Wasted salted duck eggshells as an alternative adsorbent for phosphorus removal

Abstract

Eggshells represent a large volume of unused calcium carbonate-rich material which can be valorized into adsorbents for wastewater treatment through a calcination process which converts calcium carbonate into calcium oxide. In this work, the properties of preserved salted eggs from Chaiya, Suratthani, Thailand and their abilities to remove phosphorus from a low phosphorus-containing wastewater (4 g L−1) were compared to those from duck eggshells. Different calcination temperatures (500 and 700 °C), calcination periods (2 and 4 h) and adsorbent dosages were tested. Chemical composition, functional groups, surface area, porosity, particle size, and morphology were determined for all samples. The specific surface area was in the range of 0.44–0.92 m2 g−1. The maximum experimental phosphorus removal from salted eggshells was equal to 96.2%. It was obtained from calcined samples at 700 °C for 4 h at adsorbed phosphorus of 3.32 ± 0.06 mg g−1 after 240 min. This study suggests that the egg preservation process enhanced the adsorption ability of eggshells at high calcination temperature. Lower calcination temperature did not result in good phosphorus removal (<4%) meaning that a minimum calcined temperature is required to “activate” the adsorbing ability. Freundlich and Langmuir isotherms were used to model phosphorus adsorption and Langmuir was a better fit. Response surface methodology (RSM) was performed and the optimum parameters for the maximum phosphorus removal were determined. The maximum phosphorus removal efficiency was obtained from adsorbents prepared at 700 °C for 4.0 h, adsorbing time of 128.5 min at a 0.96 gadsorbent L−1.