During the last decades, intense human activities have been involved in the pollution of the environment by many hazardous pollutants, such as heavy metals. These pollutants can interact with soil particles or remain dissolved in water. Additionally, the development of plant fiber filters in hydraulic structures is a method utilized to reduce heavy metal content in urban water, contributing to environmental remediation efforts. Recently, an environmentally friendly method to clean up polluted waters is to use bio-based adsorbents. In this study, flax fibers are used to remove Cd(II), Cu(II), Hg(II), Ni(II), Pb(II), and Zn(II) from simulated polluted waters at concentrations close to polluted runoff water samples (µg L−1 range). Several parameters affecting the adsorption efficiency were investigated in two systems, single and multi-element: adsorbent concentration, initial metal concentration, contact time, and the competition effect. Kinetic study was performed for each metal for both systems. The Competitive Langmuir isotherm model was used to analyze the experimental data. It encompassed regression and validation steps to study the simultaneous adsorption of metals. At this concentration range (µg L-1), results suggested no competition between heavy metals for adsorption sites on flax fibers: variation coefficients obtained between maximum adsorption capacities in single (each metal) and multi-element (all metals simultaneously) systems were less than 4%.