Effect of equilibration time on estimates of the maximum phosphorus sorption capacity of industrial by-products using the Langmuir model

Abstract

Using reactive industrial by-products (IBPs) to reduce phosphorus (P) losses associated with diffuse water pollution is a potentially cost-effective mitigation strategy. However, IBPs must be screened to assess their effectiveness and optimum application rates. This requires accurate estimates of parameters such as the maximum P sorption capacity. Traditionally, these parameters have been derived from the Langmuir model applied to data from batch sorption experiments following a 24-h equilibration period. In this paper, we examined (i) how equilibration time can influence estimates of the maximum P sorption capacity for IBPs and (ii) the relative P sorption characteristics of a range of IBPs available in the UK. Four IBPs containing different reactive components including ochre, aluminium (Al)-based water treatment residual (WTR), iron (Fe)-based WTR and Fe-lime (CaO)-based WTR were selected for this study. The maximum P sorption capacities of these IBPs were determined using a linearized Langmuir model applied to batch sorption data collected at different equilibration times of 24 h, 5 days and 10 days. The maximum P sorption capacity of ochre, Al-based WTR, Fe-based WTR and Fe-CaO-based WTR estimated from the linearized Langmuir model following a 24-h equilibration period was 10.1, 13.7, 2.4 and 9.3 mg P g−1, respectively. However, extending the equilibration time from 24 h to 5 days increased the estimated maximum P sorption capacity for these IBPs by factors of 2.2, 2.1, 6.8 and 2.3, respectively. No significant increase was found in estimates of the maximum P sorption capacity when further extending the equilibration time to 10 days. A minimum equilibration period of 5 days is recommended to avoid underestimating the maximum P sorption capacity of the IBPs examined in this paper. Each of the IBPs we evaluated was able to sorb P from solution, although with variable capacity (maximum sorption capacity after 5 days of equilibration ranged from 16.3–28.5 mg P g−1). These findings emphasise the importance of accurate quantification of the P sorption capacity of IBPs before application.