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.

Maybe I'm wrong, I hope I helped.

See:

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).

A negative RL value suggest that desorption will takes place instead of adsorption.

@Laura Bulgariu ,please give me some clarification or références

I have similar data Laura Bulgariu , Could you please provide a literature reference that I can cite for the same?

For RL (separation factor) to be negative may possibly indicate no adsorption or desorption.

Either no adsorption, or description or absorbent is not active.

Visit the link below,

https://youtu.be/wWIZyxrp04I

Unfavorable biosorption.