Purification of platinum group metals process stream using saccharomyces cerevisiae waste yeast biomass
Date
2015-04-28
Authors
Oke, Doris Oluwafunmilayo
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Abstract
The removal of platinum group metals (PGMs) from dilute process streams is essential due to
their economic value and the increasing need to recycle process water. This study presents the
removal of Pd (II), Ir (III), Pt (II), Rh (III) and Au (III), from synthetic multicomponent aqueous
solution in a batch system using untreated, NaOH treated and ethanol treated Saccharomyces
cerevisiae waste biomass obtained as a byproduct of brewery fermentation industry. A twolevel
four factor full-factorial experimental design and analysis was successfully employed for
the testwork. The effects of pH, initial metal ion concentration, temperature and biomass
dosage on PGMs removal were assessed using the design of experiment (DOE) approach.
Solution pH and initial metal concentration were found to be statistically significant for the
adsorption of PGMs tested. Ethanol treated biomass gave the highest adsorption uptake for all
the PGMs tested. The sorption uptake is in the order of Au3+>Pd2+>Pt2+>Ir3+>Rh3+. The overall
maximum sorption attained was 99.9% for Au (III), 91% for Pt (II), 99.5% for Pd (II), 81.9%
for Ir (III) and 81.1% for Rh (III).
The desorption of the tested PGMs from the loaded biomass was carried out using different
concentrations of hydrochloric acid, sulphuric acid, thiourea and hydrochloric – thiourea
solution. Effective desorption was obtained using acidified 0.1M thiourea which can be
explained as due to both stable complex formation and the electrostatic interaction between
some of the PGMs species and charged species from elution. The biosorbent was successfully
regenerated and reused up to 5 cycles.
Equilibrium data for metal removal in the batch study conformed well to the Langmuir
isotherm except for the fitting of Pd (II) which complied with Freundlich isotherm model.
Kinetic data were fitted to Lagergren first –order, pseudo second – order, Intraparticle diffusion
and Elovich models. The kinetic parameters, rate constants, equilibrium adsorption capacities
and related correlation coefficients for each kinetic model were calculated and discussed.
Consequently, the adsorption of Pd (II), Ir (III), Pt (II), Rh (III) and Au (III) were found to
follow a pseudo- second order kinetic model suggesting chemisorption of all the tested PGMs.
In addition, column studies were conducted for the purification of real dilute process stream
from Impala Platinum using ethanol treated yeast immobilized on Plaster of Paris (POP). These
studies focused not only on the PGMs but base metals and other trace elements present in the
dilute solution. The breakthrough curve for most of the element do not resemble those of an
ideal breakthrough curve due to the complexity of the solution and the normalized
concentration (C/Co) for most of the metals in solution remained high even in the early stages
of treatment when the sorbent material was most pristine. Generally, the affinity of the sorbent
for the elements considered follow the order Pd2+> SO42->Te2+>Pt2+>Ir3+>Ni2+>Cl-
>.Ru3+>Se2+> Na+.
The data from the column studies was fitted into Adam-Bohart and Thomas model and the data
conformed well to Adam – Bohart model. In general, ethanol treated S.cerevisiae yeast biomass
appeared to be a good sorbent for the effective removal of PGMs but the complexity of the
industrial wastewater requires that pretreatment steps be taken to reduce chloride and sulphate
concentration in the solution before this process can be applied successfully.