Dynamics of Flocculation of Lignocellulosic Hydrolyzates by Polymers

Pretreatment of lignocellulosic materials involves the solubilization of hemicelluloses, small portions of the lignin, and such components. The hemicellulose solutions are hydrolyzed for downstream fermentation into biofuels or biomaterials. Hydrolyzates produced by pretreatment contain significant colloidal material that is anionically charged. Many of the compounds that are present in the hydrolyzates are inhibitory to fermentation and interfere with downstream separations. The flocculation of this colloidal material makes separations easier by sedimentation and can reduce the fouling tendencies of membranes. It can also reduce the toxicity of the hydrolyzates to fermentation micro-organisms. We studied the dynamics of flocculation of lignocellulosic hydrolyzates with a variety of flocculating agents: electrolytes (alum) and polymers (PEI, pDADMAC, CPAM). It was found that trivalent cations were the most effective suspension destabilizers among the electrolytes, while the cationic polymers could cause flocculation and also redispersion depending on their dosage levels. Flocculation reduced the hydrolyzates' turbidity from >10000 to under 20. With PEI and pDADMAC, flocculation occurred rapidly when the zeta potential of the colloid was close to zero showing that charge neutralization is the significant destabilizing mechanism. At higher dosages, redispersion occurred indicating that patching is also important in flocculation. Flocculation by PEI was sensitive to pH (from hydrolyzate pH of 3 to 8) with increased dosage necessary at higher pH values. The cationicity of PEI is reduced at higher pH which results in loss of its effectiveness. On the other hand, the zeta potential was largely unaffected with CPAM dosage indicating the dominance of bridging flocculation. Floc sizes ranged up to 3 mm, depending on flocculant dose and pH.