Activated Carbon Manufacture: An opportunity for sustainable management of problematic waste biomass

Abstract

The management of waste biomass is an increasing problem. The high water content and mixed composition make its wider utilisation difficult. In addition, there are certain invasive species of flora in the UK which cause significant damage to the natural and built environment. One of the most publicised and problematic species is Japanese Knotweed (Fallopia japonica) or JKW. There is a growing recognition that global resources are being consumed at an increasing rate that is not sustainable for the long-term stability of our society. One such commodity material is activated carbon, which is mostly made from coal and imported to the UK in significant quantities (16,000 tonnes per annum). Thus, this research has focussed on investigating the feasibility of utilising waste biomass as a potentially novel and renewable source of activated carbon. This research required the characterising and assessment of the suitability of green (biomass) waste and JKW as potential feedstock for the production of activated carbon. The JKW biomass was separated into rhizomes (JKWR) and stems (JKWS) and experiments were conducted independently. Some of the characterisation tests conducted included CHNS analysis, thermal degradation behaviour and lignocellulosic content analyses. Using high temperature carbonisation and steam activation, waste biomass was converted into activated carbon. For the purpose of this work, the parameters that were investigated were activation temperature, time and water flow rate. The resulting adsorbents were characterised using established international as well as peer-reviewed methods in order to determine their capabilities as activated carbons. These included BET surface area measurement, butane working capacity, phenol adsorption tests as well as iodine number determination. Green waste was not suitable to undergo steam activation as it had high ash content and low carbon content. As a result, the research focussed on the carbonisation and subsequent steam activation of JKW. JKW biomass was carbonised at 600°C for 1h and the JKW chars were activated under optimised conditions at 925°C for 1h under 0.5ml/min water flow rate. BET surface areas in excess of 900m2/g were achieved and this compared very favourably with a benchmark international commercial carbon, Filtrasorb 400. It was observed that increased steam activation temperatures resulted in activated carbons with high degrees of microporosity and the relative microporosity decreased with activation temperature. The carbons were also superior to Filtrasorb 400 in phenol and iodine adsorption tests. Further, Langmuir phenol adsorption capacities of 256 mg/g and 222 mg/g and iodine numbers of 1307 mg/g and 1190 mg/g, respectively were measured for JKWS and JKWR carbons. There are growing concerns over new water pollutants which arise from plastic use. These molecules are known to exhibit oestrogenic activity and hence, their removal is becoming a priority. The application of these carbons for this purpose was tested, using bisphenol-A (BPA) as a surrogate chemical of interest. JKWS activated carbon was able to perform as well as Filtrasorb 400 in BPA adsorption tests achieving at least 300 mg/g capacity, which is extremely promising for a novel, biomass derived activated carbon. Overall, this research work has clearly demonstrated that rather than regarding waste biomass, and particularly Japanese knotweed, as a problem, it may be an opportunity to provide the UK with a home-sourced and renewable feedstock for commercial activated carbon. Adopting this feedstock would be a major step towards resolving two UK government goals; reducing waste arisings and creating a more sustainable economy by reducing demand for imported products.