Grantee Research Project Results
Final Report: Lessons from Nature - Synthetic Humic Acid Materials for Improved Water Purification
EPA Grant Number: SU835311Title: Lessons from Nature - Synthetic Humic Acid Materials for Improved Water Purification
Investigators: Webster, H. Francis , Burkhardt, Cindy A , Slate, Craig , Godward, Dennis , Crook, Elizabeth , Shelton, Jacob , Freeman, James , Ford, Maddie , Sublett, Matt , Webster, Rebekah
Institution: Radford University
EPA Project Officer: Hahn, Intaek
Phase: I
Project Period: August 15, 2012 through August 14, 2013
Project Amount: $14,917
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2012) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Chemical Safety , P3 Challenge Area - Safe and Sustainable Water Resources , P3 Awards , Sustainable and Healthy Communities
Objective:
Due to population increases and rapid industrialization around the world, water quality and the ability to ensure the availability of reliable clean drinking water will be an increasingly important global concern. The lack of clean water is most keenly felt in developing countries where more expensive water purification technologies are not available. The need for clean water has spurred a variety of research efforts related to the development of new methods and materials for water purification. Our project focused on the development of an inexpensive, multi-functional adsorbent material that will improve existing sand filtration technology to better remove a wide range of water contaminants, including arsenic, other heavy metals, and organic compounds. The adsorbent is based on our recent discovery of a novel synthetic humic acid-like carbon material (sHAC) that is economical, easy to prepare, and based on renewable starting materials. The overarching goal of this project is to mimic the humic acid found in nature, a natural polyelectrolyte that easily binds to minerals and serves as an excellent chelator of metals in the environment. Our purpose is to overcome the main limitation of natural humic acid variability by synthesizing a highly functionalized humic acid-like material with reproducible chemical composition for use in water purification.
Our Phase I project had four major objectives: (1) synthesize and fully characterize a sHAC material with heterogeneous, but defined, chemical structure using renewable starting materials, including sugar and sugar alcohol sources, and a low temperature one-pot synthesis; (2) prepare three composite materials, including sHAC/sand, sHAC/iron-coated sand, and sHAC/magnetic iron nanoparticles, and fully characterize these materials; (3) investigate the potential of the composite materials to bind arsenic, heavy metals, and a model organic dye in controlled water systems using batch and column methods; and (4) perform an initial cost/benefit analysis to assess the feasibility for use in the developing world. Aspects of this project were incorporated as a novel experiment in the Radford University Physical Chemistry course and two students from a regional math and science Governor’s School worked on natural extensions of this project for a science fair competition. This research supports the goal of the Environmental Protection Agency to undertake research leading to improved methods of water purification and providing a safe supply of drinking water (EPA; SDWA: Safe Drinking Water Act - Section 1442).
Summary/Accomplishments (Outputs/Outcomes):
Synthetic humic acid (sHAC) was produced through the dehydration of various sugars using sulfuric acid as a dehydrating agent. Characterization of this material included a number of techniques, and the number of acid sites available was evaluated through the determination of ion-exchange capacity (IEC) using standard methods for humic acid. The results show a highly functionalized material with almost 10 mmol/g total acid sites. The number of strong acid sites was found to range from 1.3-2.6 mmol/g, which compares favorably to commercial cationic exchange resins. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition of the surface (~50 nm sampling depth) for strong and weak surface acid sites. Results show a sulfur photopeak at 168 eV indicating the presence of sulfonic acid groups in the material. The thermal stability of our adsorbent material was evaluated using thermal gravimetric analysis (TGA) and results showed that the sHAC carbon contained approximately 10-15% water and was stable up to 200ºC. Samples were analyzed by SEM to characterize the surface topology and showed macro-sized carbon particles with a relatively smooth surface structure. At a much higher magnification (100,000x) the presence of a rough nano-particle surface structure was seen. Light scattering was used to determine the particle size distribution for dilute suspensions (1 ppm) and showed a relatively narrow distribution of nanoparticle-sized structures with a mean radius of approximately 100 nm.
To form the composite materials used for adsorption studies, sand was coated with sHAC carbon using the slow evaporation of a slurry at 50ºC with stirring. To form sHAC/goethite coated sand, the carbon gel was mixed with ferric chloride and the pH increased until the formation of iron hydroxides (goethite) occurred. The slurry was again allowed to evaporate under low temperature conditions with stirring. sHAC/magnetite composites were prepared using a method where ferric and ferrous chloride were first mixed with sHAC gel and the pH adjusted to 10 using ammonium hydroxide. After heating at 80ºC, the now magnetic composite particles were harvested with a magnet and dried before use.
Coated sand samples were analyzed by x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS analysis for both carbon and carbon/goethite coatings showed the presence of sulfonic acid groups on the surface. SEM microscopy was useful in the evaluation of the surface morphology of the coatings and for the sHAC coated samples, the surface coating was not evident except at very high magnifications (100,000x). The carbon/goethite coatings were much more apparent, but appeared as non-uniform coatings with considerable nano-sized structure at high magnification. Analysis of sHAC/magnetite composite samples was done using the same techniques as described above. TGA analysis showed that the composite was composed of approximately 40-50% carbon material and XPS analysis showed the presence of sulfonic acid and oxidized carbon mixed with typical magnetite iron signals. SEM showed large 1-10 μm plate-like structures at low magnifications, while higher magnifications indicated a surface composed of very uniform nanoparticles ranging in size from 10-30 nm.
Adsorption Studies
In a typical experiment for sand-based adsorption experiments, 12 mL of 10 ppm metal ion solutions at pH values ranging from 3-6 was added to a 15 mL conical tube and 250 mg of sand was added. For methylene blue tests, 0.1 mM stock solutions were used. Samples were equilibrated on a rotating shaker for 24 hours. After allowing the sand to settle, the liquid was removed and analyzed by atomic adsorption spectroscopy (Cu2+, Pb2+, Cr2+, Cd2+), inductively coupled plasma spectroscopy (As (III), As (IV)), or visible spectroscopy (methylene blue). In a typical experiment for magnetite and sHAC/magnetite adsorption experiments, 50 mL of 10 ppm metal ion solutions at pH values ranging from 3-6 was added to a 50 mL conical tube and 25 mg of the adsorbent added. Samples were equilibrated on an orbital shaker for 24 hours. A neodymium magnet was used to remove the magnetic adsorbent, the liquid removed, and analyzed as above.
Control sand samples showed limited ability to remove all contaminants investigated with only a small capacity for lead and chromium at higher pH values. The sHAC-coated sand had little adsorption capacity for arsenic, but the adsorption for all metals was dramatically increased, especially for lead and cadmium ( > 80% removed). The adsorption capacity was increased somewhat by repeating the coating process (2 layers). Methylene blue was completely removed at all pH values. The iron/carbon composite-coated sand showed a dramatic increase in the adsorption of all metals with lead, copper, and cadmium being completely removed under the conditions studied. Sixty percent of the chromium was removed and methylene blue was completely removed. The adsorbents also demonstrated the ability to remove arsenic, particularly arsenic (III), which is neutral at the pH tested. Repeating the coating process improved the adsorption performance for chromium and arsenic. Another striking feature was that the adsorption shows a reduced dependence on pH when compared to iron-coated sand in the 4-6 pH range.
The magnetite control samples showed a high capacity to remove arsenic for both oxidation states at the pH of 5 investigated. This adsorbent showed little ability, however, to remove the other contaminants investigated. The sHAC/magnetite retained significant ability to remove arsenic, but the capacity to remove the other metals investigated was dramatically improved (approximately 80% removal) and relatively independent of pH in the range of 4-6.
To test our adsorbents under flow conditions, the Rapid Small Scale Column Test (RSSCT) was used. For this test, a small column size was used to predict the results for pilot or full-scale column systems. If the hydrodynamic characteristics of the small column are similar to the large column, breakthrough curves are expected to be similar and large pilot or full-scale columns can be designed using the results of the RSSCT. The advantage of this approach was the much lower test cost for required materials, shorter time for analysis, and the much lower amount of water needed to test potential adsorbents. The results showed that under the flow conditions of the RSSCT study here, both sHAC and sHAC/goethite show dramatic improvement in the amount of all metal contaminants removed compared to sand (greater than 10 times the breakthrough volume in all cases). sHAC-coated sand seemed to have superior performance compared to sHAC/iron under flow conditions, but a more in-depth study under various conditions must be done to fully compare the two.
Conclusions:
The objective of balancing the elements of people, prosperity, and the planet was paramount in all aspects of the Phase I research. To reduce the cost of production, an adsorbent for water purification was prepared using simple one-pot methods with bio-renewable sugars and other readily available starting materials. This increases the possibility of providing clean water to a wider range of people by the development of technology that is accessible to everyone. In addition, the demonstrated use of glycerol to make the sHAC carbon now leads to the possibility of using waste glycerol from biodiesel production. Using this waste material increases the value of glycerol, decreases the energy costs to purify or dispose of it, and decreases the cost of making the sHAC carbon. Progress toward sustainability will happen when scientists learn to design materials using renewable resources, require less energy to prepare them, and generate fewer by-products with directed uses for each.
Journal Articles:
No journal articles submitted with this report: View all 2 publications for this projectSupplemental Keywords:
water purification technologies, water treatment, water filtration, coated-sand adsorbents, synthetic humic acidRelevant Websites:
P3@Radford University ExitP3 Phase II:
Lessons from Nature - Synthetic Humic Acid Materials for Improved Water PurificationThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.