R. del T. Déniz, M. S. Echevarría, Z. F. Terra, A. M. P. Espinosa, and H. de Moya, “Use of modified silica gel for concentrating Pb (II) and Cd (II) occurring in form of complex anions”, Eclet. Quim., vol. 32, no. 2, pp. 13–16, Jun. 2007.