Field experiments on remediation of coastal sediments using granulated coal ash

Effective capping of dissolved sulfide generated in Ulva prolifera-rich marine sediments by iron-rich red soil

Bloom-induced macroalgal enrichment on the seafloor can substantially facilitate dissolved sulfide (DS) production through sulfate reduction. The reaction of DS with sedimentary reactive iron (Fe) is the main mechanism of DS consumption, which however usually could not effectively prevent DS accumulation caused by pulsed macroalgal enrichment. Here we used incubations to investigate the performance of Fe-rich red soil for buffering of DS produced from macroalgae (Ulva prolifera)-enriched sediment. Based on our results, a combination of red soil additions (6.8 kg/m2) before and immediately after pulsed macroalgal deposition (455 g/m2) can effectively cap DS within the red soil layer. The effective DS buffering is mainly due to ample Fe-oxide surface sites available for reaction with DS. Only a small loss (4 %) of buffering capacity after 18-d incubation suggests that the red soil is capable of prolonged DS buffering in macroalgae-enriched sediments.

Effective abatement of ammonium and nitrate release from sediments by biochar coverage

Inorganic forms of N from sediments and runoff water, among others, remain some of the key sources of pollution of water bodies. However, the release of NH4+-N from sediment to water can be effectively reduced by biochar coverage due to high adsorption capacity, unlike NO3-N, where biochar has a low affinity. The feasibility of biochar coverage to abate NO3--N release needs to be evaluated. This study collected four sediments from Lake Taihu (China). Three types of biochar pyrolyzed from ordinary wastes, coconut shell (coBC), algal and excess sludge, were prepared to cover them and were incubated for 90 days. Results showed that the terminal total nitrogen (TN) and NO3--N concentrations decreased from 5.35 to 2.31-3.04 mg/L, 3.05 to 0.34-1.11 mg/L, respectively. CoBC coverage showed the best performance for reducing NO3--N release flux from 26.99 ± 0.19 to 9.30 ± 0.02 mg/m2·d (63.6 %). Potential denitrifiers, such as Flavobacterium and Exiguobacterium, were enriched in the biochar-coverage layer, and the absolute abundance of N-related functional genes (narG, nirS, nosZ and anammox) was increased by 1.76-4.21 times (p < 0.05). Jar tests by 15N isotope labeling further indicated that biochar addition increased the denitrification and anammox rates by 53.5-83.4 %. Experiments combining exogenous organic‑carbon addition and 15N labeling demonstrated that biochar's key role was regulating organic matter's bioavailability. Analysis with partial least square path modeling (PLS-PM) implied biochar with higher adsorption enhanced the denitrification and anammox processes in sediments via modifying the niche with suitable DOC, TN, and pH. This study suggested that biochar coverage could effectively abate NO3--N release from sediments by affecting the denitrification and anammox processes.

Effect of sediment deposition on phosphate and hydrogen sulfide removal by granulated coal ash in coastal sediments

Granulated coal ash (GCA) is a strong in-situ capping material for removing PO₄-P and H₂S-S in contaminated coastal sediments. Although GCA performance is weakened by sediment deposition, related research is rare. To evaluate sediment deposition effects on PO₄-P and H₂S-S removal by GCA, GCA was placed on the top of sediment (C-GCA), was partially mixed with sediment (M-GCA), and was fully covered by sediment (N-GCA). Effective PO₄-P and H₂S-S removal from sediments occurred in the order of C-GCA > M-GCA > N-GCA. C-GCA and M-GCA significantly decreased PO₄-P and H₂S-S concentrations by 84- 90% and 100%, respectively, through calcium phosphate and iron sulfide precipitation. N-GCA was less effective in PO₄-P and H₂S-S removal than the control after 2.5 months, as fine sediment particles blocked the GCA pores, decreasing calcium and iron elution. The results provide a better understanding of how sediment deposition negatively impacted GCA performance.

Feasibility of Remediation of Heavy-Metal-Contaminated Marine Dredged Sediments by Active Capping with Enteromorpha Biochar

Enteromorpha biochar (BC) has been proposed as a potential absorbent in the marine environments. This study attempts to understand the process of active capping using Enteromorpha BC to prevent the release of heavy metals (Pb and Cd) from contaminated marine dredged sediments. The capping efficiency was assessed with a series of lab-scale column experiments. Results showed that the Enteromorpha BC exhibits rough pore structure and higher specific surface area, as well as more surface organic functional groups, which is favorable for its adsorption capacity and selectivity towards heavy metals. The capping thickness of 2 cm for Enteromorpha BC was sufficient to prevent the release of heavy metals from sediments, with the capping efficiency of 47% for Pb and 62% for Cd. Kinetic studies showed that heavy metals released into the overlying water can be described by a three-parameter sigmoidal kinetic model. Importantly, the fractions of heavy metals in the dredged sediments below the capping layer were analyzed to reveal the capping remediation mechanism. The outcomes of the present study indicate that capping with Enteromorpha BC is a promising method to regulate the water environment by preventing the release of heavy metals from the contaminated dredged sediments.

Utilizing a granulated coal bottom ash and oyster shells for nutrient removal in eutrophic sediments

Various in-situ capping materials have been studied to remediate contaminated sediments for sustaining a healthy ecosystem in a coastal area. We developed Granulated coal bottom ash and oyster shells (GBO) with different mixing ratios of OS. Pyrolyzed and grounded coal bottom ash and oyster shells were used to produce GBO, which the main chemical elements were analogous to cement. The nutrient-removal abilities of GBO were evaluated through long-term mesocosm experiments. It was found that GBO was an effective in-situ capping material for remediation of eutrophic coastal sediments, decreasing PO₄-P and SiO₂-Si concentrations in pore water by 88.4% and 56.5%, respectively. The most efficient mixing ratio of coal bottom ash and oyster shells was at a weight ratio of 1:1 for PO₄-P and SiO₂-Si removal.

Effect of Bacillus subtilis Zeolite Used for Sediment Remediation on Sulfide, Phosphate, and Nitrogen Control in a Microcosm

Eutrophication is an emerging worldwide issue concerning the excessive accumulation of various pollutants in sediments, owing to the release of industrial or household wastewaters to coastal areas. The coastal sediment of Goseong Bay in the Republic of Korea is organically enriched with pollutants, including heavy metals, sulfide, phosphate, and ammonia. Microbial remediation and capping techniques have been suggested as effective routes for sediment remediation. In this study, Bacillus subtilis zeolite (BZ) was used as a sediment capping material, and effective remediation of coastal sediment was observed in a 40-day laboratory microcosm experiment. A significant decrease in the sediment water content and reduced concentration of acid volatile sulfide were observed in the BZ-capped sediment. In the overlying water and pore water, significant decreases in phosphate and dissolved inorganic nitrogen (DIN; NO₂-N + NO₃-N and NH₄-N) concentrations were observed in the BZ-treated experiment. Based on our findings, we conclude that BZ could be an effective capping material for coastal sediment remediation.

Application of iron [Fe(0)]-rich substrate as a novel capping material for efficient simultaneous remediation of contaminated sediments and the overlying water body

Release of contaminants from sediments has been one of the main pollution sources causing eutrophication and malodorous black of ponds. In this study, an iron-rich substrate (IRS) was developed based on iron‑carbon micro-electrolysis and applied for simultaneous sediments and overlying water remediation. IRS obtained high ammonia and phosphate adsorption capacities (Langmuir isotherm) of 13.02 and 18.12 mg·kg^-1, respectively. In the 90-day long-term remediation, IRS reduced NH₄⁺-N, PO₄^3--P, organic-N, organic-P, TN and TP in overlying water by 48.6%, 97.9%, 34.2%, 67.1%, 53.2% and 90.4%, respectively. In sediments, IRS reduced NO₃^--N, NH₄⁺-N and organic-N by 98.5%, 26.5% and 6.3%, respectively. The unstable P-compounds (i.e., organic-P, Ca-bounded-P and labile-P) were effectively transferred (20.1%, 54.3% and 98.2%, respectively) into inert P-compounds (i.e., Fe-bounded-P and residual-P). Meanwhile, flux rates of nitrogen and phosphorus from sediments to overlying water were reduced from 7.02 to 4.92 mg·m^-2·d^-1 (by 29.9%) and from 7.42 to 2.21 mg·m^-2·d^-1 (by 70.2%), respectively. Due to micro-electrolysis, Fe²⁺/Fe³⁺/[H] were in-situ generated from IRS and NO₃^--N was effectively reduced. Additionally, the generation of O₂^· was promoted by Fe²⁺/[H] and strengthened the NH₄⁺-N, organic-N/P oxidation. Fe³⁺ enhanced the immobilization of PO₄^3- (e.g., as FePO₄·H₂O and Fe_nPO₄(OH)_3n-3). The released Fe²⁺/Fe³⁺ from IRS were finally stabilized as poorly reactive sheet silicate (PRS)-Fe and magnetite-Fe in the sediments and hardly showed side effect to sediments and water body. The developed IRS obtained advantages of high efficiency, ecologically safe and cost-effective in contaminated sediments and overlying water remediation.

Using biochar capping to reduce nitrogen release from sediments in eutrophic lakes

The effects of reduced nitrogen release from sediments were studied using biochar (BC) capping in simulated water-sediment systems. Dried solid waste of Phyllostachys pubescens was used to produce BC, which was then pyrolyzed at 500 °C. Subsequently, 14 sediment cores were collected, including the sediment-water interface and some overlying water, from two sites in Baiyangdian Lake (China). The sediment cores were split into two batches (A and B), and then two each were capped with soil, BC or a BC/soil mixture, and incubated for 30 days. In the BC capped cores, the ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N) and total nitrogen (TN) concentrations decreased from 0.90 mg·L^-1 to 0.05 mg·L^-1, 0.88 mg·L^-1 to 0.18 mg·L^-1, 6.93 mg·L^-1 to 2.81 mg·L^-1, respectively, in batch A and 3.51 mg·L^-1 to 0.11 mg·L^-1, 0.92 mg·L^-1 to 0.61 mg·L^-1, 8.88 mg·L^-1 to 3.32 mg·L^-1, respectively, in batch B. The sediments to water fluxes of NH₄⁺-N, NO₃^--N and TN were greatly reduced or reversed. Compared with other cappings, the BC layer was shown to absorb more NH₄⁺-N from the pore water, thereby breaking the diffusion gradient of NH₄⁺-N at the sediment-water interface, and has a good inhibitory effect on the endogenous release of NH₄⁺-N from the sediments. Additionally, in the BC capped cores, the redox potential remarkably increased and dissolved oxygen was comparatively high. This study suggests that BC capping can reduce the amount of nitrogen released from polluted sediments because the diffusion of nitrogen to the overlying water is chemically blocked by the cap.

Relaxing the formation of hypoxic bottom water with sediment microbial fuel cells

The method of improving bottom water environment using industrial wastes to suppress diffusion substances from bottom sediment has recently captured the attention of many researchers. In this study, wastewater discharge-derived sediment was used to examine an alternative approach involving the use of sediment microbial fuel cells (SMFCs) in relaxing the formation of hypoxic bottom water, and removing reduced substances from sediment. Concentrations of dissolved oxygen (DO) and other ions were measured in overlying water and sediment pore water with and without the application of SMFCs. The results suggest that SMFCs can markedly reduce hydrogen sulfide and manganese ion concentrations in overlying water, and decrease the depletions of redox potential and DO concentration. In addition, SMFCs can dissolve ferric compounds in the sediment and thereby release the ferric ion available to fix phosphate in the sediment. Our results indicate that SMFCs can be used as an alternative method to relax the formation of hypoxic bottom water and to remove reduced substances from the sediment, thus improving the quality of both water and sediment environments.

Numerical evaluation of the use of granulated coal ash to reduce an oxygen-deficient water mass

Granulated coal ash (GCA), which is a by-product of coal thermal electric power stations, effectively decreases phosphate and hydrogen sulfide (H2S) concentrations in the pore water of coastal marine sediments. In this study, we developed a pelagic-benthic coupled ecosystem model to evaluate the effectiveness of GCA for diminishing the oxygen-deficient water mass formed in coastal bottom water of Hiroshima Bay in Japan. Numerical experiments revealed the application of GCA was effective for reducing the oxygen-deficient water masses, showing alleviation of the DO depletion in summer increased by 0.4-3mgl(-1). The effect of H2S adsorption onto the GCA lasted for 5.25years in the case in which GCA was mixed with the sediment in a volume ratio of 1:1. The application of this new GCA-based environmental restoration technique could also make a substantial contribution to form a recycling-oriented society.

Estimation of hydrogen sulfide removal efficiency with granulated coal ash applied to eutrophic marine sediment using a simplified simulation model

Hydrogen sulfide generated in eutrophic marine sediment is harmful for living organisms. It is therefore necessary to remove hydrogen sulfide from the sediment to restore benthic ecosystems. Previous studies revealed that granulated coal ash, which is a by-product of coal thermal electric power stations, could remove and oxidize hydrogen sulfide. In this study, we propose a simplified simulation model to estimate the hydrogen sulfide removal efficiency of granulated coal ash. Hydrogen sulfide concentrations in eutrophic marine sediment pore water with and without the application of granulated coal ash were calculated by the proposed model, and the outputs were compared with semi-field or field observation data. The model outputs reproduced the observed data well. Using the proposed model outputs, we suggest an optimum application dosage of granulated coal ash for remediating eutrophic marine sediment.