Location: Commodity Utilization Research2013 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.
3. Progress Report:
This three-year project started on February 2012 to study the removal and detoxification of engineered nanomaterials (NMs; very small materials having lengths below 100 nm) by biochar, a value-added product made from agricultural wastes. NMs made from silver, cerium oxide, and carbon (carbon nanotubes and fullerene) are already used in textile, medical, cosmetic, and other commercial products. Various international organizations are explicitly calling for basic understandings in NMs’ impact on agriculture and environment. NMs in sludge and pesticides are making their way directly into agricultural soils. Biochar is produced by heating agricultural wastes without air (charring). Biochar content of soil is likely to increase as a result of intentional addition to improve fertility and to mitigate global warming. Biochar contains very stable forms of carbon that will not degrade in soil for thousands of years. Biochar contains very small, nm-sized pores, and are water-repelling. These properties will likely cause NMs to attach on the surface of biochar and become less water-soluble. Such biochar-NMs interactions will in turn control the effect of NMs on crop growth and environmental pollution. In order to produce value-added products having high stability in soil, pecan shell wastes were heated for 2 hours at 300, 350, 400, 500, 600, and 700 degrees Celsius without air. One kilogram of each biochar sample was mailed to the collaborator to study (1) physical attachment of cerium oxide NMs and (2) toxicity and uptake of NMs by food crops. Carbon dioxide and nitrogen gas-based surface area measurements indicated the abundance of nanometer-size pores, especially for biochars made at higher temperatures. Stability of biochar was estimated from (1) weight change at elevated temperature and (2) hydrogen and oxygen contents, and showed a progressive increase with charring temperature. These analyses confirmed that we prepared the expected materials. Because of known difficulty in recovering water-soluble fullerene NMs, reliable analytical methods needed to be established first. Water-soluble fullerene NMs were prepared by stirring bulk fullerene in water for several weeks in the dark. In addition, concentrated and more regularly shaped fullerene NMs were prepared by adding naturally occurring surfactants. Additional stock solution was prepared by sonicating fullerene-containing toluene layer in a large volume of water. After testing different recovery methods, repeated recovery using toluene and salt was determined to be most reliable. Then, analytical techniques were developed to separate and recover fullerene NMs in toluene layer using liquid chromatography with photodiode array detector. We then conducted experiments to test how fast, how strongly, and how much of fullerene NMs will become trapped in biochar. After 3 days of mixing in water, fullerene NMs were strongly attached to biochar, and could not be removed from biochar, even using a harsh recovery method involving hot toluene. Smaller fullerene NMs were especially tightly bound on biochar, suggesting penetration into pores. All biochars strongly removed fullerene NMs from water, but biochars made at higher temperatures were most effective. Interestingly, biochars made at low temperature decreased the size of water soluble NMs by releasing surfactants. Structure and amount of biochar surfactants were studied using a fluorescence-based imaging technique. Further experiments are being conducted using different salts and pH to understand the chemistry of biochar-NMs interactions, and to visualize the attachment of NMs on biochars using transmission electron microscopy (TEM). Parallel batch experiments were conducted on cerium oxide NMs by the collaborator having an access to a highly sensitive mass spectrometric analyzer for metal NMs. Because smaller NMs are expected to be more toxic, separate methods were developed to prepare cerium oxide NMs having different size. Concurrently with the decrease in size, electric charge (zeta potential) of cerium oxide NMs became more negative with higher pH and surfactant concentrations. The property of NMs suspension did not change over time, making it suitable for the use in biochar experiments. These observations along with TEM imaging confirmed that higher pH and surfactant concentration made NMs more soluble in water. Based on these results for NMs sequestration by biochar, greenhouse experiments were started to test bioavailability of cerium oxide and fullerene NMs on corn, soybean, lettuce, and tomato by the collaborator. Effects of 0.5 to 5% biochar (with respect to the weight of soil) are studied on the crop yield, NMs accumulation, reactive oxygen species production, water evaporation, and photosynthesis. This project was monitored via conference calls, email communications, and face-to-face discussions at two technical conferences and a review panel meeting. Members of this project sponsored a symposium at a professional conference, and contributed a book chapter on topics addressing both NMs and biochar. Agricultural Research Service' Principal Investigator is monitoring activities.