Leaching of hazardous substances from a composite construction product--an experimental and modelling approach for fibre-cement sheets

Applicability of machine learning models for the assessment of long-term pollutant leaching from solid waste materials

Column leaching tests are a common approach for evaluating the leaching behavior of contaminated soil and waste materials, which are often reused for various construction purposes. Standardized up-flow column leaching tests typically require about 7 days of laboratory work to evaluate long-term leaching behavior accurately. To reduce testing time, we developed linear and ensemble models based on parametric and non-parametric Machine Learning (ML) techniques. These models predict leachate concentrations of relevant chemical compounds at different Liquid-to-Solid ratios (LS) based on measurements at lower LS values. The ML models were trained using 82 column leaching test samples for Construction and Demolition Waste materials collected in Germany during the last two decades. R-Squared values measuring models' performance are as follows: Sulfate = 0.94, Vanadium = 0.97, Chromium = 0.82, Copper = 0.92, group of 15 (US-EPA) PAHs = 0.98 (values averaged over predictive models for LS 2 and 4). Sensitivity analysis utilizing the Shapley Additive Explanation value indicates that in addition to the concentrations of the considered compound at LS<=1, electrical conductivity and pH are the most critical features of each model, while concentrations of other compounds also play a minor role.

Environmental Impact of Construction Products on Aquatic Systems—Principles of an Integrated Source–Path–Target Concept

Buildings exposed to water can release undesirable substances which, once transported to environmental compartments, may cause unwanted effects. These exposure pathways need to be investigated and included in risk assessments to safeguard water quality and promote the sustainability of construction materials. The applied materials, exposure conditions, distribution routes and resilience of receiving compartments vary considerably. This demonstrates the need for a consistent concept that integrates knowledge of emission sources, leaching processes, transport pathways, and effects on targets. Such a consistent concept can serve as the basis for environmental risk assessment for several scenarios using experimentally determined emissions. Typically, a source–path–target concept integrates data from standardized leaching tests and models to describe leaching processes, the distribution of substances in the environment and the occurrence of substances at different points of compliance. This article presents an integrated concept for assessing the environmental impact of construction products on aquatic systems and unravels currently existing gaps and necessary actions. This manuscript outlines a source–path–target concept applicable to a large variety of construction products. It is intended to highlight key elements of a holistic evaluation concept that could assist authorities in developing procedures for environmental risk assessments and mitigation measures and identifying knowledge gaps.

Leaching of Carbon Reinforced Concrete-Part 1: Experimental Investigations

The composite material ‘carbon concrete composite (C³)’ is currently capturing the building sector as an ‘innovative’ and ‘sustainable’ alternative to steel reinforced concrete. In this work, its environmental compatibility was investigated. The focus of this research was the leaching behavior of C³, especially for the application as irrigated façade elements. Laboratory and outdoor exposure tests were run to determine and assess the heavy metal and trace element emissions. In the wake of this work, the validity of laboratory experiments and the transferability to outdoor behavior were investigated. The experimental results show very low releases of environmental harmful substances from carbon concrete composite. Most heavy metal concentrations were in the range of <0.1–8 µg/L, and higher concentrations (up to 32 µg/L) were found for barium, chromium, and copper. Vanadium and zinc concentrations were in the range of 0.1–60 µg/L, boron and nickel concentrations were clearly exceeding 100 µg/L. Most of the high concentrations were found to be a result of the rainfall background concentrations. The material C³ is therefore considered to be environmentally friendly. There is no general correlation between laboratory leaching data and outdoor emissions. The results depend on the examined substance and used method. The prediction and evaluation of the leaching of building elements submitted to rain is therefore challenging. This topic is debated in the second part of this publication.

A mathematical approach for the analysis of data obtained from the monitoring of biocides leached from treated materials exposed to outdoor conditions

Leaching processes are responsible for the release of biocides from treated materials into the environment. Adequate modeling of emission processes is required in order to predict emission values in the framework of the risk assessment of biocidal products intended for long-term service life. Regression approaches have been applied to data obtained from the long-term monitoring of biocide emissions in experiments involving semi-field conditions. Due to the complex interaction of different underlying mechanisms such as water and biocide diffusion and desorption, however, these attempts have proven to be of limited usefulness - at least, for the available biocide emission data. It seems that the behavior of the biocide emission curve depends to a considerable extent on whichever underlying mechanism is slowest at a given point in time, thus limiting the amount of biocide available for release. Building on results obtained in the past few years, the authors propose a criterion for determining which mechanism controls the leaching process at a given point in time based on the slope of the log-log emission curve. In addition, a first-order approximation of this slope value is presented which displays advantages both in terms of computability and interpretability. Finally, an algorithm for the determination of breakpoints in the slope of the log-log emission curve is presented for the demarcation of phases within which one mechanism acts as a limiting factor.

Reduction of undesirable element leaching from fly ash by adding hydroxylated calcined dolomite

Fly ash always contains many toxic elements which can be released into environment, thereby easily leading to environmental contaminations. In order to dispose fly ash safely, related strategies are needed. In this investigation, two kinds of hydroxylated calcined dolomites (HCD60 and HCD100) were used as the additives and compared with lime on the leachabilities of anionic species from fly ash. Both additives were found effective in reducing the leaching concentrations of these elements, which was better than that of only lime addition. Mg(OH)₂ and MgO were believed to play important roles in the hydration reaction of fly ash. In the presence of Mg(OH)₂ and MgO, there were more hydration products including calcium silicate hydrate, ettringite, hydrocalumite and other Layered double hydroxides (LDHs) generated which were effective candidates for anion removal. Thus, the final leaching results were controlled by these newly formed phases through adsorption, incorporation or encapsulation. On the other hand, compared with Mg(OH)₂, MgO can promote the formation of hydration products in a larger extent because of the hydration process of MgO into Mg(OH)₂. There was no systematic trend in the promotion of fly ash hydration by Mg(OH)₂ or MgO because it had a close relationship with the properties of original fly ash. Objectively, hydroxylated calcined dolomites can be promising candidate additives for reduction of toxic elements leaching from fly ash.

Impact of pre-equilibration and diffusion limited release kinetics on effluent concentration in column leaching tests: Insights from numerical simulations

Column leaching tests have become a standard method for assessing leaching of pollutants from materials used, e.g., for road and railway constructions and in landscaping measures. Column tests showed to be practical in laboratories yielding robust and reproducible results. However, considerable uncertainty still exists related particularly to the degree of equilibration of the pore water with the solids during preparation (pre-equilibration) and percolation of the column. We analyse equilibration time scales and sensitivity of concentrations in column leachate with respect to initial conditions in a series of numerical experiments covering a broad spectrum of material and solute properties. Slow release of pollutants from solid materials is described by a spherical diffusion model of kinetic sorption accounting for multiple grain size fractions and sorption capacities. Results show that the cumulative concentrations are rather independent of the pre-equilibration level for a broad spectrum of parameter settings, e.g. if intra-particle porosity is high, grain size is small, or if the sorption coefficient is large. Sensitivity increases with decreasing liquid solid ratios and contact time during percolation. Significant variations with initial column conditions are to be expected for material and compound properties leading to slow release kinetics. In these cases, sensitivity to initial conditions may have to be considered.

Pervious concrete reactive barrier for removal of heavy metals from acid mine drainage - column study

This paper presents a column study conducted to investigate the potential use of pervious concrete as a reactive barrier for treatment of water impacted by mine waste. The study was done using acid mine drainage (AMD) collected from a gold mine (WZ) and a coalfield (TDB). Pervious concrete mixtures consisting of Portland cement CEM I 52.5R with or without 30% fly ash (FA) were prepared at a water-cementitious ratio of 0.27 then used to make cubes which were employed in the reactor columns. It was found that the removal efficiency levels of Al, Fe, Mn, Co and Ni were 75%, 98%, 99%, 94% and 95% for WZ; 87%, 96%, 99%, 98% and 90% for TDB, respectively. The high rate of acid reduction and metal removal by pervious concrete is attributed to dissolution of portlandite which is a typical constituent of concrete. The dominant reaction product in all four columns was gypsum, which also contributed to some removal of sulphate from AMD. Formation of gypsum, goethite, and Glauber's salt were identified. Precipitation of metal hydroxides seems to be the dominant metal removal mechanism. Use of pervious concrete offers a promising alternative treatment method for polluted or acidic mine water.

Protection of Reinforced Concrete Structures of Waste Water Treatment Reservoirs with Stainless Steel Coating Using Arc Thermal Spraying Technique in Acidified Water

Waste water treatment reservoirs are contaminated with many hazardous chemicals and acids. Reservoirs typically comprise concrete and reinforcement steel bars, and the main elements responsible for their deterioration are hazardous chemicals, acids, and ozone. Currently, a variety of techniques are being used to protect reservoirs from exposure to these elements. The most widely used techniques are stainless steel plating and polymeric coating. In this study, a technique known as arc thermal spraying was used. It is a more convenient and economical method for protecting both concrete and reinforcement steel bar from deterioration in waste water treatment reservoirs. In this study, 316L stainless steel coating was applied to a concrete surface, and different electrochemical experiments were performed to evaluate the performance of coatings in different acidic pH solutions. The coating generated from the arc thermal spraying process significantly protected the concrete surface from corrosion in acidic pH solutions, owing to the formation of a double layer capacitance—a mixture of Cr³⁺ enriched with Cr₂O₃ and Cr-hydroxide in inner and Fe³⁺ oxide on the outer layer of the coating. The formation of this passive film is defective owing to the non-homogeneous 316L stainless steel coating surface. In the pH 5 solution, the growth of a passive film is adequate due to the presence of un-dissociated water molecules in the aqueous sulfuric acid solution. The coated surface is sealed with alkyl epoxide, which acts as a barrier against the penetration of acidic solutions. This coating exhibits higher impedance values among the three studied acidic pH solutions.

PAHs in leachates from thermal power plant wastes and ash-based construction materials

The focus of the current study is to characterise the leaching behaviour of polycyclic aromatic hydrocarbons (PAHs) from oil shale ashes (OSAs) of pulverised firing (PF) and circulating fluidised-bed (CFB) boilers from Estonian Thermal Power Plant (Estonia) as well as from mortars and concrete based on OSAs. The target substances were 16 PAHs from the EPA priority pollutant list. OSA samples and OSA-based mortars were tested for leaching, according to European standard EN 12457-2 (2002). European standard CEN/TC 15862(2012) for monolithic matter was used for OSA-based concrete. Water extracts were analysed by GC-MS for the concentration of PAHs. Naphthalene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene and pyrene were detected. Still, the release of PAHs was below the threshold limit value for inert waste. The amount of the finest fraction (particle size <0.045 mm), the content of the Al-Si glass phase and the surface characteristics were the main factors, which could affect the accessibility of PAHs for leaching. The mobility of PAHs from OSA of CFB and PF boilers was 20.2 and 9.9%, respectively. Hardening of OSA-based materials did not lead to the immobilisation of soluble PAHs. Release of PAHs from the monolith samples did not exceed 0.5 μg/m(2). In terms of leaching of PAHs, OSA is safe to be used for construction purposes.