Flocculants are widely used to decrease the settling time of solutions/suspensions and/or to increase the amount of settled materials. Effective flocculants require good adsorption onto particles through both surface charge and molecular weight.
Lignosulfonates and sulfonated lignin have long been exploited as flocculants.One study employed lignosulfonates to flocculate sulfur slurry used in copper heap leaching at a dosage of 0.2 wt % and improved its density from 40 to 67 %. The lignosulfonates used in this application did not interfere with other aspects of copper leaching.
However, unmodified lignosulfonates and sulfonated lignin are often insufficiently effective to be commercially viable flocculants, and as such should be used along with other flocculants.In one study, lignosulfonates were also applied to various food processing wastewater streams at dosages of 20–40 ppm in combination with xanthan gum and carrageenan.
Increasing the dosage of lignosulfonates provided clearer effluent; however, the size of flocs was adversely affected. Including xanthan gum at 8 ppm increased the floc size and allowed for 85 % removal of biological oxygen demand (BOD), 67 % removal of NH3, and 91 % removal of suspended solids.
Preparing an amphoteric flocculant copolymer of 50 wt % acrylamide, 25 wt % lignosulfonates, and 25 wt % chitosan resulted in dye removal of 50–100 % at a dosage of 300 mg L−1 for various dyes.These values were comparable with those of commercial flocculants with increased environmental advantages because lignosulfonate was used.
Lignosulfonates and sulfonated lignin are effective flocculants when their molecular weights are increased through cross-linking. In one study, lignosulfonates were modified by using polyethylene glycol and tosyl chloride, and the products were tested in a 4 wt % clay suspension containing 1000 ppm NaCl.
This modification resulted in settling times of 80–136 s, whereas the settling time for unmodified lignosulfonates was 570 s.Another study increased the molecular weight of the lignosulfonates by cross-linking with formaldehyde and dimethylamine under alkaline conditions, followed by mesyl disulfonate ester.The product was used as a flocculant in a clay water slurry; it decreased the settling time to 50 s at a 10 ppm dosage relative to a settling time of 600 s when no additive was used.