Dredging and disposal have been the most widely used strategies for remediation of contaminated sediments, but these methods are costly, damaging, and may spread contaminants. These limitations have led to the development of in situ methods including passive and active capping. Passive caps composed of inert materials (e.g., sand) physically isolate contaminated sediment and control sediment resuspension but may permit contaminant migration by diffusion, advection, and/or bioturbation. Active caps, which contain sequestering agents, can avoid these problems by reducing the mobility or bioavailability of sediment contaminants. The materials comprising most active caps, however, are susceptible to erosion, which can occur even in low-energy environments as a result of unpredictable dynamic events such as storms and floods. Therefore, current capping technologies may not represent a secure, long-term solution. There is a need for remedial technologies that provide a more permanent solution by degrading or sequestering contaminants while resisting erosion and other types of physical disturbance such as bioturbation. Such technologies must treat a broad range of contaminants in a variety of benthic environments and have acceptable impacts on benthic habitats.
The objective of this SERDP Exploratory Development (SEED) project was to develop a permeable active amendment concrete (PAAC) consisting of apatite, limestone, organoclays, zeolite, sand, and cement. PAAC has the potential to produce a barrier that combines high structural integrity with the ability to stabilize a variety of contaminants.
The PAAC was evaluated through four tasks: 1) development of PAAC; 2) assessment of PAAC for contaminant removal; 3) evaluation of promising PAAC formulations for potential environmental impacts; and 4) assessment of the hydraulic, physical, and structural properties of PAAC.
Conventional permeable concrete (often referred to as pervious concrete) is concrete with high porosity as a result of an extensive and interconnected void content. It is made from carefully controlled amounts of water and cementitious materials used to create a paste that forms a coating around aggregate particles. The mixture has a substantial void content (e.g., 15% - 25%) that results in a highly permeable structure that drains quickly. In PAAC, the aggregate material is partly replaced by chemically-active amendments that precipitate or adsorb contaminants in water that flow through the concrete interstices. PAAC combines the relatively high structural strength, ample void space, and water permeability of pervious concrete with the contaminant sequestration ability of chemically-active amendments to produce a new material with superior durability and ability to control contaminant mobility. The high surface area provided by the concrete interstices in PAAC provides significant opportunity for contaminants to react with the amendments incorporated into the concrete matrix. PAAC has the potential to immobilize a large variety of organic and inorganic contaminants by incorporating different active sequestering agents including phosphate materials (rock phosphate), organoclays, zeolite, and lime individually or in combinations.
The results of this work can be summarized as follows:
- Active amendments were successfully incorporated into permeable concrete (PC). PAACs with apatite, zeolite, organoclay or limestone and apatite effectively removed metals.
- The replacement of a small amount of crushed stone by amendments (e.g., 10%) is sufficient to effectively remove metals from the aqueous phase.
- A static column study was conducted with PAAC containing 20% apatite (PAAC–A), PAAC containing a mixture of 10% apatite, 5% zeolite, and 5% MRM (PAAC–AZM), and permeable concrete without amendments (PC). This study showed that concentrations of metals were significantly (P<0.05) lower in leachates from PC, PAAC-A, and PAAC-AZM than in control leachates (uncapped sediment) for a test period of five months. Results also showed that the concentrations of Cd, Co, Pb, and Zn in the sediment beneath the caps (down to 5 cm) were substantially lower than in uncapped sediment.
- Three flow-through columns (PAAC–A, PAAC–AZM, and PC) were tested under saturated conditions at high and low flow rates. Leachates from the columns were analyzed for concentrations of As, Cr, Co, Cd, Cu, Co, Ni, Se, Pb, and Zn for six weeks. All tested materials removed almost 100% of all metals from the spike solution at low flow rates through the column. There was no difference between the tested materials except that PAAC–AZM was more effective at removing As and Se. At the high flow rate, concentrations of Cr, Cd, Co, Ni, Pb, and Zn in the leachates from the PC column increased to levels found in the spike solution. However, the PAAC-A column effectively removed up to about 40% of almost all tested metals. These results contrasted with the findings of the static column study in which all treatments (PC, PAAC-A, and PAAC-AZM) performed similarly. The better performance of PAAC-A was the result of better metal binding sites in this material. The flow-through column study indicated that PAAC-A constitutes a better capping material than PC (permeable concrete without amendments) because its metal removal capacity is greater – a factor that could be important over long periods of time or in situations where there is substantial movement of water through a cap.
- PAAC exhibited high retention (90% or more) of most tested metals indicating low potential for remobilization based on the Toxicity Characteristic Leaching Procedure (TCLP) and 1 M MgCl2 desorption data.
- All developed PAAC materials exhibited high porosity and hydraulic conductivity values compared to ordinary concrete, and the observed property ranges are consistent with typical permeable concrete. The PAAC-AZM formulation exhibited the highest porosity and hydraulic conductivity. Substantial porosity and high hydraulic conductivity make PAAC ideal for flow through treatment of waters contaminated with heavy metals. PAAC porosity could be modified by changing the ratio of crushed stone to sand.
- PAAC has the potential to create structural barriers that contain contaminants while resisting physical disturbance and permitting the passage of water.
Mixtures of permeable concrete and chemically active amendments have the potential to produce active caps that prevent the migration of sediment contaminants by diffusion, advection, bulk sediment dispersal, and bioturbation in a variety of benthic environments, including sloping shorelines and environments subject to dynamic forces. Existing technologies lack the ability to control simultaneously all of these mechanisms of contaminant migration. This project provided data and proof-of-concept of PAAC for construction of sediment caps at Department of Defense and other sites.