According to the equation RL = 1 / (1+b*C0), "RL" would only be negative if "b" is negative. In the nonlinear Langmuir equation, I can't see negative b.
"b" is related to affinity, It may be a very small number, but I don't think it would be negative.
When the value of RL lies between 0 and 1, indicate the favorable biosorption. The value of RL > 1 represents the unfavorable biosorption, the value of RL equal to 1 represents the linear biosorption and specifies the irreversible biosorption process in case of RL value equal to 0 (Rangabhashiyam and Balasubramanian, 2019).