Wannawek, A., P. Pookmanee, S. Satienperakul, R. Putharod, N. Laorodphan, and S. Phanichphant. “Removable Pb (II) from Aqueous Solutions by Adsorption onto Natural and Modified Leonardite”. Engineering and Applied Science Research, vol. 43, Nov. 2016, pp. 477-9, https://ph01.tci-thaijo.org/index.php/easr/article/view/70839.