- Determine if heavy metals present in some AMD sources would be incorporated into the coating and characterize their leaching potential;
- Determine the operational parameters for the use of coated sand in the horticultural industry; and
- Quantify any additional benefits from using coated sand in potting media such as improved root structure, increased flowering and improved post-production success.
Coated sand, made from AMD, is a cost-effective phosphate sorbent that will allow the capture and reuse of dissolved phosphate in the greenhouse/nursery industries. This will result in reduction in fertilizer application and non-point source phosphate inputs to aquatic systems while reducing the amount of AMD sludge to be disposed of. It will also provide a constant source of phosphorus during production and post-production that will lead to increased profits for the growers.
Coated sand is made using acid mine drainage (AMD) which may contain toxic heavy metals such as lead, copper or arsenic. The procedure used to make coated sand will exclude many of the toxic heavy metals. Additionally, heavy meals that are in the coating could be bound to the sand and are not at risk of leaching into aquatic and terrestrial environments. However, it is essential that we understand which, if any, of the toxic heavy metals in the AMD might be present in coated sand and whether they can be leached from the coating. In this experiment we added any of the heavy metals not already present in the AMD to the highest concentrations found in the literature and followed our standard coating procedure. Both the sand and leachate were evaluated for heavy metal concentrations.
Seventy-five liters of raw AMD was collected from the Omega mine site south of Morgantown. The raw AMD was spiked with heavy metals at concentrations found in the literature. The pH was adjusted and the precipitated metals allowed to settle overnight. The supernatant was siphoned off and the precipitated was applied to sieved sand in the standard coating procedure. Supernatant and sludge samples were analyzed for aluminum (Al) , arsenic (As), calcium (Ca), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), nickel (Ni), lead (Pb), antimony (Sb), selenium (Se), and zinc (Zn) via ICP-OES to determine the partitioning of metals.
Coated sand (250 ml/414 g) was placed in each of six 2 inch pots and watered with 250 ml tap water to thoroughly wet the sand. On subsequent days the sand was watered daily with 100 ml tap water. Leachate was collected on day 1, 14, 28, and 42 and analyzed for the metals listed above. Tap water was also analyzed for the same metals to differentiate between the contribution from the sand and what existed in the tap water.
A second leaching experiment was done with both coated sand spiked with heavy metals (n=2) and coated sand that had not been spiked (n=3). In this experiment we monitored leachate pH, sulfate, Fe, Al, Ca, Mg, and Mn concentrations.
The metals in AMD were primarily Fe (42%), Ca (26 %), Al (20%), and Mg (8%). The percentage of these metals changes when the pH is adjusted during the precipitation process. Fe was still the predominant metal accounting for 63% while Al accounted for 31% of the mass. The metals partitioning between supernatant and precipitate was metal-specific. The metals Ca, Co, Mg, Mn, Ni, Sr, and Zn remained predominately in the supernatant while the metals Al, As, Cd, Cr, Cu, Fe, Pb, and Sb were predominately in the precipitate. Despite being major components of the AMD, Ca and Mg accounted for just 1.7 and 0.6% respectively of the precipitate mass. The majority of the As, Cr, Cu, Pb, and Sb precipitated; however, due to low concentrations in the raw water, they comprised just 3.6% of the mass of the precipitate. These results likely over estimate the percentage of metals precipitated as we used the highest concentrations reported in the literature. Additionally, there may have been interactions between metals that would not naturally occur as they would not be found together.
The pots of coated sand were watered daily with tap water with samples being taken on days 1, 14, 28 and 42. Metal concentrations in the pot leachate were highest on the first day with most metals decreasing in concentration over time (Table 1). The metals with the highest concentrations were Ca and Mg with concentrations of 6,501 and 18 mg/L, respectively on day 1. These concentrations decreased rapidly over time with concentrations around or below tap water concentrations by day 42. Fe, the metal making up most of the coating had concentrations of around 0.1 mg/L through the entire experiment. Concentrations of Al ranged from 0.7 to 1.0 mg/L. Concentrations of most of the other metals were less than 0.05 mg/L with Cd, Co, and Cu concentrations generally below detection. However, As concentrations ranged between 0.20 – 0.45 mg/L, Cr concentrations were initially 0.8 mg/L but dropped quickly and most troubling, initial Sr concentrations were 10 mg/L. These results indicate that AMD that contains heavy metals such as Al, Fe, Mn, and Co can be used to coat sand without concern as they are either not incorporated into the coating or they do not leach. However, AMD that contains Sr, Cr or As should not be used.
BDL – Below Detection Limit, *reported value is from the one sample in five that was above the detection limit
A second leaching experiment was done where pot leachate was collected daily to examine how leachate varied over shorter time periods. We used the remaining spiked coated sand (2 pots) and regular coated sand (3 pots ) and measured pH, and sulfate, Fe, Al, Ca, Mg, and Mn concentrations.
The pH of the leachate varied between 7.5 and 8.8 with a mean of 8.2. The pH of tap water ranged between 7.5 to 8.0. Initial sulfate concentrations were between 434 to 494 mg/L and then increased to a maximum of 2443 mg/L on day 5. The concentrations decreased such that by day 18 the sulfate concentrations were 170 to 300 mg/L. Sulfate concentrations continued to decrease but at a slower rate and were 70 mg/L at the end of the 30 day experiment. Tap water concentrations ranged between 55-75 mg/L.
Initial calcium and magnesium concentrations were extremely high on the first day, 8899 and 534 mg/L, respectively. Calcium concentrations decreased to around 100 on the next 3 days before increasing again to a high of 610 mg/L on day 8 after which concentrations decreased slowly to 116 mg/L by day 20. Concentrations at the end of the experiment were comparable to tap water at 30 mg/L. Magnesium concentrations followed the same pattern with concentrations around 30 mg/L on days 2 through 5 increasing to 40 on day 8 and they decreasing to 19 by day 11. Concentrations decreased to 7.3 by day 22 and then increased to 12 at day 30.
Fe, Al, and Mn were all at low concentrations. Initial iron concentrations were 0.57 mg/L but dropped to 0.35 mg/L by day 3 and further decreased to 0.10 mg/L by day 10. From day 20 to day 30 concentrations were 0.03 to 0.05 mg/L. Aluminum concentrations varied between 0.23 to 1.1 mg/L with no apparent pattern. Initial manganese concentrations were 0.24 but dropped to 0.03 mg/L on day 2 and remained less than 0.06 for the remainder of the experiment with an average concentration of 0.02 mg/L
The leaching experiments demonstrated that the coated sand needs to be rinsed before use to remove sulfate, calcium and magnesium which leach at high concentrations. The rinsing will also remove heavy metals that are loosely bound. The pH of the leachate was generally stable. The experiments confirmed previous experiments where the major metals in AMD (Fe, Al, and Mn) were retained within the coating with insignificant losses to the leachate. A washing protocol for the coated sand has been developed to avoid the initial flush of metals when the coated sand is used. Leaching analysis will be done on washed coated sand that was spiked with heavy metals to ascertain efficacy of the procedure in removing those metals. Additionally, SEM will be done on both washed coated sand and washed coated sand spiked with heavy metals to evaluate the presence of metals within the coating.
Education & Outreach Activities and Participation Summary
We will be staffing a booth at the 16th annual Small Farms Conference in Charleston, WV, February 20 – 22, 2020. The conference attracted over 600 attendees from across the state and region last year and serves as a vehicle to assist farmers in a wide range of subjects. Our booth will provide educational material regarding coated sand and the experiments we will be conducting. We will survey attendees regarding their views on coated sand and solicit feedback regarding our planned activities.