Variation of Copper Adsorption with Initial pH and Pyrolysis Temperature by Saw Dust and Paddy Husk Biochar Made in an Industrial Type Pyrolizer

Copper (II) ions (Cu2+) are one of the widely spread heavy metals in the environment causing ecological and human health risk. Among the conventional methods of Cu2+ removal, adsorption is observed as the most favorable. Biochar, a product of pyrolysis, has a relatively structured carbon matrix with high degree of porosity and extensive surface area, like activated carbon suggesting its suitability to be used as an adsorbent material. In this study, biochar was synthesized using an industrial type pyrolyzer and low-cost agricultural waste materials; saw dust and paddy husk as feedstocks. Six sets of biochar were made adhering to three temperature ranges: 350–450, 450–550 and 550–650 °C with a resident time of 30 min. Further, the effect of solution pH on the adsorption of Cu was tested. All the biochar materials exhibit similar trend of maximizing adsorptive removal at pH = 6. At lower pH values the surface charge of the biochar materials is positive creating unfavorable grounds for Cu2+ removal. As pH increases above 6.5, Cu2+ precipitates as Cu(OH)2 lowering the amount of soluble Cu2+ in the solution. Thus, the negative adsorptive removal efficiencies can be observed at the pH values above 6. The trend followed the shape of a maximum graph for almost all biochar materials where the removal maximizes at 450–550 °C pyrolysis temperature as lower temperatures favor more functional groups while the higher temperatures favor the development of higher specific surface area. At most pH values, the saw dust biochar shows higher removal percentage compared to the paddy husk biochar. Hence, a positive indication of Cu removal is present at lower pH values. The kinetics follow the pseudo-second order model suggesting that chemisorption is dominant. However, isotherms of adsorption, surface characterization of the materials, tests for leaching of adsorbed pollutants must be performed before field applications are done with the developed biochar materials.