Evaluation of characteristics of raw tea waste-derived adsorbents for removal of metals from aqueous medium

Chemical modification and microwave activation are cost-effective and eco-friendly engineering methods to improve biochar’s physicochemical and adsorption performance. Therefore, a series of biochars samples was produced by pyrolysis at 500 °C in the presence of phosphoric acid (H3PO4) without and with microwave (MW) irradiation of the raw tea waste (RTW). Effect of acid and MW activation on the characteristics of biochar was compared with biochars produced from pyrolysis of RTW at 300, 500, 700, and 900 °C. Pyrolysis in the presence of H3PO4 promoted the formation of oxygen-containing functional groups and increased the surface area of the biochar product (931 m2 g–1 compared with 13.6 m2 g–1). Compared with biochars obtained from acid-assisted pyrolysis, MW activation nearly doubled the measured surface area (1763 m2 g–1) by the formation of a micropore structure and required seconds instead of hours to perform. Heavy metal adsorption capacities observed for the acid-activated chars were comparable to biochar samples produced in the temperature range of 500–900 °C, but acid-promoted activation required much lower temperatures, and the effects of activation depended on the metal, the solution pH, and the activation treatment. Specifically, acid promotion increased Cu and Cd sorption capacity, attributable to the increased surface area and increased surface densities of oxygen functional groups (C = O and OH). The results of ternary-metal adsorption results demonstrated the competitive adsorption between three metals, and the affinity sequence of biochar produced at 900 °C (BC900) was found as Pb(II) > Cd(II) > Cu(II). However, the metal affinity sequence of BC900 to single metal is Pb(II) > Cu(II) > Cd(II). FTIR and SEM–EDS results revealed that Pb(II) had the same adsorption sites with Cu(II), but Pb(II) has a greater affinity than Cu(II). Kinetic measurements and spectroscopic analysis indicated that Pb, Cu, and Cd form chemical complexes on the surface. These results provide new insight on the utilization of tea wastes for heavy metal removal from aqueous streams.