We investigated the adsorption of aqueous Cu2+ and Cr3+, as model heavy metal ions, by the root powder
of Eichhornia crassipes, followed by combustion to establish an economical route suitable for large-scale
recovery of heavy metal ions from wastewater. In the optimal pH range of 5.0–6.0, the adsorption reached
equilibrium after 30 min and could be described by Langmuir isotherms with maximum adsorption capacities
of 32.51 mg/g for Cu2+ and 33.98 mg/g for Cr3+. A pseudo-second-order kinetic model was applied
to describe the adsorption kinetics. We concluded from Fourier transform infrared spectroscopy that
functional groups containing OH and COOH contributed to the adsorption. During adsorption, Ca2+,
Mg2+, and K+ were discharged from the root powder. Electrostatic interactions also played an important
role in the absorption process. X-ray photoelectron spectroscopy analysis suggested that this long-root
adsorbent appeared to chelate Cr3+ more strongly than Cu2+, both of which appeared on the surface and
in the interior of the adsorbent; both were eluted best with H2SO4. Combusting the saturated adsorbent
generated a product with high concentrations of metal ions: 23% w/w Cu2+ and 30% w/w Cr3+, values
equal or higher than the regular contents of mine ore (20–25% w/w). Thus, this process created a product
that was favorable for subsequent processing.