Potential of Azadirachta indica Leaves for Removal of Hexavalent Chromium from Aqueous Solution with Reference to Adsorption Isotherm

Hexavalent chromium is considered as a serious environmental pollutant through industries. The main objective of present studies is to investigate the adsorption behaviour of Cr⁶⁺ ions on to Azadirachta indica leaves. Main parameters considered are pH, contact time, adsorbent dose and initial hexavalent chromium concentration during removal of Cr⁶⁺ using natural adsorbent by adsorption technique with Batch mode experiments. Present study shows that the Azadirachta indica leaves were found as an effective biosorbent for the removal of hexavalent chromium from aqueous solution. The optimum  percentage removal of hexavalent chromium from aqueous solution obtained is 99.68 %, 99.63%, 98.79%, 98.99% for  initial Cr (VI) concentration 50 mg/L ,75 mg/L, 100 mg/L, 125 mg/L at pH  6 , adsorbent dosage 5 g, contact time 4 hrs in jar test apparatus with 160 rpm. The results showed that the maximum adsorption capacity of hexavalent chromium on  Azadiracta indica leaves observed is 6.202 (50mg/L), 8.307 (75mg/L), 9.546 (100mg/L) and 12.68 (125mg/L) at pH 6 after 4 hr. The values of coefficient of correlation (R²) obtained was nearer to 1 for Azadirachta indica leaves  are in good agreement with Langmuir, BET , Freundlich and Temkin adsorption isotherm models. For Langmuir and BET adsorption isotherm the adsorption capacity (Q₀) is observed highest 10.194 mg/g and (q_max) 5.8754 mg/g for 125 mg/L initial Cr(VI) concentration. Rate of adsorption (b) was also observed highest 7.0469 L/mg and 344.82 L/mg in case of Langmuir and BET adsoption  isotherm for 125 mg/L initial Cr(VI) concentration, suggest better applicability of BET isotherm for Azadirachta indica  leaves as adsorbent for Cr(VI) from aqueous solution. Azadirachta indica  leaves data for all chromium concentration(50mg/L, 75mg/L, 100mg/L and 125mg/L) followed Freundlich model due to value of n obtained is 3.690, 4.115, 3.875 and 4.975 respectively . Freundlich adsorption isotherm is also better applicable for 125 mg/L initial Cr(VI) concentration as the highest value of adsorption capacity (K_f) is obtained 4.518 mg/g and intensity of adsorption (n) is 4.9751 L/mg. The Azadirachta indica leaves  shows good agreement with Temkin isotherm  for all initial concentration of Cr (VI)   = 50 mg/L, 75 mg/L ,100 mg/L, 125 mg/L as the value of rate of adsorption (b) = 1.049 , 1.292, 1.646, 1.580 L/mg respectively. Here also, the value of adsorption capacity (a) was observed highest 3.939 mg/g for initial Cr(VI) concentration of 125 mg/L.  The natural water/wastewater/effluent can be treated by adding suitable dose of low-cost, natural adsorbent Azadirachta indica leaf powder and by application of flocculator/other instrument with 160 rpm. The settled residue remained at the bottom can be taken out. The adsorbent and Cr⁶⁺ can be recovered by giving H₂SO₄ treatment to this sludge. The sludge of Azadirachta indica leaves is biodegradable. By using this cost effective, eco-friendly technique we can get rid of the hazardous sludge disposal problem-generated by chemical treatment of Chromium removal/recovery.