Biosorption of U(VI) from aqueous systems by malt spent rootlets. Kinetic, equilibrium and speciation studies

Abstract

Α low-cost and environmentally friendly clean-up technique is evaluated in this study, based on the use of a brewery by-product, malt spent rootlets, as potential biosorbent for U(VI) sequestration from aquatic systems. Uranium uptake was rapid (2.5 h at 25 °C), and MSR exhibited capability of removing U(VI) from effluents of high acidity (pH 1.5) and salinity (0.5 M NaCl and NaNO₃). Maximum uptake was 157 mg U(VI) g⁻¹ at 25 °C. The pseudo-second-order model gave the best fit for kinetic data, whereas film diffusion was the rate-controlling step. Langmuir adsorption isotherm was the best fitting model. Activation energy, thermodynamic data and the extent of sorption reversibility implied that sorption of U(VI) is predominantly chemical. FTIR studies showed that lignin moieties are mainly responsible for U(VI) uptake. Speciation modeling showed that only positively charged and uncharged uranium species can be retained on the biomass. Finally, desorption studies revealed that Na₂CO₃ was the most efficient eluent with 78 % of U(VI), previously bound on the biosorbent’s surface, recovered.

Nomenclature

Bt

Dimensionless constant of Boyd model

C

Vertical axis intercept of Morris–Weber model (mg g⁻¹)

C _i

U(VI) initial concentration (mg L⁻¹ or mol L⁻¹)

C _fin

U(VI) final concentration (mg L⁻¹ or mol L⁻¹)

E _a

Activation energy (kJ mol⁻¹)

k ₁

Rate constant of pseudo-first-order reaction kinetic model (min⁻¹)

k ₂

Rate constant of pseudo-second-order reaction kinetic model (g mg⁻¹ min⁻¹)

k _d

Intraparticle diffusion rate constant (mg g⁻¹ min^−1/2)

K _L

Langmuir constant (L mol⁻¹)

K _F

Freundlich constant

K′

Dubinin–Radushkevich mean adsorption energy constant (mol² kJ⁻²)

m

Dry weight of biomass (g)

q

U(VI) uptake per biosorbent mass unit (mg g⁻¹ or mol g⁻¹)

q _e

U(VI) bound per biosorbent mass unit at equilibrium (mg g⁻¹ or mol g⁻¹)

q _t

U(VI) bound per biosorbent mass unit at time t (mg g⁻¹ or mol g⁻¹)

q _max

Maximum U(VI) uptake as predicted by isotherm models (mol g⁻¹)

R

Gas constant (J K⁻¹ mol⁻¹)

T

Temperature (K)

R(%)

Percentage removal of U(VI) from the aqueous solution

V

Solute volume (L)

α

Initial sorption rate for Elovich model (mg g⁻¹ min⁻¹)

β

Constant related to the extent of surface coverage and activation energy for chemisorptions for Elovich model (g mg⁻¹)

ΔΗ ^o

Apparent enthalpy change (kJ mol⁻¹)

ΔS ^o

Apparent entropy change (J mol⁻¹ K⁻¹)