Sediment Ripening with Biochar

Abstract

The growing demand for sustainable construction materials has prompted interest in reusing dredged sediments as an alternative to traditional raw materials in dike construction. Before they can be reused, sediments must undergo the lengthy ripening process, during which they are transformed into stable soil. Accelerating this transformation could significantly improve the feasibility of sediment reuse. This study investigates whether biochar amendment can enhance the biophysicochemical ripening of dredged sediments, focusing on the influence of biochar application rate and particle size.

Dredged material from the port of Hamburg, Germany, that was dewatered and processed at the METHA plant, was mixed with biochar produced by Bio Energy Netherlands from the gasification of wood waste at 800-1000°C for 90-120 minutes. The mixtures contained biochar with varying application rates (2%, 4%, 6%) and particle sizes (<2 mm, 2-5 mm, >5 mm). Over the course of 15 weeks of field ripening, the sediment-biochar mixtures were exposed to natural weather conditions and turned weekly. Biochar amendment introduced additional porosity which increased water holding capacity by 33-72% compared to the control after 15 weeks of ripening, resulting in values of 24-72% DW. The oven-dried COLE, ranged from of 2.2 to 5.4% which represents a decrease of up to 54% relative to the unamended sediments. This improvement can be attributed to the non-plastic behavior of biochar and explains the decreasing shrinkage observed with an increasing application rate. Increasing particle size was correlated to decreasing shrinkage (p <0.05) which could be due to the interrupting effect of coarse biochar particles on tensile load propagation in the rods. A qualitative assessment of the structure development of the experimental variants suggests an acceleration of structure formation with higher biochar application rate and larger particle size when combined with weekly turning. This resulted in a faster breakdown of the dense and platy METHA material into smaller and more aerated aggregates. Overall, the physical ripening of the dredged material was improved with the addition of biochar at increasing application rates and particle size, which promoted a faster stabilization of sediment aggregates and enhanced physical properties beneficial for construction applications.

The occurrence of sulfur oxidation, the main chemical ripening reaction, was evidenced by a loss in the total sulfur content of samples and an increasing electrical conductivity during dry periods. The pH was expected to decrease as a result of the release of protons from this reaction, however this was not observed. Instead, increasing biochar application rates was correlated to a higher pH (p <0.05) and was evidence of the material's buffering capacity which can be attributed to its high functional group and mineral content. The total sulfur content reduced on average by 5% and 22% in the amended samples and the control, and this smaller decrease compared to the control could be explained either by a slower chemical ripening in amended sediments or by measurement limitations. Furthermore, the evolution of electrical conductivity over the 15 weeks of field ripening evidenced the accumulation of chemical reaction products in dry periods.

The influence of biochar on sediment physical and chemical properties, including the increased pore structure, water holding capacity, aeration and buffering capacity, all contributed to creating conditions favorable to microbial activity. A priming effect of biochar application could be observed in the first six weeks of ripening, with high respiration rates, high decomposition rates, and decreasing stabilization of organic matter. In this period, total organic carbon content decreased on average by 30\% in amended samples, compared to only 6% in the control. At the same time, nitrogen content decreased on average by 13% in the samples with biochar, further confirming the high microbial activity. This was followed by a period of decreasing microbial activity until the end of the experiment, which was marked by 14-32% lower respiratory carbon release of the amended samples compared to the control, decreasing decomposition rates and increasing stabilization of organic matter. Thus, biochar application accelerated the decomposition of labile carbon and enhanced the biological stabilization of organic matter in sediments.

These findings suggest that biochar amendment can significantly improve sediment ripening processes and can result in a material with properties desirable for dike construction.