Franz-Hubert Haegel

The knowledge of the structural and chemical properties of biochars is decisive for their application as technical products. For this reason, methods for the characterization of biochars that are generally applicable and allow quality control are highly desired. Several methods that have shown potential in other studies were used to investigate two activated carbons and seven biochars from different processes and feedstock. The chars were chosen to cover a wide range of chemical composition and structural properties as a hardness test for the analytical methods used in this study. Specific problems connected with the pretreatment of samples and drawbacks of some methods for some types of chars could be identified in an integrated consideration of the results from different methods. None of the spectroscopic methods was found to be suitable for the quality control of all types of chars. The most valuable results were obtained by chemical analysis that, however, required the complete determination of the main elements, including that of oxygen, and of inorganic components for adequate results. The combination of X-ray photoelectron spectroscopy (XPS) and FT-IR spectroscopy allows a rough characterization of surface functional groups, but cannot discriminate aliphatic and aromatic OH groups. FT-IR might be a suitable method for the quality control of biochars made at lower temperature. The results of Raman spectroscopy did not well correlate with the amount of sp² hybridized carbon determined by XPS. A better correlation of XPS data was found with the electrical polarization determined by the method of spectral induced polarization that was used for the first time in conjunction with extensive analytical characterization.

Core Ideas: Water retention and spectral induced polarization were measured for biochar in sand. Water retention curves could be fitted by using a dual porosity model. Residual water content was found to increase with the addition of biochar. The polarization of sand–biochar mixtures was much larger than that for sand. Spectral induced polarization may be suitable for monitoring biochar in the field. Biochar is considered a promising soil amendment, but an effective method to detect and characterize the spatial distribution and temporal dynamics of biochar in soil is still missing. The aim of this study is to investigate the ability of the spectral induced polarization (SIP) method for the noninvasive detection of biochar in unsaturated sandy media. In particular, a pure sand and two sand–biochar mixtures with 1 and 2% biochar made from pine ( Pinus spp.) wood by pyrolysis at 800°C were investigated. The measured SIP spectra as a function of saturation were interpreted by fitting a Cole–Cole model to the low‐frequency part of the SIP measurements. The porous nature of the biochar particles strongly affected the SIP response of the partially saturated sand–biochar mixtures. Due to the high residual water content of the biochar in a dry background, the relationship between bulk electrical conductivity and water saturation was nonlinear in a log–log representation. This nonlinear behavior could adequately be explained with a dielectric mixing model that considered the drainage of the biochar particles. Both the measured phase and chargeability of the sand–biochar mixtures showed a complex dependence on water saturation. This was attributed to the decrease in polarization strength of the biochar particles with desaturation and the simultaneous increase in phase of the sand background. Overall, the results of this study suggest that field SIP measurements may be a promising tool for the characterization and monitoring of biochar amendments to agricultural soils.

In current soil remediation processes, the contaminants are usually removed by abrasion from soil particles using mechanical energy and water as washing medium. In this manner, a fine-particle pollutant-enriched fraction is always formed during the washing process that cannot be remediated by present techniques. In particular, organic contaminants with very low water solubilities, like polycyclic aromatic hydrocarbons (PAH), are concentrated in this soil fraction.

The use of biochar as a soil amendment attracts increasing research interest. However, the lack of methods to detect and monitor biochar in situ limits the validation of the field‐scale application of biochar. Spectral induced polarization is a potential tool to characterise biochar in soil. The aim of this study is to investigate the sensitivity of spectral induced polarization to biochar in sand and to understand how the physicochemical properties of both the biochar and the surrounding matrix influence the spectral induced polarization response. To this end, spectral induced polarization measurements were conducted on four types of biochar with different mass fractions disseminated in saturated sand as a host media with changing electrical conductivity. In addition, it was investigated how the spectral induced polarization response depends on the particle size of biochar. The measured SIP data were interpreted by Debye decomposition to obtain values for the peak relaxation time, ; total chargeability, ; and normalised total chargeability, . Spectral induced polarization showed a clear and specifically differentiated response to the presence of all four types of biochars. was found to be proportional to the mass fraction of biochars, although relationships varied for each type of biochars. of biochars increased with increasing particle size. Increased electrolyte concentration enhanced for all biochars, although again, the specific response was different for each biochar. In addition, higher electrolyte concentrations decreased for biochars derived from wood through pyrolysis but did not affect of biochar derived from miscanthus through hydrothermal carbonisation. It was concluded that the spectral induced polarization response of pyrolytic biochars resembled that of conductors or semiconductors, whereas the spectral induced polarization response of hydrothermal carbonisation biochar more closely resembled that of clay. Overall, the findings in this study suggest that spectral induced polarization is a promising method for the detection and characterisation of biochar in soil.

The influence of additives (alkyl polyglucoside, Glucopon 600 CS UP and alcohol ethoxylate C18E100) on the behavior of the water/toluene/Lutensol ON 50 (technical oxoalcohol, i-C10E5) microemulsion system as a function of temperature and composition has been investigated. The phase behavior of the microemulsions was determined by vertical sections through the Gibbs phase prism (fish-like phase diagrams). Alkyl polyglucoside shifts the one phase region to lower temperatures compared with water/toluene/Lutensol ON 50 mixtures. This is contrary to the expectation, considering the extreme hydrophilic nature of the sugar headgroup. The addition of hydrophilic alcohol ethoxylate (C18E100) to the water/toluene/Lutensol ON 50 system increases the solubilization capacity of the surfactant, even if the co-surfactant is used in small quantities, and shifts the one-phase region to higher temperature by a few degrees Celsius.

The concept of specific polarizability cp, being the ratio between imaginary conductivity and specific surface area, can be used to represent the polarization of the mineral-fluid interface per unit pore-volume-normalized surface area Spor and to account for the control of the fluid chemistry and/or mineralogy on induced polarization (IP) measurements. We used a database of IP measurements on sands and sand-clay mixtures to investigate the variation in cp as a function of clay content and/or mineralogy. We found an apparent variation in cp between sands and sand-clay mixtures when Spor was calculated using the nitrogen adsorption (Brunauer-Emmett-Teller — BET) method, with clays having an apparently higher cp than sands. However, this variation was considerably reduced when Spor was calculated using a wet-state methylene blue (MB) method that also sensed the surface area associated with internal layers of clay minerals inaccessible with the dry-state BET method. Furthermore, the imaginary conductivity was significantly better correlated with Spor determined from the MB method relative to Spor determined from the BET method. We found no evidence for a strong difference in the specific polarizability of quartz and clay minerals. This finding contradicted predictions from recent mechanistic formulations of the IP response of the Stern layer. Our findings have significant implications for improving and simplifying the interpretation of IP measurements in near-surface materials.

Microemulsion systems with water, toluene and nonionic surfactants are interesting for application due to the excellent solvent properties of aromatic hydrocarbons. In this study, the pseudo-binary phase diagram (?fish-diagram?) of such a ternary system was investigated using a branched technical alkyl polyethoxylate. Lutensol ON 50 (i-C10E5) was considered a suitable surfactant. The system with technical branched i-C10E5 exhibits very long and for compositions near optimum solubilization, i.e. the minimum content of surfactant needed for a bicontinuous microemulsion, extremely long times for equilibration. In addition to visual observation, qualitative measurements of turbidity were performed with a UV-Vis spectrometer for characterizing the behavior of this unusually sluggish system. Isothermal phase diagrams at 20, 25 and 30?C yielded inconsistent results after the change of temperature and the application of mechanical stress for different treatment of the samples. Ternary mixtures of water, toluene, and Lutensol ON 50 seem to form long-living metastable states. The sluggishness and the ambiguous phase behavior of the system are discussed. Inconsistent results are attributed to the slow kinetics of the formation and destruction of liquid crystals and the ability of the system to form miniemulsions within an unusual concentration range.

Over the last 15 years significant advancements in induced polarization (IP) research have taken place, particularly with respect to spectral IP (SIP), concerning the understanding of the mechanisms of the IP phenomenon, the conduction of accurate and broadband laboratory measurements, the modelling and inversion of IP data for imaging purposes and the increasing application of the method in near‐surface investigations. We summarize here the current state of the science of the SIP method for near‐surface applications and describe which aspects still represent open issues and should be the focus of future research efforts. Significant progress has been made over the last decade in the understanding of the microscopic mechanisms of IP; however, integrated mechanistic models involving different possible polarization processes at the grain/pore scale are still lacking. A prerequisite for the advances in the mechanistic understanding of IP was the development of improved laboratory instrumentation, which has led to a continuously growing data base of SIP measurements on various soil and rock samples. We summarize the experience of numerous experimental studies by formulating key recommendations for reliable SIP laboratory measurements. To make use of the established theoretical and empirical relationships between SIP characteristics and target petrophysical properties at the field scale, sophisticated forward modelling and inversion algorithms are needed. Considerable progress has also been made in this field, in particular with the development of complex resistivity algorithms allowing the modelling and inversion of IP data in the frequency domain. The ultimate goal for the future are algorithms and codes for the integral inversion of 3D, time‐lapse and multi‐frequency IP data, which defines a 5D inversion problem involving the dimensions space (for imaging), time (for monitoring) and frequency (for spectroscopy). We also offer guidelines for reliable and accurate measurements of IP spectra, which are essential for improved understanding of IP mechanisms and their links to physical, chemical and biological properties of interest. We believe that the SIP method offers potential for subsurface structure and process characterization, in particular in hydrogeophysical and biogeophysical studies.

Biochar is currently of emerging interest, because it is a by‐product of the increasing energetic use of organic waste and energy crops via pyrolysis. It is discussed as a modifier for soil to improve fertility and store carbon for long‐term. The benefit of carbon for fertility is well known from terra preta (anthropogenic “black soil”) in Amazonia since centuries, but many questions are still open, e.g. concerning the influence of charcoal on the hydrology in different soils. The large variety of feed material and pyrolysis techniques may result in different effects concerning the properties of soil. Spectral induced polarization (SIP) was considered as a method which can contribute to answer some of the questions. Therefore, a series of studies was performed on mixtures of 4 different biochars and commercial active carbon with sand. The results obtained provide additional information on the characterization of different types of charcoal and on the fertilizing effect of biochar. Considerable differences were found for biochars depending on the pyrolysis technique for both the real and the imaginary part of complex conductivity. The time dependence of the release of ions from the carbonaceous material could also be monitored. Based on the results of SIP, a 2D experiment was performed with electrical impedance tomography (EIT) which proved the suitability of the method to locate biochar and active carbon in sand. Since EIT can be applied at different scales, it has high potential for studying the influence of biochar on the hydraulic properties of soil and on its fertilizing effect in the laboratory and in the field.