Citric Acid Enhanced Metal UptakeThree Plant Species Studied at Bioretention Field Site

Utah State University, Department of Civil and Environmental EngineeringM. Rycewicz-Borecki, Dr. R.R. Dupont, Prof. J.E. McLean

Outline• Quick Introduction

• Design Methodology

• Results and Discussion

• Conclusions

INTRODUCTION

Pollutants in Stormwater• Untreated Stormwater Runoff Detrimentally Affects

Downstream Water Bodies and Groundwater Recharge

• Planted Bioretention Systems Remove Significantly More Pollutants From Runoff than Unplanted Systems

Plant Uptake• Aboveground Plant Tissue Takes Up some Portion of Pollutants

and can be Harvested and Disposed of Off-Site (Uptake)

• Uptake can be Enhanced with the Use of Synthetic Chelators or Low Molecular Weight Organic Acids (i.e., Citric Acid)

Critical Needs• Design and Develop Inexpensive and Environmentally

Responsible Systems for Maximum Stormwater Pollutant Removal

• Maximize Metal Uptake to Decrease Soil Metal Accumulation and Prevent Hazardous Soil Levels

Research Objective• Evaluate potential for citric acid-enhanced metal uptake as a

stormwater management technique in bioretention

-Does Citric Acid: Increase Soluble Metals in Soil Pore Water?

Increase Leaching of Metals into Groundwater?

Enhance Plant Uptake?

-Identify Plant Species Influence on Uptake Potential

METHODS

Field-Site Design• Green Meadows Subdivision• Built with Logan City• Initially Planted Oct 2010

600

Sou

th

North

Cattail

Sunflower

Sedge

• Small-wing Sedge• Maximilian Sunflower • Broadleaf Cattail

Field Site

Field-Site Design

Vertical separators

Aug 2014 – Oct 2014

Control = 0Low =10 mmol/kgHigh = 50 mmol/kg

Field-Site Design

Citric Acid Applied (August Only) Irrigated

3” depth 6” depth

Suction Cup Samplers Installed

Field-Site Design+ 24 hours

Suction Cup Pore Water Samples

Soil and Pore Water

1.5”

4.5”

7.5”

3” Pore Water

6” Pore Water

Surface Soil

Field-Site Design5 days later

Suction Cup Pore Water Samplers

Above Ground and Root Tissue (3)

Soil and Pore Water

Above Ground Harvest (3 x 1sf)

Root Samples (auger, 3 x 1sf)

Field-Site Design• No Citric Acid Applied for October Sampling

• All Other Experimental Procedure Repeated

Soil and Pore Water

Above Ground and Root Tissue (3)

Same Suction Cup Location

RESULTS

Results – Laboratory Soil Investigation

• Soils analyzed for metal solubility with citric acid and HCl

pH + chelation:

AlAsCaCrCuZn

pH effect

*

*

chelation effect

*

*

Results – Field Study Citrate Recovery

• Linear Relationship in unplanted• At high dose, planted has lower citrate at 6 inch depth

Unplanted Treatment, Aug (24 hours after CA application)

Aug Citrate

3 inch6 inch3 inch6 inch

3 inch depth 6 inch depth

3 inch depth

6 inch depth

Speculation: • Microbes in plant root

environment enhance citric acid degradation

Results – pH

• No pH effect with citric acid addition• calcareous soil with high buffer capacity

• Only pH differences @ 6” depth Planted Treatment

• pH differences (at 3” and 6”) continue in October

pH influenced by root environment microbial activity, not Citric Acid

3 inch6 inch3 inch6 inch

* All CA levels

Results – Pore Water Metal Solubility (3 inch depth, Aug)

Planted:• Increasing trend (Seven metals)

Unplanted: • Increasing trend

(Four metals)• Lower concentrations

than planted

Citric acid increases microbial activity in root environment, and metal solubility

Planted Unplanted Planted Unplanted

Results – Groundwater Leaching Potential

• Aug Citrate Decreased with Depth; Completely Degraded in Oct

3 inch

6 inch3 inch6 inch

3 inch6 inch3 inch

6 inch

plan

ted

unpl

ante

d

• No Risk of Leaching Soluble Metals Under these Conditions

Also:

AlCaCuMgZn .

Results – Above Ground Biomass and Uptake

• Citric Acid Influence NOT Significant

Speculation:• Reducing Conditions in Calcareous Soil Solubilize

Carbonate Minerals (Meng, 2015)• Abundance of Ca cations out compete other metals

Results – Above Ground Biomass and Uptake

Iron Uptake (mg/m2)

August October

SedgeSunflowerCattail

Also Al, Cr, Cu, Pb

•Consistent Species Differences Found

August October

AG b

iom

ass (

g m

-2)

Biomass (g/m2)

Results – Sunflower Speculated Arsenic Toxicity

10 mmol/kg

50 mmol/kg

SedgeSunflowerCattail

Arsenic

August October

Above Ground

16th

SedgeSunflowerCattailUnplanted

SedgeSunflowerCattailUnplanted

3”

dept

h6”

de

pth

Pore Water (Aug 12th)

Conclusions• Soluble Metals in Soil Pore Water Increase with Citric Acid

Dose (Fe, Al, As, Ca, Cr, Cu, Mg).

• Citric Acid Application Does Not Pose a Threat to Groundwater Leaching.

• Citric Acid Does NOT Enhance Plant Uptake Potential Under these Conditions. Species Selection Does.

• Citric Acid is Speculated to Induce Arsenic Toxicity in Sunflower.

Engineering Significance• Citric Acid Application is Not Suggested as a Enhanced Uptake

Technique (Calcareous Soils with Low-Level Metal Concentrations)

• Plant and Harvest Small Wing Sedge (over Cattail and Sunflower) to Slow Soil Metal Accumulation, Optimize Metal Recovery, and Lengthen Bioretention Operating Life

Future Work• Expand Site Conditions

• Experiment earlier in the Growing Season• Increase Replications• Investigate Sunflower Arsenic Toxicity Further

• Calcareous and Acidic Soils• Soil Metal Concentrations (Low, Med, High)

THANK YOU!

Funded by: • Utah Mineral Lease Fund• EPA 2010 Source Reduction Assistant Grant Program

THANK YOU!Questions?