  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 non-local
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.
