2012 Annual Report
1a.Objectives (from AD-416):
The objective of this research is to elucidate fundamental mechanisms of nanoscale interactions between engineered nanomaterials and biochar in soil.
1b.Approach (from AD-416):
Various porisimetry and microscopy techniques will be employed to understand the surface characteristics of biochar. In addition to sorption/desorption/elusion behaviors of nanomaterials with and without soil, plant/worm assays will be conducted.
This three-year project started on February 2012 to understand how engineered nanomaterials (NMs), very small materials having lengths below 100 nanometer, will affect current and future agricultural practices in the United States. NMs made from silver, cerium oxide, and carbon (carbon nanotubes and fullerene) are already used in textile, medical, cosmetic, and other commercial products off the shelf, and various international organizations explicitly call for research investigating NMs impact on agriculture and environment. Specifically, NMs in sludge and pesticides are making their way directly into agricultural soils. Biochar is formed by heating agricultural wastes like crop residues and nutshells in the absence of oxygen. Biochar contains very stable forms of carbon that will not degrade in soil for thousands of years. Biochar content of soil is likely to increase as a result of intentional addition to improve fertility and stable carbon content of soil. Biochar contains very small, nm-sized pores, and are also water-repelling. These properties will likely cause NMs to attach on the surface of biochar and become less water-soluble. These biochar-NMs interactions will in turn determine the role of NMs on crop growth and environmental pollution. Pecan shell wastes were heated at 300, 350, 400, 500, 600, and 700° Celcius in the absence of oxygen to produce six different kinds of biochar. Water-soluble fullerene NMs were prepared by stirring in water for several weeks with and without humic substance, an organic carbon that is naturally present in all soils. Experiments were conducted to investigate how fast, and to what extent fullerene NMs will be retained by biochar.
To make materials that are stable in soil, pecan shell wastes were heated for 2 hours at 300, 350, 400, 500, 600, and 700° Celcius in the absence of oxygen to produce 1.5 kg biochar at each temperature. Portions of these biochars were mailed to the collaborator to conduct experiments on cerium oxide and other NMs. All biochars were characterized for carbon, hydrogen, nitrogen, sulfur, and oxygen contents; moisture, ash, and stable vs. easily degradable carbon contents; surface area; and by infrared spectroscopy. The collaborator measured the positive and negative charges of biochar by pH titration and zeta potential measurements. These analyses confirmed that we prepared the expected materials.
Water-soluble fullerene NMs were prepared by stirring to grind large clusters of fullerene in water for several weeks in dark. In addition, concentrated and more regularly shaped fullerene NMs were prepared by adding humic substance as a surfactant in this procedure. Analytical techniques were developed to separate and recover fullerene NMs that are dissolved in water and attached biochar, using liquid chromatography with photodiode array detector. Results from this analysis were compared with total organic carbon measurements, and ultraviolet visible spectral features. These complementary techniques confirmed that we prepared the expected water-soluble NMs. We then conducted experiments to test how fast, and to what extent fullerene NMs will be retained by biochars in water. After 3 day shaking of biochar-fullerene mixture, fullerene NMs dispersed more than 30,000-times greater weight of biochar to form a uniform suspension. Fullerene NMs were mainly dissolved in water, and only small portion was attached to biochar. These results suggested that fullerene NMs made biochar less water-repelling by uniformly surrounding biochar that will otherwise sink in water. Further experiments are being conducted using different salt concentrations and pH to understand chemistry of biochar-NMs interactions, and to visualize attachment of NMs on biochars using transmission electron microscopy. In addition, three-dimensional images of biochar’s pore structures are obtained by a collaborator using synchrotron x-ray computed microtomography. These advanced imaging techniques will help us see how NMs move across the pores once attached to the biochar. This project was monitored via conference calls, email communications, and discussions at the annual grantees’ workshop.