Martínez, G. A., Traina, S. J. and Logan, T. J. (1998) “Evaluation of proton and europium (III) binding reactions in a sewage sludge humic acid”, The Journal of Agriculture of the University of Puerto Rico, 82(3-4), pp. 121–140. doi: 10.46429/jaupr.v82i3-4.4071.