Synthesis and characterisation of carbon-based adsorbents from some ligno-cellulose agricultural residues and their application in the removal of cadmium(II) and lead(II) ions from aqueous systems

Abstract

Adsorption using activated carbon has become an important separation process applied in industrial wastewater treatment. Presently, the need to reduce the cost of this process has driven the search for low cost adsorbents such as those obtained from agricultural by-products in effluent treatment especially in developing countries. In this study, seven agricultural residues were investigated as adsorbents for the removal of cadmium(II) and lead(II) ions from aqueous systems using a batch sorption. The residues were: oil palm fruit fibre (OPFS), cocoa pod (CCPS), coconut shell fibre (CNFS) plantain peel residue (PTHS) cocoyam peel (CCYBS), sweet potato peel (PTPS) and white yam peel (YTBS). These residues were further subjected to hydrothermal carbonisation at 200°C & 170°C, carbonisation at 400°C in limited air, pyrolysis at 700°C in N2 and chemical activation using K2CO3 and Na2CO3 in N2 to develop a number of carbon adsorbents that improve their sorption capabilities. Adsorbent characterisation studies included: functional group analysis; BET surface area; morphology, thermal degradation profile, zeta potential and chemical composition. The pHpzc was used to determine the most favourable pH for sorption and this was 6.5 for the residues and 7 for the prepared adsorbents. The results show that the BET surface area of the adsorbents did influence the rate of uptake of the two metal ions. The kinetics of sorption showed a two-stage profile for all adsorbents- initial uptake time was 180 minutes (residues) and 30-60 minutes for the other adsorbents. Sorption kinetics indicated that for Cd(II) ion sorption, optimum uptake was obtained using the CNFS; CNFS-HTC 200; CCPPCA and CNFKCA adsorbents, while for Pb(II) ion this was observed for the OPFS; CNFS-HTC; YTBPCA and CNFNCA adsorbents. Kinetic modelling indicated that the pseudo-first order (PFO) model described the sorption better than the pseudo second order (PSO) for both ions implying that the rate limiting step of sorption depends on concentration. Equilibrium isotherm modelling for metal ion sorption for the unmodified residues; Na2CO3 activated adsorbents and a commercial activated carbon (CGAC) was also studied from which the Langmuir isotherm parameter (qmax) to rank their adsorption capacities. The results obtained showed that the CCPNCA was the best adsorbent for Cd(II) ion removal with a qmax value of 29.4 mgg-1, while for Pb(II) ion, the CCYBNCA adsorbent was the best with an optimum qmax value of 41.9 mgg-1.These values were higher than those of the CGAC adsorbent used to benchmark this sorption study which were; 20.3 mgg-1 and 27.2 mgg-1 for Cd(II) and Pb(II) ions respectively. Finally, spent adsorbent ash stabilization using polymer encapsulation was successful carried out as leached samples after 60 days showed no leaching of the two ions.

Date of Award	1 Aug 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Xiaolei Fan (Supervisor) & Flor Siperstein (Supervisor)