We study a two-dimensional disordered system consisting of Dirac fermions coupled to a scalar potential. This model is closely related to a more general disordered system that has been introduced in conjunction with the integer quantum Hall transition. After disorder averaging, the interaction can be written as a marginal osp(2 2) current-current perturbation. The osp(2 2) current-current model in turn can be viewed as the fully renormalized version of an osp(2 2) ( 1 ) Toda-type system (at the marginal point). We build nonlocal charges for the Toda system satisfying the U q [osp(2 2) ( 1 ) ] quantum superalgebra. The corresponding quantum group symmetry is used to construct a Toda S -matrix for the vector representation. We argue that in the marginal (or rational) limit, this S -matrix gives the exact (Yangian symmetric) physical S -matrix for the fundamental ''solitons'' of the osp(2 2) current-current model.