Speciation of Recovered Yttrium from the Remediation of Acid Mine Drainage.

[NNadir]

The paper I'll discuss in this paper is this one: Solid and Aqueous Speciation of Yttrium in Passive Remediation Systems of Acid Mine Drainage (Lozano et al Environ. Sci. Technol. 2019, 53, 19, 11153-11161).

Besides consuming, as an accelerating rate, dangerous fossil fuels and deliberately dumping dangerous fossil fuel waste in the atmosphere, the seas, and the land, we are also consuming workable resources of many of the important elements in the periodic table. Thus future generations will have very low quality ores - which will require ever larger quantities of energy to refine per unit mass - with which to remediate the environmental disaster that this current generation of human beings represent. This will have implications on our glib predictions, all of which are contemptible nonsense, of so called "renewable energy" nirvanas "by 2050," or "by 2030," or "by 2075" or "by 2100." These nirvanas have not appeared, are not present and will not appear.

The insurmountable fact about so called "renewable energy" is its low energy to mass ratios, which requires massive amounts of materials to produce systems which economically, technologically and most importantly environmentally require redundant systems working at reduced thermodynamic and economic efficiency. (This is why the highest household electricity prices among the OECD nations belong to Germany and Denmark.)

One type of low grade ore which elements may be recovered is water, particularly polluted water, which is often polluted precisely because of the elements in it. If the water flows naturally, such recovery can involve lower expenditures of energy, and thus future generations may wish to exploit this fact since in our contempt for them we have doomed to live on our garbage, while we flatter ourselves for owning useless solar cells, electric cars, and what have you.

That brings me to the current paper. From the introduction:

Yttrium belongs to the rare earth element (REE) group, also composed of lanthanides and scandium. REEs are necessary for the development of modern technologies, and specifically, yttrium has important applications, for instance, in fluorescent lamps as phosphors,(1) and in the aircraft industry, used in the thermal barrier coatings for jet engines.(2) The increasing demand for REEs and their low worldwide supply have led to considering REEs as critical raw materials, boosting searches for alternative resources, such as recycling used stocks or identifying new geological sources of these elements. Because the REE concentrations in acid mine drainage (AMD) are from one to two orders of magnitude higher than the average concentrations in natural waters,(3) it may be possible to perform secondary REE recovery from precipitates from AMD neutralization in passive remediation systems. These active systems were developed to minimize the environmental impacts of AMD and they are used worldwide.(4,5) However, due to the high water content, sludge storage has substantial operational costs and environmental concerns.(6,7) In contrast, passive remediation systems, which have been developed extensively in recent decades,(8?11) allow the AMD neutralization generating lower amounts of solid waste precipitates. Ayora et al. documented nearly complete aqueous REE retention in two laboratory columns, simulating a disperse alkaline substrate (DAS), a passive treatment already implemented in the field, for two highly acidic AMDs (SW Spain).(12,13) The REEs were scavenged by basaluminite, a mineral precipitated in the columns, which also presented Y enrichment due to the higher yttrium concentration with respect to the rest of REEs in the two treated AMDs. Basaluminite, an aluminum oxyhydroxysulfate (Al4(SO4)(OH)10·5H2O), precipitates in acidic environments as a consequence of the natural attenuation of the AMD when mixed with more alkaline waters, or due to the induced neutralization of the acid waters, when the solution pH reaches ?4.(14) Basaluminite is considered a nanomineral, with a short-range order, around 1 nm of coherent domain size, which is described as layers of Al-octahedra with structural point defects and with sulfate groups as outer-sphere complexes between the Al layers.(15)

Similarly to the REE uptake by basaluminite in DAS treatments, Gammons et al. reported the precipitation of hydrous aluminum oxides accompanied by a decrease in REE concentration from AMD when mixed with natural water.(16) Recently, the scavenging of REEs by basaluminite precipitates has been described as a sorption mechanism.(17) AMD is characterized to contain high loads of dissolved sulfate and the affinity of REEs to form aqueous species with sulfate is very high, the MSO4+ aqueous complex being more abundant in AMD solutions.(18) Sorption of dissolved REEs from sulfate-rich waters onto basaluminite is thus described as the sorption of the MSO4+ aqueous complex via ligand exchange with a surface site of basaluminite, forming a monodentate surface complex with the Al-octahedron as one proton is released.(17) Here, a structural description of the aqueous YSO4+ complex and of the local environment of the surface complex formed upon adsorption onto basaluminite are reported...

...The objective of this study is to elucidate the structure of Y adsorbed onto basaluminite. Its chemical similarities with HREEs allow us to assume similar structural configuration for this subgroup. Moreover, this element was one of the most concentrated in waste samples allowing performing X-ray absorption spectroscopy experiments. Since the YSO4+ aqueous complex is adsorbed onto the mineral,(17) a previous characterization of the geometry of the aqueous complex has been carried out. Finally, a quantification of Y-species in basaluminite solids precipitated from AMD treatments has been performed. Structural studies were performed using EXAFS and pair distribution function (PDF) analyses of aqueous and solid samples combined with ab initio molecular dynamics (AIMD) simulations of the aqueous YSO4+ complexes.

Two hypotheses are used to investigate the local structure of the aqueous YSO4+ ion pair: (1) an outer-sphere complex, with water located between Y3+ and SO42–, and (2) an inner-sphere complex. In the latter case, two more hypotheses must be considered: (a) a monodentate complex, with one oxygen atom shared between the sulfate and the first coordination sphere of Y3+, and (b) a bidentate complex, with two oxygen atoms shared between the yttrium hydration sphere and the sulfate group.

Once the structure of the aqueous solution is fully described, different hypotheses have been considered to interpret the YSO4+ surface complexation onto the Al-oxyhydroxysulfate:

Pair distribution analysis and EXAFS techniques are analytical methods that depend on the absorption and scattering of x-rays.

A reference in this text indicates that a calculated quantity of yttrium (and perhaps other lanthanide (rare earth) elements would be on the order of a few hundred kg per year from remediating acid mine drainage that pollutes two Iberian rivers.

Some pictures from the text:



The caption:

Figure 1. (A) Top: Experimental PDFs of the YSO4-sol and Y-sol samples. Bottom: Simulated (AIMD) PDF (YSO4-calc) and partial PDFs of a YSO4+ aqueous complex. (B) Fourier-filtered signal from 1.8 to 4.2 Å for the EXAFS data. (C) EXAFS Fourier transform (FT) amplitude functions of the YSO4-sol sample. Black lines: experimental; red lines: fits. Simulated (AIMD) PDF and partial PDFs have been multiplied for visualization purposes: YSO4-calc (3×), Y–S (5×), and Y–O and S–O (2×). Dashed lines indicate the position of the Y–O, Y–S, and S–O bonds in the YSO4-sol sample.

The caption:

Figure 2. PDFs of basaluminite coprecipitated in the presence of Y (B-Ycop) and pure basaluminite (B-pure) and differential PDF (d-PDF). The d-PDF spectrum has been amplified (3×) for visualization purposes.

The caption:

Figure 3. k3-Weighted EXAFS (A) and FT amplitude functions (B) for four waste samples from column treatments, W-MR-C1-3, W-MR-C1-4, W-Alm-C3-8, and W-Alm-C3-9 (upper part); solid standards (basaluminite sorbed with YSO4 (B-YSO4), basaluminite sorbed and coprecipitated with Y: B-Yads and B-Ycop, respectively); and aqueous solution (free ion and sulfate complex: Y-sol and YSO4-sol, respectively) (bottom part). The dashed lines in the EXAFS signals of the column samples represent LCF with B-Yads (basaluminite with sorbed yttrium) and YSO4-sol (solution of Y with SO4) standards as the most representative references (results in Table 3). The arrows indicate a frequency present in the solid standards.

Perhaps these technical graphics don't mean much to the average person, but may stimulate some thinking for those interested in analytical techniques, but this cartoon shows how to yttrium bonds to the surface of the aluminate mineral being utilized to remediate the acid mine dicharges:



The caption:

Figure 4. Atomistic representations of the three models of YSO4 aqueous complexes adsorbed on the basaluminite–water interface. The different atomic positions of YSO4 to octahedral-Al are used to fit the EXAFS signal of the B-YSO4 sample. The three models show different inner-sphere surface complexes: (A) monodentate, (B) bidentate mononuclear, and (C) bidentate binuclear.

These results suggest that the monodentate structure is most important:




The caption:

Figure 5. (A) k3-Weighted EXAFS spectra at the Y K-edge of the basaluminite with YSO4 sorbed (B-YSO4-ads reference) and (B) its Fourier transform amplitude. The experimental and fitted curves are shown in black and red, respectively.

The authors propose the following environmental implications:

The YSO4+ aqueous species has been characterized combining PDF analyses of aqueous solutions and AIMD simulations, confirming the formation of an inner-sphere Y–SO4 ion pair with a monodentate configuration, with a Y–S interatomic distance of 3.5 Å. Results from the thermodynamic sorption model describe REE sorption onto basaluminite via sorption of aqueous REESO4+. The use of an atomistic model using this positively charged ion yields the best results for the EXAFS fitting of Y sorbed on basaluminite. However, the EXAFS technique cannot confirm the presence of YSO4+ sorbed onto basaluminite by itself, due to the low sensitivity to discern between Al and S neighbors. However, the EXAFS fitting, together with the PDF, can confirm the strong interaction and the formation of inner-sphere surface complexes of Y on basaluminite precipitates, via ligand exchange with AlO6 units of its structure. EXAFS analyses of column waste samples show that most of the Y is retained as the same inner-sphere sorbed species, YSO4+, with a low proportion of YSO4+ in the outer-sphere configuration.

The description of the local structure of yttrium sorbed onto the basaluminite surface provided here complements the atomic configuration studies of other trace metals, such as As and Se elements in this mineral.(44) The chemical similarity between yttrium and other HREEs (from Tb to Lu) suggests that similar environments could be present for the other elements of the same group. This fact has important environmental consequences, as the HREE would be strongly sorbed, forming inner sphere complexes, which could result in their long-term immobilization at least until the host phase is dissolved or re-precipitated. A key question emerges about the long-term stability of the complex, particularly with an increase in the solution pH.

I feel for future generations, I really do.

It's nice that, despite our obvious contempt for them, as evidenced by our focus on our own wishful thinking (on the left) and denial (on the right), that we may leave them with some knowledge that may be useful.

Enjoy the remainder of your Sunday afternoon.