Sorption properties of a new hydrosoluble polymer containing carbamoylmethylphosphonic diacid (hcmp) moieties were investigated. This polymer was obtained by hydrolysis of the poly(diethyl-6-(acrylamido)hexylcarbamoylmethylphosphonate) (P(CPAAm6C)). The study allowed the determination of an experimental maximum capacity Qmax equal to 1.5 mmol⋅g−1 which is three times higher than the one obtained for P(CPAAm6C), as previously described (i.e. 0.6 mmol⋅g−1). Experimental data were fitted with the Langmuir equation and parameters Qmax L and KL, were found to be 1.72 ± 0.067 mmol⋅g−1 and 0.302 ± 0.063 L⋅mmol−1, respectively. Additionally, this capacity depends on different parameters (concentration of metals and sorbent, pH, and ionic strength). The study proved that the hP(CPAAm6C) was selective of gadolinium in Gd/Ni mixtures. Additionally, the precipitation of the polymer was observed simultaneously to the Gd complexation. The results described in this paper proved that the hP(CPAAm6C) is a very promising hydrosoluble material for the purification of lanthanide elements or in the field of nuclear fuel reprocessing by using new solvent free and low energy consuming processes.