Analysis of the Adsorption-Release Isotherms of Pentaethylenehexamine-Modified Sorbents for Rare Earth Elements (Y, Nd, La)

Abstract

Waste from electrical and electronic equipment (WEEE) is constantly increasing in quantity and becoming more and more heterogeneous as technology is rapidly advancing. The negative impacts it has on human and environment safety, and its richness in valuable rare earth elements (REEs), are accelerating the necessity of innovative methods for recycling and recovery processes. The aim of this work is to comprehend the adsorption and release mechanisms of two different solid sorbents, activated carbon (AC) and its pentaethylenehexamine (PEHA)-modified derivative (MAC), which were deemed adequate for the treatment of REEs deriving from WEEE. Experimental data from adsorption and release tests, performed on synthetic mono-ionic solutions of yttrium, neodymium, and lanthanum, were modelled via linear regression to understand the better prediction between the Langmuir and the Freundlich isotherms for each REE-sorbent couple. The parameters extrapolated from the mathematical modelling were useful to gain an a priori knowledge of the REEs-sorbents interactions. Intraparticle diffusion was the main adsorption mechanism for AC. PEHA contributed to adsorption by means of coordination on amino groups. Release was based on protons fostering both a cation exchange mechanism and protonation. The investigated materials confirmed their potential suitability to be employed in real processes on WEEE at the industrial level.

Keywords: Freundlich isotherm; Langmuir isotherm; WEEE; activated carbon; adsorption; linear regression; pentaethylenehexamine; rare earth elements; release.

Figure 1

Adsorption and release experimental procedures.

Figure 2

(a) Linear regression using Linear Langmuir 2 (full line) and corresponding extrapolated parameters; (b) Linear regression using Linear Langmuir 2 on the partial set (dashed line) and corresponding extrapolated parameters; (c) Comparison of experimental data and predicted data (full and dashed lines) with the Langmuir isotherm for the adsorption of Y (■) on AC.

Figure 3

(a) Linear regression using Linear Freundlich (full line) and corresponding extrapolated parameters; (b) Comparison of experimental data and predicted data (full line) with the Freundlich isotherm for the adsorption of Nd (●) on AC.

Figure 4

(a) Linear regression using Linear Langmuir 2 (full line) and corresponding extrapolated parameters; (b) Linear regression using Linear Langmuir 2 on partial sets A and B (dashed line) and corresponding extrapolated parameters; (c) Comparison of experimental data and predicted data (full and dashed lines) with the Langmuir isotherm for the adsorption of La (▲) on AC.

Figure 5

(a) Linear regression using Linear Freundlich (full line) and corresponding extrapolated parameters; (b) Linear regression using Linear Freundlich on partial sets A and B (dashed line) and corresponding extrapolated parameters; (c) Comparison of experimental data and predicted data (full and dashed lines) with the Freundlich isotherm for the release of Y (■) from AC.

Figure 6

(a) Linear regression using Linear Langmuir 3 (full line) and corresponding extrapolated parameters; (b) Comparison of experimental data and predicted data (full line) with the Langmuir isotherm for the release of Nd (●) from AC.

Figure 7

(a) Linear regression using Linear Langmuir 1 (full line) and corresponding extrapolated parameters; (b) Comparison of experimental data and predicted data (full line) with the Langmuir isotherm for the release of La (▲) from AC.

Figure 8

(a) Linear regression using Linear Freundlich (full line) and corresponding extrapolated parameters; (b) Linear regression using Linear Freundlich on partial sets A and B (dashed line) and corresponding extrapolated parameters; (c) Comparison of experimental data and predicted data (full and dashed lines) with the Freundlich isotherm for the adsorption of Y (■) on MAC.

Figure 9

(a) Linear regression using Linear Langmuir 2 (full line) and corresponding extrapolated parameters; (b) Comparison of experimental data and predicted data (full line) with the Langmuir isotherm for the adsorption of Nd (●) on MAC.

Figure 10

(a) Linear regression using Linear Langmuir 2 (full line) and corresponding extrapolated parameters; (b) Comparison of experimental data and predicted data (full line) with the Langmuir isotherm for the adsorption of La (▲) on MAC.

Figure 11

(a) Linear regression using Linear Langmuir 3 (full line) and corresponding extrapolated parameters; (b) Comparison of experimental data and predicted data (full line) with the Langmuir isotherm for the release of Y (■) from MAC.

Figure 12

(a) Linear regression using Linear Langmuir 3 (full line) and corresponding extrapolated parameters; (b) Comparison of experimental data and predicted data (full line) with the Langmuir isotherm for the release of Nd (●) from MAC.

Figure 13

(a) Linear regression using Linear Langmuir 1 (full line) and corresponding extrapolated parameters; (b) Comparison of experimental data and predicted data (full line) with the Langmuir isotherm for the release of La (▲) from MAC.

Figure 14

Quantity of adsorbed Y, Nd, and La cations (Q_eq) on (a) AC and (b) MAC with respect to initial concentration (C₀); Quantity of released Y, Nd, and La cations (Q_rel) from (c) AC and (d) MAC with respect to quantity of adsorbed cations (Q_eq).