Adsorption of 17ß-Estradiol on to Bone char derived from Cattle bones

Abstract

Soils as our natural capital assets need to be well preserved to maintain their quality to be productive for farmland use and livestock rearing. Water is the most vital ingredient which influences the soil quality to a significant level, hence its quality need to be controlled to be free of pollutants and by economical means. There is also a huge pressure on the pastoral landscape which is becoming more dynamic and sophisticated by the day. Micro pollutants in water are a rapidly emerging global problem that seriously threatens the environmental and human health. There are some natural and synthetic compounds that are attracting attention due to their interference with the normal functioning of the endocrine system in humans and animals commonly known as endocrine disrupting chemicals. Steroid estrogens are one such class of endocrine disruptive chemicals which are considered to be harmful at extremely low levels if not monitored strictly. Water Pollution is a matter of grave concern these days. One of the steroid estrogen contaminant, 17β-estradiol found in waste water effluents was selected for this study. Bone char was used as an adsorbent for the adsorption of 17β-estradiol from water. Bone char was obtained from cattle bones which are considered as a novel low cost adsorbent. A systematic process was designed to eliminate this compound from water and develop an understanding of the possible reactions taking place: 1) Optimising the operating parameters for the preparation of bone char; 2) Conducting sorption studies on the prepared bone char; 3) Comparative sorption studies on the adsorbents (raw bone powder, bone char and commercial hydroxyapatite) and monitoring the adsorption reactions. The hydroxyapatite nanocrystals were studied in detail using Transmission electron microscopy (TEM) and understand the activation mechanism, the bone chars were activated using KOH (Potassium Hydroxide) as the activating agent. The operating parameters such as temperature and holding time play a significant role in the preparation of bone char. The physiochemical characteristics were studied using various characterisation techniques such as TGA, XRD, SEM, FT-IR and BET. The results indicate that the microstructure, pore diameter and the crystal size have strongly been influenced by the increasing temperature and residence time. The bone char prepared at 400 oC with 2 h residence time had the highest surface area of 114.15 m2 g-1 which was considered as the novel low cost adsorbent. This adsorption experiments involve studying the batch adsorption process with respect to the effect of contact time, initial concentration and dosage of adsorbent. The surface functionality plays a dominant role in determining the adsorption efficiency of the prepared carbon. Based on the experimental results, the point of zero charge (pHzpc) for the bone char was measured to be 5.9. The adsorption studies were conducted below the pHzpc; i.e pH 5 since the surface is positively charged and may enhance the removal of E2 from aqueous solution. The adsorption of E2 was affected by the concentration of adsorbent. The adsorption kinetic studies revealed that the adsorption of E2 on bone char obeys the pseudo second order kinetic model. In addition, the adsorption kinetics was also assessed for the intraparticle diffusion model. This study shows that the adsorption of E2 on bone char has a very complex mechanism involving the diffusion process. The experimental adsorption isotherm data was fitted to Langmuir and Freundlich isotherm models due to the presence of both homogenous and heterogenous surface. The saturated bone char was regenerated with ethanol/ water (1/1; v/v) solvent and the regenerated bone char was effective up to three cycles of adsorption and regeneration and the adsorption capacity was restored to 85% of its initial capacity in the third cycle. Comparative adsorption studies were conducted using the raw bone powder, bone char and the commercial hydroxyapatite. The bone char showed the higher adsorption capacity when compared to the other two adsorbents. The adsorbents mostly followed the pseudo- second order kinetics model. The contact time between the adsorbent and adsorbate played a significant role in the entire adsorption process. The FT-IR analysis indicated that the E2 molecules get bonded to the phosphate group which became intense after the adsorption process. Transmission electron microscopy (TEM) and X-ray diffraction was used to study the microstructure before and after the heat treatment. The heat treatment influences the crystallinity and growth of these hydroxyapatite nano- crystals. The bone char was then activated using potassium hydroxide (KOH) as an activating agent using varying char: KOH ratio. It was found that the bone char prepared with char: KOH ratio 1:2 has a comparatively higher surface area along with the average pore volume and the micropore volume.