Addition of activated carbon (AC) or biochar (BC) to sediment to reduce the chemical and biological availability of organic contaminants is a promising in-situ remediation technology. But concerns about leaving the adsorbed pollutants in place motivate research into sorbent recovery methods. This study explores the use of magnetic sorbents. A coal-based magnetic activated carbon (MAC) was identified as the strongest of four AC and BC derived magnetic sorbents for polycyclic aromatic hydrocarbons (PAHs) remediation. An 8.1% MAC amendment (w/w, equal to 5% AC content) was found to be as effective as 5% (w/w) pristine AC in reducing aqueous PAHs within three months by 98%. MAC recovery from sediment after three months was 77%, and incomplete MAC recovery had both, positive and negative effects. A slight rebound of aqueous PAH concentrations was observed following the MAC recovery, but aqueous PAH concentrations then dropped again after six months, likely due to the presence of the 23% unrecovered MAC. On the other hand, the 77% recovery of the 8.1% MAC dose was insufficient to reduce ecotoxic effects of fine grained AC or MAC amendment on the egestion rate, growth and reproduction of the AC sensitive species Lumbriculus variegatus.
Applying
activated carbon (AC) to contaminated sediments is an
in-situ approach to remediation with great potential. The bioavailability
of persistent organic pollutants can be rapidly reduced and kept low
over long periods of time. However, there are limitations to the method.
The high buoyancy of AC particles makes their application difficult
in the field, and AC retention on the amended site can be low in turbulent
waters. Furthermore, the fine particles of powdered AC (PAC) can have
adverse effects on organisms, but their remediation potential is superior
to coarser, granular ACs (GAC). To tackle these shortcomings, a novel
sorbent material was developed, consisting of PAC embedded into a
stable, granular clay–matrix, significantly reducing buoyancy.
These AC–clay granules (ACC-G) were tested for remediation
potential (PCB-bioaccumulation reduction) and adverse effects on the
benthic invertebrates
Chironomus riparius
and
Lumbriculus variegatus
. The novel ACC-G material was compared
to GAC of the same particle size, the clay–matrix, and PAC.
The findings show that ACC-G has a significantly higher remediation
potential than GAC, allowing for reductions in PCB-bioaccumulation
of up to 89%. Adverse effects could not be totally eliminated with
ACC-G, but they were less severe than with PAC, likely due to the
increased particle size.
The in situ remediation of aquatic sediments with
activated carbon (AC)-based thin layer capping is a promising alternative
to traditional methods, such as sediment dredging. Applying a strong
sorbent like AC directly to the sediment can greatly reduce the bioavailability
of organic pollutants. To evaluate the method under realistic field
conditions, a 300 m2 plot in the PCB-contaminated Lake
Kernaalanjärvi, Finland, was amended with an AC cap (1.6 kgAC/m2). The study lake showed highly dynamic sediment movements
over the monitoring period of 14 months. This led to poor retention
and rapid burial of the AC cap under a layer of contaminated sediment
from adjacent sites. As a result, the measured impact of the AC amendment
was low: Both the benthic community structure and PCB bioaccumulation
were similar on the plot and in surrounding reference sites. Corresponding
follow-up laboratory studies using Lumbriculus variegatus and Chironomus riparius showed that
long-term remediation success is possible, even when an AC cap is
covered with contaminated sediment. To retain a measurable effectiveness
(reduction in contaminant bioaccumulation), a sufficient intensity
and depth of bioturbation is required. On the other hand, the magnitude
of the adverse effect induced by AC correlated positively with the
measured remediation success.
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