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In situ amendments are a promising approach to enhance removal of metal contaminants from diverse environments including soil, groundwater and sediments. Apatite and chitin were selected and tested for copper, chromium, and zinc metal removal in marine sediment samples. Microbiological, molecular biological and chemical analyses were applied to investigate the role of these amendments in metal immobilization processes. Both apatite and chitin promoted microbial growth. These amendments induced corresponding bacterial groups including sulfide producers, iron reducers, and phosphate solubilizers; all that facilitated heavy metal immobilization and removal from marine sediments. Molecular biological approaches showed chitin greatly induced microbial population shifts in sediments and overlying water: chitin only, or chitin with apatite induced growth of bacterial groups such as Acidobacteria, Betaproteobacteria, Epsilonproteobacteria, Firmicutes, Planctomycetes, Rhodospirillaceae, Spirochaetes, and Verrucomicrobia; whereas these bacteria were not present in the control. Community structures were also altered under treatments with increase of relative abundance of Deltaproteobacteria and decrease of Actinobacteria, Alphaproteobacteria, and Nitrospirae. Many  of these groups of bacteria have been shown to be involved in metal reduction and immobilization. Chemical analysis  of pore and overlying water also demonstrated metal immobilization primarily under chitin treatments. X-Ray absorption spectroscopy (XAS) spectra showed more sorbed Zn occurred over time in both apatite and chitin treatments (from 9% - 27%). The amendments improved zinc immobilization in marine sediments that led to significant changes in   the mineralogy: easily mobile Zn hydroxide phase was converted to an immobile Zn phosphate (hopeite). In-situ amendment of apatite and chitin offers a great bioremediation potential for marine sediments contaminated with heavy metals.

Purpose This study characterized the chemical transport potential of polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPH) in the vicinity of a sand cap placed in the nearshore zone of a tidal marine embayment. Materials and methods Groundwater seepage was investigated along the perimeter and within the footprint of the sand cap, with results verifying the presence of significant freshwater upwelling shoreward of the sand cap boundary. The depth distribution of PAHs and TPH was assessed in sediment cores collected from within the cap area footprint.Results and discussion The depth distribution of PAHs and TPH demonstrated a spatial pattern of elevated chemical concentrations in the shoreward zone of the capped area, consistent with the spatial pattern of elevated freshwater flux. Visual inspection of recovered cores confirmed the presence of a fine-grained, low-conductivity sediment layer underlying the sand cap, with material properties of this layer potentially suggesting compaction following placement of the sand cap. This fine-grained sediment layer was not evident in the shoreward zone of the capped area.Conclusions The presence of the aquitard under the sand cap, coupled with the apparent erosion of this fine-grained layer in the higher energy shoreward zone, suggests the potential for enhanced groundwater seepage in the shoreward zone of the sand cap. It is hypothesized that enhanced groundwater flux is responsible for the elevated concentrations of PAH and TPH observed in core profiles collected from the zone characterized by elevated freshwater seepage and tidal pumping and that the fine-grained sediment layer that serves as an aquitard impedes groundwater flux within the cap area footprint. In effect, the absence of groundwater seepage observed for those stations within the footprint of the sand cap has likely resulted from compaction of the native sediment strata, whether or not compaction resulted directly from cap placement.