Water is our most important resource, and decreasing the costs of water treatment is essential not only to increase the accessibility to this resource but also to encourage companies to reuse their liquid waste. Lignocellulosic agricultural waste is of great interest for this purpose due to their relatively high adsorption capacities and their vast availability as a residue. However, the molecular mechanism of the sorption processes happening on these materials has not yet been properly reported in the literature, which limits the advance of their application in water treatment. This research project had the objective of using molecular dynamics simulations (GROMACS package) to study the adsorption mechanism of wastewater pollutants on lignocellulosic biomass. The project aimed to provide sorption kinetic and thermodynamic data that could be used to optimize low-cost water treatment systems that use these materials. 

Lignocellulosic biomass consists of a cellulosic fiber with lignin and hemicellulose attached to it. However, studies in the literature showed that the biomass most interesting to water treatment had a relatively low hemicellulose content. Therefore, a cellulose fiber with only lignin attached to it was modeled in the computer and then put in a box with water and different pollutants. Simulations of this system under different conditions were run to observe the adsorption mechanism. The results showed that cellulose can form stable electrostatic interactions with pollutants while lignin serves as a molecule “catcher”, attracting adsorbents, bringing them close to cellulose, then physically blocking their way out. By varying the temperature of the system, it was also observed that for each pollutant, there is an optimum temperature in which adsorption is maximized. This temperature dependence agrees with experimental data in the literature, which supports the accuracy of the model. Finally, the modeled structure was used to test the interactions between lignocellulosic biomass and enzymes. Simulations evidenced that lignocellulosic materials can form stable electrostatic interactions with PETase, which is an enzyme that catalyzes the decomposition of PET plastics. This indicates that lignocellulosic agricultural waste could be used with these enzymes to develop low-cost systems that could remove microplastics from the water.

This study provides valuable data for developing more effective applications of waste agricultural biomass in wastewater treatment. Furthermore, it provides a framework for biomimetics studies that may use similar adsorption mechanisms to develop artificial adsorbents. Finally, it provides a physical understanding of biosorption, which can help better model molecular interactions of plant cells. However, future studies are needed to observe the adsorption behavior in these materials at a larger range of system conditions, and at different pollutant concentrations.