precipitated by acidifying the liquor with then washed (other methods for isolation exist). Reaction with sodium sulfite or sodium bisulfite and an aldehyde under a basic environment completes sulfonation. Here the sulfonic acid groups end up on the aromatic ring instead of the aliphatic sidechain.
Uses
LS and SL have a wide variety of applications. They are used to stably Dispersant pesticides, dyes, carbon black, and other insoluble solids and liquids into water. As a binder it suppresses dust on unpaved roads. It is also a humectant and a in water treatment.
Dispersant
The single largest use for lignosulfonates is as plasticizers in making concrete,
Lignosulfonates are also used for the production of Drywall to reduce the amount of water required to make the stucco flow and form the layer between two sheets of paper. The reduction in water content allows lower kiln temperatures to dry the plasterboard, saving energy.
The ability of lignosulfonates to reduce the viscosity of mineral slurries (deflocculation) is used to advantage in oil drilling drilling fluid, where it replaced from Quebracho tree (a tropical tree). Furthermore, lignosulphates are being researched for use in enhanced oil recovery (EOR) due to their ability to reduce interfacial tension in foams, allowing for improved sweep efficiency, and hence increased recovery factor.
Binder
Besides their use as dispersants lignosulfonates are also good binders. They are used as binders in well-paper, particle boards, linoleum flooring, coal briquettes, and roads.
They also form a constituent of the paste used to coat the lead-antimony-calcium or lead-antimony-selenium grids in a Lead-acid battery.
Aqueous lignosulfonate solutions are also widely used as a non-toxic dust suppression agent for unpaved road surfaces, where it is popularly, if erroneously, called "tree sap". Roads treated with lignosulfonates can be distinguished from those treated with calcium chloride by color: lignosulfonates give the road surface a dark grey color, while calcium chloride lend the road surface a distinctive tan or brown color. As lignosulfonates do not rely on water to provide their binding properties, they tend to be more useful in arid locations.
It is used as a soil stabilizer.
Chemical feedstock
Redox of lignosulfonates from softwood trees produced vanillin (artificial vanilla flavor).
Dimethyl sulfide and dimethyl sulfoxide (an important organic solvent) are produced from lignosulfonates. The first step involves heating lignosulfonates with sulfides or elemental sulfur to produce dimethyl sulfide. The methyl groups come from methyl ethers present in the lignin. Oxidation of dimethyl sulfide with nitrogen dioxide produces dimethyl sulfoxide (DMSO).
Other uses
The anti-oxidant effect of lignosulfonates is utilized in feeds, ensilage and flame retardants.
The UV absorbance of lignosulfonates is utilized in sun screens and bio-pesticides.
Lignosulfonate is used in agriculture as an analogue of humic substances. As a soil conditioner, it is mainly used to enhance the absorption and retention of fertilizers and other nutrients.
See also
