Evaluation of chemical compounds in Plectranthus barbatus leaves extract for application in reduction of tannery wastewater toxicity

Abstract

Plectranthus barbatus Andr. grows in Kenya, sub-Saharan Africa, Australia, Brazil and Asia and is one of the most important species of the Lamiaceae family. The plant is widely mentioned in traditional medicine in Africa and Asia. Some of the compounds previously isolated from the plant include diterpenoids, phenolic acids and essential oils. The objective of the study was to evaluate and determine the presence of chemical compounds in Plectranthus barbatus Andr. leaves extract of tannin nature that can be beneficial ingredients in processing clean leather and at the same time reduce environmental pollution. The Plant’s leaves were collected from Nyamira County then dried and ground to desired mesh before extraction was done with water and 80 % methanol in water. The methanolic solid extract that was recovered after freeze drying was subjected to column chromatography. The pure fractions were analyzed using Nuclear Magnetic Resonance spectroscopy and compared to those previously analyzed using High Pressure Liquid Chromatography. Some of the compounds isolated include phenolics e.g. Para-Hydroxybenzoic acid and Diterpenes. Water solid extract was used in pre-tanning and re-tanning applications followed by analysis of Biological Oxygen Demand, Chemical Oxygen Demand, pH and total chrome content of respective wastewaters. Tannery liquor from re-tanning and pre-tanning combinations with Plecranthus barbatus leaves extract showed reduced toxicity to the environment with pH of 7.97 and 2.96 respectively as compared to chrome tannage liquor that recorded a relatively more acidic pH of 2.30. Biological Oxygen Demand, Chemical Oxygen Demand and chrome content levels were 230.00 mg/l, 4520.00 mg/l and 9.69 ppm respectively for effluent from Plectranthus barbatus retannage against 320.10 mg/l, 2331.20 and 5.03 ppm respectively from pretannage of the same extract. Permissible limits for pH, Biological Oxygen Demand, Chemical Oxygen Demand and total chrome in discharged tannery wastewater are 5.5 – 10.0, 125 mg/l – 1000 mg/l, 300 mg/l – 3000 mg/l, and 1.0 ppm – 10.0 ppm respectively. Pretanning with Plectranthus barbatus leaves extract during leather processing was more effective is reduction of tannery wastewater toxicity although, the use of pretanning and retanning in a single tannage might yield more promising results.