Cationic Lignin Polymers as Flocculant for Municipal Wastewater.

Courtney Moore,Pedram Fatehi,Weijue Gao

doi:10.3390/polym13223871

Abstract

The radical polymerization of acid-washed and unwashed softwood kraft lignin with [2-(methacryloyloxy) ethyl] trimethylammonium chloride (METAC) was attempted to investigate the production of lignin-based flocculants for simulated wastewater. The incorporation of METAC onto lignin resulted in a cationic charge density (2.3–3.3 meq/g), increased water solubility (89–96% in neutral pH), and increased molecular weight (70,000–210,000 g/mol) of lignin. The lignin–METAC polymers generated from acid-washed lignin had higher molecular weights than those generated from unwashed lignin. The lignin–METAC polymers showed lower resistance to thermal decomposition than unmodified lignin due to the inclusion of PolyMETAC. The unmodified acid-washed lignin samples did not significantly affect the COD of the wastewater, while the unmodified unwashed lignin samples contributed to the COD, implying that unmodified lignin was not suitable for wastewater treatment. The flocculation of wastewater with lignin–METAC led to the chemical oxygen demand (COD) reduction of 17–23% and total organic carbon (TOC) drop of 51–60%. The lignin–METAC polymer with the highest molecular weight (produced from acid-washed lignin) reached the highest COD removal, while lignin–METAC polymer with the highest charge density (produced from unwashed lignin) reached the highest TOC removal. Focused beam reflectance measurement (FBRM) studies revealed that the lignin–METAC polymer produced from acid-washed lignin with a high molecular weight generated larger and more flocs in wastewater than the lignin–METAC polymer produced from unwashed lignin. The comparison of theoretical and experimental dosages required for neutralizing the charges of wastewater demonstrated that charge neutralization was the main flocculation mechanism, although a bridging mechanism was also involved for component removals from wastewater. The use of 1 mg/L of alum along with 65 mg/L lignin–METAC in a dual coagulation–flocculation system led to higher average phosphorous (42%) and COD (44%) removals than the singular flocculation system only using 65 mg/L of lignin–METAC (with phosphorous removals of 3.4% and COD removals of 18.7%). However, lignin–METAC flocculant slightly increased the ammonia–nitrogen content in both singular flocculation and dual coagulation–flocculation systems due to the residual ammonia content of lignin–METAC. The coagulation–flocculation system determined that the use of lignin–METAC (65 mg/L) could reduce the alum dosage significantly while maintaining a similar organic content reduction of 44% for wastewater.

Highlights

IntroductionCoagulants and flocculants are often considered the second most expensive operational costs of municipal wastewater treatment facilities next to electricity [1,2]
A slight increase in ammonia–nitrogen is observed when the wastewater is treated with a lignin–METAC flocculant, which can be attributed to the residual ammonia content of lignin–METAC
The radical graft polymerization of acid-washed and unwashed kraft lignin samples with METAC was attempted, and the products were successfully used as flocculants in synthetic municipal wastewater

Summary

Introduction

Coagulants and flocculants are often considered the second most expensive operational costs of municipal wastewater treatment facilities next to electricity [1,2]. Organic removal from municipal wastewater with polymer flocculants often reaches >90% efficiency [3,4]. Coagulation often requires high dosages, resulting in the production of large volumes of sludge and high disposal and landfilling costs [4]. In this regard, coagulation–flocculation, in which polymer flocculants and coagulants are used together, is often utilized for municipal wastewater treatment to reduce the coagulant dosage while still achieving effective organic removal [4]. As common flocculants and coagulants are often considered toxic, corrosive, and non-biodegradable [4], a more cost-effective and environmentally friendly flocculant would benefit wastewater treatment facilities greatly, as it could reduce the secondary contamination of wastewater

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Conclusion