Effect of changes in the physicochemical properties of sand-alternatives on bacterial community structure in coastal sediments

- Ca elution from SA (sea-sand alternatives) in sea sediment was stabilized in 6 months.
- Effects of chemicals eluted from the SAs were negligible for bacterial community.
- The physicochemical properties of the SAs affected bacterial community.
- Bacterial communities become almost the same as in natural sediment after 1 year.

Steel slag and its granulated agglomerates, and the granulated agglomerates of ash from thermal power stations are reserved for and partially used in the construction of sand-alternatives (SAs) in coastal restoration and creation projects. These SAs contain CaO and leach calcium and alkalis that may affect biological activity of microorganisms colonizing surrounding sediments. We evaluated the chemical behavior of four SAs, namely, decarburized slag (DCSlag), dephosphorized slag (DPSlag), granulated agglomerates of DCSlag (GSlag), and granulated agglomerates of coal fly ash (GAsh) in coastal sediment and the impact of these SAs on bacterial community structure in the sediment. The SAs eluted calcium with a decrease in the surface calcium content in the sediment. The elution from slag and slag-based SA (DCSlag, DPSlag, and GSlag) continued for 8 weeks, whereas the elution from GAsh continued for 24 weeks. The different elution periods are explained by the depth of eluted calcium found in the SAs; 0.19 ± 0.07, 0.04 ± 0.02, 0.27 ± 0.12 and 2.4 ± 0.97 mm in DCSlag, DPSlag, GSlag and GAsh, respectively. After 1 year, the DGGE (Denaturing Gradient Gel Electrophoresis) profiles were different among all the SAs except for GSlag and GAsh. The taxon richness of bacterial communities was estimated by the number of DNA bands found in the sediment mixed with the SAs, and it was lower than that in the natural sediment. Physicochemical properties of the SA's have been shown to affect the bacterial community structure. On the other hand, the communities recovered to almost the same level as that in the natural sediment after 1 year. The long-term effects of chemicals eluted from the SAs on bacterial community structure would be limited or negligible.