A negative n suggests that the adsorption process is unfavorable. This means that as the concentration of adsorbate increases, the adsorption capacity decreases, which is contrary to typical adsorption behavior.

I guess there is a mistake; the Freundlich exponent cannot be negative. As correctly noted by Bairam Lohan, this means when concentration of a sorbing species increases, its sorbed concentration decreases. Generally speaking, it is thermodynamically forbidden (if any other variables are maintained constant). Perhaps, there is a confuse: if sorption data were plotted in log log scale, to linearized the Freundlich model, the intercept (log Kf) may become negative