Pecan nut shell biochar

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.

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?

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.

  1. 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)
  2. Expose granules to air for 2 weeks to complete oxygen chemi-sorption.
  3. 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
  4. Add Biochar to CeO2 NP-amended agricultural and residential soil

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..

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.

X-ray absorption near edge structure ,XANES, defines edges in terms of the orbitals that many of us are more familiar with.

These images were taken from a Libretext.org article on X-ray Absorption Near Edge Structure (XANES), The interesting thing to note is that the availability of more excited state transitions is more than UV/Vis fluorescence that we are accustomed to thinking of only involving outer shell electrons. It is assumed that the Ce-L-alpha edge used in this study is the same as the L1 edge in this image.

Total Ce content was detected with ICP-MS. Like the activated carbon, mitric oxide was used in this protocol but EDTA was not.

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.

plantag 350oC biocharag 600oCres 350oCres 600oC
cornnot signot sigBC, both ↓ @ 2000BC 0.5% ↓@ 2000ppm
lettuce0.5% ↑ @ 2000ppm0.5% ↓ 2000ppmnot signot sig
soy both ↓@ 1000ppm5% worse @2000ppmboth ↓@ 1000ppm5% ↓ @ 2000ppm
zucchiniboth BC ↑ @2000ppmboth BC ↓ @ 2000pmnot sig0.5% ↓@ 2000ppm
↓ reduce Ce accumulation, ↑ Ce accumulation BC pecan shell biochar. . ag agriculture soil, res, residential soil

Note that the nano particles are aggregating.

Figure 3. Images of BC-600 exposed to 500 mg/kg of CeO2 ENPs in residential soil for 28 days. A) Optical microscopy image of biochar sample B) SEM image of biochar sample (x200). Black Pink box marked areas indicate where x800 magnification images were acquired C-F) SEM images (x800) of BC-600 sample displaying Ce distribution on soil- biochar surfaces.

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.

Figure 4. Images of BC-600 exposed to 500 mg/kg of CeO2 ENPs in residential soil for 28 days. (A) Tricolor micro-XRF map. Red color stands for cerium, green for calcium, and blue for silicon. The µXRF map was acquired at 5.8 KeV, 100 ms dwell time, 2 µm2 pixel size. White box marked areas indicate where µXANES was acquired. (B) Map acquired at 5.8 KeV with 200 ms dwell time, and 0.5 µm2 pixel. (C) Ce temperature map, color scale units are raw intensity, numbers indicate areas where µ-XANES was acquired (D) Map acquired at 5.8 KeV with 100 ms dwell time, and 1 µm2 pixel. (E) Ce temperature map, color scale units are raw intensity, numbers indicate areas where µ-XANES was acquired. (F) µ-XANES spectra of reference materials and 9 and 6 spots from Figure 4C and E, respectively

These CeO2 nanoparticles seem to be sticking to the pecan nut shell biochar.

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.

Not all of these variables made a difference. The significant trends in Figure 1 were

CeO2 mg /kg soil
ag 350oC biocharag 600oCres 350oCres 600oC
0not signot signot signot sig
500not signot signot signot sig
1000not sigCe down to baseline5% ↑1&5% ↓
2000not signot sig1&5% ↓ not sig
effect of treatments on CeO2 content of earth worms

These results were quite variable as one would expect for soils with contain mineral metal oxides in addition to the CeO2 nanoparticles.

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.

Figure 2. Images of cross sections of the digestive system of an earthworm exposed to 5% BC-600 and 500 mg/kg of CeO2 NPs in residential soil for 28 days. (A, D, and G) Micrograph of earthworm cross sections. (B, E, H, F, and I) Tricolor μ-XRF maps of earthworm cross sections after depuration for 12, 48, and 72 h. μ-XRF maps were acquired at 5.8 keV, 100 ms dwell time per pixel, and 25 μm2 pixel size (E and H) or 2 μm2 pixel
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.

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. ,

Figure 3. Images of the earthworm cross section exposed to 5% BC-600 and 500 mg/kg of CeO2 NPs in residential soil after depuration for 12 h. (A) Ce temperature map; color scale units are raw intensity, where red represents the maximum Ce content and blue represents the absence of Ce. Red rectangles indicate regions where μ-XANES in fluorescence mapping mode was performed. (B, D, and F) Bicolor μ-XRF maps of earthworm cross sections after depuration for 12 h. Red color stands for cerium (total Ce+4 and Ce+3), and green color stands for Ce3+. White circles indicate the areas where μ-XANES with the predominant Ce3+ oxidation state were extracted to display in panel H. (C, E, and G) Ce temperature maps obtainedat 5.75 keV; color scale units are normalized intensity. (H) μ-XANES spectra of reference materials, sum of all pixels from panels B, D and F, and theCe3+-rich regions marked in panels B and F. The detected contributions from Ce3+ in the spectra ranged from 55 to 70% obtained from linearcombination fitting to CeO2 and Ce3+2(CO3)3.

In Panel H we are seeing proof of concept to detect changes i oxidation states of CeO2 nano particles in an earthworm.

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.

  1. 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
  2. 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
  3. 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.

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