  Mean-field theory and scaling arguments are presented to model
polyelectrolyte adsorption from semi-dilute solutions onto charged surfaces.
Using numerical solutions of the mean-field equations, we show that adsorption
exists only for highly charged polyelectrolytes in low salt solutions. Simple
scaling laws for the width of the adsorbed layer and the amount of adsorbed
polyelectrolyte are obtained. In other situations the polyelectrolyte chains
will deplete from the surface. For fixed surface potential conditions, the salt
concentration at the adsorption--depletion crossover scales as the product of
the charged fraction of the polyelectrolyte f and the surface potential, while
for a fixed surface charge density, \sigma, it scales as \sigma^{2/3}f^{2/3},
in agreement with single-chain results.
