A. Wannawek, P. Pookmanee, S. Satienperakul, R. Putharod, N. Laorodphan, and S. Phanichphant, “Removable Pb (II) from aqueous solutions by adsorption onto natural and modified leonardite”, Eng Appl Sci Res, vol. 43, pp. 477–479, Nov. 2016.