Effects of Phosphate Rock On Long-Term Arsenic Hyperaccumulation by Pteris Vittata.

In the U.S., there are 1,210 Superfund sites contaminated with arsenic (As). Due to its toxicity and carcinogenicity, arsenic is ranked by the Agency for Toxic Substances & Disease Registry as the #1 dangerous chemical in the environment. Development of novel soil remediation techniques are required to meet the demands of a burgeoning society.

             Phytoremediation using Pteris vittata (Chinese brake fern) offers a simple, non-invasive, and cost-effective method to remediate arsenic-contaminated soil compared to traditional clean-up techniques. A major disadvantage of phytoremediation is the lengthy remediation period (years – decades). However, the ability of P. vittata to remediate As contaminated soils are based primarily on experiments with short growing periods (~12 weeks) under conditions not representative of the soil environment (i.e., glasshouse conditions; <1.5 kg soil; small pots <15 cm h × <15 cm d ). While useful, these studies may underestimate the remediation capacity of P. vittata.

            This study aims to replicate natural soil conditions to determine the potential of P. vittata to remediate As-contaminated soils over several growing seasons. In addition, we evaluated the effect of phosphate rock (PR) on arsenic uptake and growth of P. vittata. The use of PR is suited because it provides a long-term supply of sparingly soluble P. Since P and arsenic compete for uptake by P. vittata roots, we hypothesized that the physiological and biochemical responses of plant roots in a P limiting environment will lead to increase arsenic solubility and uptake by P. vittata.

            Three soils were collected from abandoned cattle dipping vats (DVA and DVB) and a chromated copper arsenic (CCA) wood treatment facility, air-dried, sieved through a 2 mm screen, and mixed with 15 g^-1 kg^-1 phosphate rock [PR, Ca₁₀(PO₄)6F₂ (CaCO₃)_x, <1 mm]. Each soil was placed into a raised bed (1.44 m²) containing four quadrants (0.36 m²) to a 30 cm depth.  Similarly-sized P. vittata (3-4 fronds ~15 cm in length) plants were planted 15 cm apart in Dec 2009 (9 per quadrant; 36 per bed).  For comparative analysis, plant-less plots containing PR were setup in addition to P. vittata ferns in plots received 10:4:10 fertilizer. Soil samples were taken at time of planting and during the first (June 2010) and second harvest (Jan 2011).

            Fronds were trimmed, leaving ~15 cm and young fiddleheads. Samples were oven dried at 65 °C for 96 h, weighed, and were ground (< 2 mm). Samples were digested and analyzed for arsenic concentration with a graphite furnace atomic absorption spectrophotometer (Perkin-Elmer). Soluble P in the soil and frond biomass was measured with the molybdenum blue method. Biomass from soils amended with PR averaged 24.5, 20.9, and 19.5 g per plant, respectively, for CCA, DVA, and DVB soils while frond biomass from soils with no PR averaged 15.1, 11.2, and 10.8 g, respectively. Frond As concentrations for CCA, DVA, and DVB averaged 3,273, 1,221, and 1,334 mg/kg while the controls averaged 1,300, 502, and 501 mg/kg, respectively. With the PR amendment, fronds had twice the biomass with a ~61% increase in arsenic concentration compared to the controls.

            Using PR amendments along with improved harvesting practices, our results showed that P. vittata can accumulate up to 11-times more arsenic than previously published studies. Pteris vittata has greater capacity to remediate As-contaminated soils than previously recognized.