Adsorption of pollutants by plant bark derived adsorbents: An empirical review

Plant barks are among the most widely applied low-cost biomass materials in the study of pollutant removal from aqueous media. This paper extensively reviews the experimental findings presented in open literature with much focus on the last 15 years. This study classified plant bark adsorbents into 5 broad groups (based on their preparation technique): unmodified biosorbent, pre-modified biosorbent, chemically modified biosorbent, physically modified biosorbent and bio-based activated carbon. It was observed that eucalyptus, pine, neem, acacia and mango are the most explored source species in tree bark adsorption studies. About two-third of target impurities reported on the subject in open literature have been on heavy metals. The review elucidated the excellent adsorption capacities of plant bark based adsorbents and biosorbents for the uptake of heavy metals, dyes, pesticides and other pollutants. Adsorption was majorly best-fit to either the Langmuir or Freundlich isotherm models and the pseudo-second order kinetic model. The thermodynamics findings revealed that the adsorption is highly spontaneous and is by a physical mechanism in most cases. It was also observed that plant barks have high reusability potential thereby underlying their usefulness for industrial application. Knowledge gaps in the research area were also discussed in line with future perspectives.