# Anaerobic Digestion of Duckweed Used to Remediate Water Contaminated with Zinc and Ammonium
## 1 Introduction
Although the presence of Zn is common in many NH\({}_{4}\)\({}^{+}\)-N-rich wastewater, very few studies have reported simultaneous bioremediation of Zn and NH\({}_{4}\)\({}^{+}\)-N using duckweed [14]. Despite advances in duckweed-based remediation, there is a critical knowledge gap regarding the integrated management of nutrient and metal removal, the impacts of metal accumulation on biogas production, and the environmental risks associated with residualmetals in digestate. Addressing these gaps is essential for developing robust, circular wastewater treatment systems that maximize pollutant removal, energy recovery, and environmental safety. In this study, we investigate the simultaneous phytoremediation of zinc and ammonium from contaminated water using duckweed, followed by the anaerobic digestion of the harvested biomass for methane production. We systematically assess the effects of zinc and ammonium on duckweed growth and contaminant removal, evaluate the biochemical methane potential of acid-hydrolyzed biomass fractions, and examine the fate of zinc throughout the process. Our findings aim to provide actionable insights for optimizing integrated phytoremediation-biogas systems and inform strategies for safe, sustainable resource recovery from metal-laden wastewater. In particular, in this research duckweed was used to phytoremediate zinc- and ammonium-containing wastewater. Subsequently, the harvested duckweed was used as feedstock for anaerobic digestion, serving the role of a carbon source and providing Zn supplementation for the production of biogas to obtain the dual benefits of pollutant remediation and biofuel production. The purpose of this work was to (1) reveal the effects of Zn and NH\({}_{4}\)\({}^{+}\)-N on duckweed growth, (2) examine the ability of Zn bioaccumulation under different initial Zn and NH\({}_{4}\)\({}^{+}\)-N concentrations, (3) evaluate the biogas production and biomethane potential of Zn-enriched biomass, and (4) provide new insights into the formation of anaerobic digestion intermediates such as volatile fatty acids (VFAs) during the anaerobic digestion of duckweed. This study represents a significant advancement in the field of integrated phytoremediation and resource recovery from contaminated wastewater, specifically through the use of duckweed. While numerous studies have established duckweed as a robust candidate for nutrient and heavy metal removal from polluted waters [18; 30; 31; 32] and others have explored its potential for protein, pigment, or vitamin E production [14; 33], the novelty of the current research lies in its comprehensive process integration and mechanistic insights. Unlike previous works [14], which have focused primarily on the effects of zinc ions on the nutrient removal efficiency and physiological responses of _Lemma aquinoctialis_, this work uniquely investigates the fate and impact of zinc and ammonium not only during phytoremediation, but also through the subsequent anaerobic digestion (AD) of the harvested biomass. This study is the first to systematically assess both the acid-hydrolyzed solid and liquid fractions of Zn/NH\({}_{4+}\)-rich duckweed as substrates for methane production, revealing the dual challenges of substrate degradability and zinc inhibition in AD, a dimension largely absent in the prior literature.