Do medical grade activated carbons really contain heavy metals? It would seem that they do and that when they are in the metal oxide form, they are hardy to detect via the ICP-MS gold standard. Can activated carbon keep the metal oxide from getting into plants and/or earth worms? It depends on the soil type, but not really.
Heavy metals in medical grade activated carbon? [1]
USS-ETV ICP-MS is an acronym for ultrasonic slurry sampling electrothermal vaporization Inductively coupled plasma mass spectrometry. ICP-MS is the gold standard for heavy metal determination. A team of scientists from Taiwan and hydrabad (India) argue that difficult to dissolve samples may need a little help to be properly analyzed by ICP-MS. Metal oxides are thought to interfere with ICP-MS detection. The divalent metal chelator EDTA was added to enhance the volitility of the heavy metals analyzed. Pyrolysis time was also a variable tested. The two methods were evaluated in relation to NISH certified fly ash. Both methods agreed with this standard. Three different brands of medical activated carbon were analyzed. We were not told the brands but can probably these Indian and Taiwanese authors used brands sourced from China and Sri Lanka as has been covered in a previous post on this site. Most of the commercial suppliers of activated carbon in North America source from these countries. The following are some approximate ranges in units of ng heavy metal per g medical grade activated carbon.
- Cd, cadmium, was only found in one of the three brands. The concentration was about 5.5 ppb.
- Hg, mercury, was found in all three brands. The range was about 4.3 to 19.5 ppb
- Pb, lead, was also found in all three brands. The range was 80 to 200 ppb.
Sri Lanka may have plenty of coconut shells to dispose of. Do we in the US have something that we need to dispose of that would be a source of activated carbon as good as or better than what we import?
Pecan nut shell biochar studies protocols [2,3]
These studies asked the question if the emerging encomronmental nano contaminant CeO2, ceria, could be kept from entering plant and animal tissue. Pecan nut shell biochar and residential vs agricultural soil were the two variables.
Pecan nut shell activated carbon (biochar) protocol [2,3]
- pyrolysis of pecan shells at 350 °C (BC-350) and 600 °C (BC-600) for 4 hr under a stream of nitrogen (1600 mL/min)
- Expose granules to air for 2 weeks to complete oxygen chemi-sorption.
- Measure the ζ potential of the biochar and the and CeO2 NPs as a function of pH. The ζ potential gradually became negative, reaching -51 mV as the pH increased from 3 to 11.16
- Add Biochar to CeO2 NP-amended agricultural and residential soil
soil protocol [2]
a residential soil (sandy loam; 69% sand, 22% silt, 8.6% clay; 4.3% organic matter; pH 5.9; cation exchange capacity 18.6 cmol/kg) collected from the top 50 cm of the Connecticut Agricultural Experiment Station in New Haven, CT; and an agricultural soil (fine sandy loam soil; 56% sand, 36% silt, 8% clay;.4% organic matter; pH 6.7; cation exchange capacity 18.6 cmol/kg) collected from the top 50 cm of the Connecticut Agricultural Experiment Station Lockwood farm in Hamden, CT. Lettuce, corn, soy, and zucchini were the bio accumulating plants of choice to study how ceria nanoparticles may affect our food..
Imaging heavy metals with X-rays
The X-ray fluorescence image came from DOEET. XRF is different from visible light fluorescence in that the inner shell electron is actually ejected from the atom or moved to a much higher orbital. Most of us are more familiar with valence electrons excited to higher non occupied orbitals. When electrons from higher orbitals “come down” to fill the vacancy they emit a photon corresponding to the difference in potential energy.
Definition of edges
X-ray absorption near edge structure ,XANES, defines edges in terms of the orbitals that many of us are more familiar with.
XANES Oxidation state sensitivity
“As the oxidation-state of the absorption site increases, the absorption edge energy increases correspondingly. This observation can be explained using an electrostatic model: atoms with a higher oxidation state require more energetic X-ray to excite its core electron because the nucleus is less-shielded and carries a higher effective charge.
However, an alternative interpretation of edge energies is more suitable. This interpretation treats the edge features as continuum resonances. A continuum resonance refers to a short lived the excitation process in which a core electron is excited into a higher energy state that is usually above the continuum. An example is the potential well created by the absorbing and scattering (between nearest neighboring) atoms. As the absorber–scatterer distance gets shorter, the energy of the continuum state increases as 1/R2. Since higher oxidation-states implies shorter bond lengths in molecules, the edge energies increases as the oxidation-states increases.
As stated above, XANES is oxidation sensitive. Moreover, multiple scattering is particularly important in the XANES region. In principle, one can argue that it is possible to determine the three-dimensional structure of the absorbing atom to its environment from analysis of the XANES features. Experimentally, this has been proven true. The XANES region is quite sensitive to small structural variations. For instance, two sites with identical EXAFS spectra can nevertheless have distinct XANES spectra. Such sensitivity is intuitively, at least in part, due to the fact that geometrical differences between sites alter the multiple scattering pathways, and thus the detailed structure in the immediate vicinity of the absorption edge.”
Corn, lettuce, soy, zucchini [2]
Total Ce content was detected with ICP-MS. Like the activated carbon, mitric oxide was used in this protocol but EDTA was not.
Summary table of Ce in plants
The total plant Ce content (µg) of corn, lettuce, soybean and zucchini whole plants
grown from residential soil amended with 0-2000 mg/ kg of CeO2 ENPs with BC-350
and BC-600 at 0-5%.
This table summarizes bar graphs in the publication.
| plant | ag 350oC biochar | ag 600oC | res 350oC | res 600oC |
| corn | not sig | not sig | BC, both ↓ @ 2000 | BC 0.5% ↓@ 2000ppm |
| lettuce | 0.5% ↑ @ 2000ppm | 0.5% ↓ 2000ppm | not sig | not sig |
| soy | both ↓@ 1000ppm | 5% worse @2000ppm | both ↓@ 1000ppm | 5% ↓ @ 2000ppm |
| zucchini | both BC ↑ @2000ppm | both BC ↓ @ 2000pm | not sig | 0.5% ↓@ 2000ppm |
These are some visible light images of biochar particles and roots
Note that the nano particles are aggregating.
imaging roots and biochar with X-rays
The L-alpha edge involves 2s electrons, presumably redox sensitive. Note that the Ce(III) carbonate has only one peak around 5730 mV. This reinforces the ability to detect redox status.
These CeO2 nanoparticles seem to be sticking to the pecan nut shell biochar.
Pecan nut shell biochar, Ceria NP, and earth worms [3]
This group used micro X-ray fluorescence (μ-XRF) and micro X-ray
absorption near edge structure (μ-XANES) that biochar could influence the absorption of ceria nanoparticles by earthworms. Earthworms (E. fetida) were exposed for 28 days to
- 500, 1000. 2000 mg/kg CeO2 in
- agricultural and residential soils amended with CeO2 NPs
- 350oC biochar was compared with 600oC biochar
- biochar was added at 0, 0.5 and 5% of the weight of the soil.
earthworm ICP-MS analyses summary
Not all of these variables made a difference. The significant trends in Figure 1 were
| CeO2 mg /kg soil | ag 350oC biochar | ag 600oC | res 350oC | res 600oC |
| 0 | not sig | not sig | not sig | not sig |
| 500 | not sig | not sig | not sig | not sig |
| 1000 | not sig | Ce down to baseline | 5% ↑ | 1&5% ↓ |
| 2000 | not sig | not sig | 1&5% ↓ | not sig |
These results were quite variable as one would expect for soils with contain mineral metal oxides in addition to the CeO2 nanoparticles.
Imaging heavy metals
Earthworms lived for 28 days in residential soil supplemented with biochar-600 and 500 mg/kg of CeO2 NPs. The worms were subjected to depuration, i.e. placed in fresh water to purged loosely adhered environmental contaminates. Even after three days in fesh water, these earthworms had some residual CeO2 nanoparticles in them.
micro-XRF, several elements at once…
step size (B, F, and I). Red color stands for cerium; green color for calcium; and blue color for sulfur. (C) Ce temperature map from panel B; color scale units are raw intensity, where red represents the maximum Ce content and blue represents the absence of Ce
Panel C is a heat map quantitation image of the Ce in Panel B. The little red dots in B appear to be about the same intensity and are more or less proven to be the same intensity in panel C.
Imaging Ce Oxidation state in earthworms, 1000mg/kg residential soil
Figure 3 of the Servin earthworm publication [3] starts off with (A) a CeO2 heat map from which three red boxed regions were chosen for further XANES inspection. (panels B, D, F) Here total Ce (II) and Ce(IV) are red and and Ce(III) only is green. ,
In Panel H we are seeing proof of concept to detect changes i oxidation states of CeO2 nano particles in an earthworm.
Concluding remarks
While heavy metals not liberated by nitric acid alone in ICP-MS standard protocols might not be liberated by stomach hydrochloric acid under “Mild’ pH 1.5 conditions, that these CeO2 nano particles came off he biochar/AC to partially enter the plant or the earthworm is a bit of concern. It would be interesting to determine if this technique could be extended to detect heavy metal redox states bound to clays like bentonite/montmorillonite or zeolite/clinoptillolite discussed on this site. The preparatio of biochar in the Servin reports [2,3] was a little less involved than the protocols on the DIY activated carbon post. Can XRF and XANES detect heavy metal oxides binding to biological membranes containing phosphates. Possible d-shell interactions was discussed on metal oxide chemistry.
References
- Chen CC, Jiang SJ, Sahayam AC. Determination of trace elements in medicinal activated charcoal using slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry with low vaporization temperature. Talanta. 2015 Jan;131:585-9. Sci-Hub free paper
- Servin AD, De la Torre-Roche R, Castillo-Michel H, Pagano L, Hawthorne J, Musante C, Pignatello J, Uchimiya M, White JC. Exposure of agricultural crops to nanoparticle CeO2 in biochar-amended soil. Plant Physiol Biochem. 2017 Jan;110:147-157. Sci-Hub free paper
- Servin AD, Castillo-Michel H, Hernandez-Viezcas JA, De Nolf W, De La Torre-Roche R, Pagano L, Pignatello J, Uchimiya M, Gardea-Torresdey J, White JC. Bioaccumulation of CeO2 Nanoparticles by Earthworms in Biochar-Amended Soil: A Synchrotron Microspectroscopy Study. J Agric Food Chem. 2018 Jul 5;66(26):6609-6618.
