1)If this is directly fed to the PCF8591 which is not meant for -ve measurements I am affraid the inputs of ADC may blow up due to this -ve input.
2)Thus I want to use clamp diode which can eat up this tiny spike and can protect the ADC by not letting it enter the input of my ADC.

1)from the horses mouth aka the datasheet PCF8591

                             min    max
VI input voltage (any input) -0.5   VDD + 0.5 V,

what is the worry?

2) if you can find a diode with a Vf of less than 40mV, please enlighten us as to the type.

Erik