Plant-Based Activated Carbons for Cobalt and Strontium Removal: Insights into Adsorption Mechanisms | Chapter 8 | Chemical and Materials Sciences: Research Findings Vol. 2

Aims: To investigate the adsorption behavior of Co(II) and Sr(II) ions on the surface of plant-based activated carbons (obtained from walnut shells (CA-N) and apple wood (CA-M), and modified by oxidation with nitric acid and nitric acid/urea mixture).

Methodology: Activated carbons were chemically modified using nitric acid (sample CA-Mox) and a nitric acid/urea mixture (samples CA-Mox-u, CA-Nox-u). Their physicochemical properties were characterized through nitrogen adsorption isotherms, SEM-EDX, FTIR spectroscopy, pH metric titrations, Boehm titration, and elemental analysis. Batch adsorption experiments were conducted to evaluate the removal efficiency of Co(II) and Sr(II) ions over a pH range of 2–10. Kinetic models (pseudo-first-order, pseudo-second-order, intraparticle diffusion) and isotherm models (Langmuir, Freundlich, Dubinin–Radushkevich, Temkin-Pyzhev) were applied to study adsorption mechanisms and estimate maximum adsorption capacities.

Results: On the surface of activated carbons the following functional groups have been identified: strongly acidic groups (CA-Mox, 0.95 meq/g; CA-Mox-u, 0.44 meq/g; CA-Nox-u, 0.32 meq/g) and weakly acidic and phenolic groups, while the pH_pzc value of the studied activated carbons increase as follows:
CA-Mox (2.3) < CA-Mox-u (3.3) < CA-Nox-u (3.9) < CA-M (6.9) < CA-N (8.3).

The results of batch experiments indicate that maximum adsorption can be achieved in broad pH ranges: 4–8 for Co(II) and 4–10 for Sr(II). The maximum adsorption capacities of Co(II) and Sr(II) on oxidized activated carbons at pH = 4 are: CA-Mox, 0.085 and 0.076 mmol/g; CA-Mox-u, 0.056 and 0.041 mmol/g; and CA-Nox-u, 0.041 and 0.034 mmol/g, respectively.

The experimental data are best described by the pseudo-second kinetic model, suggesting that the adsorption of metal ions (cobalt(II) and strontium(II)) onto oxidized activated carbons is based on chemisorption. The constants determined for the Freundlich and Langmuir models and the energy values estimated by Dubinin–Radushkevich demonstrate that the adsorption process of strontium(II) and cobalt(II) ions onto oxidized activated carbons is favorable and dominated by chemisorption of the energetically homogenous surface in the modeled system.

Conclusion: The investigations highlight the important role of the solution pH, of the activated carbons' functional groups, and the pH_pzc value for the adsorption process of cobalt(II) and strontium(II) ions. By summarizing the obtained results, the adsorption mechanisms of toxic metal ions on activated carbons were proposed.

 

 

Author (s) Details

Raisa Nastas
Institute of Chemistry of Moldova State University, 3, Academiei Str. Chisinau, MD-2028, Republic of Moldova.

 

Irina Ceban
Institute of Chemistry of Moldova State University, 3, Academiei Str. Chisinau, MD-2028, Republic of Moldova.

 

Tudor Lupascu
Institute of Chemistry of Moldova State University, 3, Academiei Str. Chisinau, MD-2028, Republic of Moldova.

 

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