Application Of Hydrochar Research Articles

Surface heterogeneity mediated transport of hydrochar nanoparticles in heterogeneous porous media.

The effects of clay particles (montmorillonite, M) and phosphate (P) on the transport of hydrochar nanoparticles (NPs) in water-saturated porous media (uncoated and aluminum (Al) oxide-coated sands) were explored in NaCl (1-50 mM) solutions. Our results showed that the deposition behaviors of hydrochar NPs affected by M and phosphate were significantly different between pH 6.0 and pH 9.0, especially in Al oxide-coated sand. This can be attributed to their distinct surface characteristics: hydrochar agglomerates with a larger pore size distribution, more carboxylate groups, and less negative charges on the surface at pH 9.0 than those at pH 6.0. In Al oxide-coated sand, block adsorption of hydrochar was alleviated appreciably with the presence of M due to the preferential preoccupies of M on these favorable retention sites. On the contrary, M substantially increased the hydrochar retention on uncoated sand due to the formation of nanoaggregates between hydrochar and M. Differently, phosphate substantially enhanced the transport of hydrochar, even in coated sand, due to the strong phosphate adsorption onto Al oxide on the surface of sand and hydrochar. Our findings will provide useful insights into designing effective strategies for land application of hydrochar while minimizing potential environmental risks. Graphical abstract.

Hydrochar reduced NH3 volatilization from rice paddy soil: Microbial-aging rather than water-washing is recommended before application

Soil amendments (e.g., hydrochar) have recently gained increasing research interest because of their potential application in agricultural soils to reduce nutrient loss to the environment. In order to improve the surface properties of hydrochar, we herein used the microbial-aging approach to produce microbially-aged hydrochar (M-SHC), which was applied in rice paddy soil columns to investigate nitrogen transformation processes and gaseous nitrogen losses by NH3 volatilization. The aim of this research was mainly to explore and understand the mechanisms by which sawdust hydrochar (SHC), M-SHC, and water-washed hydrochar (W-SHC) influence NH3 volatilization in rice paddy fields. Compared with the control, M-SHC treatment considerably reduced both cumulative NH3 and yield-scale NH3 volatilization by 11.52% and 24.40% (p<0.05), respectively. This reduction could be attributed to a decrease in NH4+–N concentrations and an alteration of floodwater pH, following the application of M-SHC. The results showed that changes in floodwater pH could considerably affect the NH3 volatilization rate. When floodwater was slightly alkaline (pH > 7.25), even a minor alteration of pH following the application of hydrochar could increase NH3 volatilization rapidly. The application of hydrochar substantially inhibited the soil urease activity and increased the soil amoA gene abundance, which, in turn, decreased the floodwater NH4+–N concentrations. These results offer strong theoretical support for the application of hydrochar in agricultural systems to decrease NH3 volatilization in rice paddy fields.

The comparison of dissolved organic matter in hydrochars and biochars from pig manure

Dissolved organic matter (DOM) has an important effect on soil fertility, activity of microorganisms and transport of contaminants. In this study, DOM released by the hydrochar and biochar prepared under various conditions from pig manure, was assessed using a combination of UV–Visible spectroscopy, fluorescence excitation-emission (EEM) spectrophotometry and 1H-nuclear magnetic resonance (1H NMR). The dissolved organic carbon (DOC) extracted from the hydrochar and biochar ranged from 3.34–11.96% and 0.38–0.48%, respectively, and the highest DOM was released by HCK0.5 (180 °C and 0.5% KOH). The aliphatic compounds were most common in DOM which mainly included three humic acid-like and one protein-like substance. The hydrochar-DOM had a larger molecular weight and lower aromaticity than biochar-DOM, but the effect of temperature on the DOM characteristics of hydrochar and biochar was opposite. The acidic treatment increased the content of functional groups containing oxygen and nitrogen in hydrochar-DOM, and alkaline treatment increased the content of aliphatic compounds. The results obtained are beneficial to select carbonation process and guide the rational application of hydrochar and biochar from pig manure in soil remediation field.

Effect of Applying Hydrochar for Reduction of Ammonia Volatilization and Mechanisms in Paddy Soil

Hydrochar, as a product of the hydrothermal carbonization of biomass, has good application prospects for the NH3 volatilization reduction in rice fields due to its rich pore structure and functional surface. In this study, hydrochar was applied as a soil conditioner to paddy soil. A soil column experiment was conducted to investigate the effect of hydrochar on NH3 volatilization throughout the growth period of rice. The experiment was conducted with three treatments:CKU (control without hydrochar); SHC (sawdust hydrochar); and W-SHC (water-washed sawdust hydrochar). The application rate of SHC and W-SHC was 0.5% (w/w). The study investigated the effects of different hydrochars on the pH and concentrations of NH4+-N in floodwater, the flux and accumulation of NH3 volatilization, and the yield-scale cumulative emission of NH3 volatilization. Results show that the SHC treatment significantly reduces cumulative emissions of NH3 volatilization and the yield-scale cumulative emissions of NH3 volatilization (P<0.05), which were 32.42% and 47.61% lower than CKU, respectively. The effect of W-SHC on ammonia volatilization reduction was slightly weaker, as the cumulative emissions of NH3 volatilization and the yield-scale cumulative emissions of NH3 volatilization decreased by 10.14% and 27.71%, respectively, compared with CKU. The NH3 volatilization reduction was possibly related to the disturbance of pH and the decrease in NH4+-N concentrations in the floodwater because of the application of hydrochar. Compared with CKU, both SHC and W-SHC treatments reduced the pH and NH4+-N concentration in the floodwater. The impacts were more obvious in the rice base fertilizer period (BF) and the first supplemental fertilizer period (SF1) than in the second supplemental fertilizer period (SF2). The soil urease activity was significantly inhibited by hydrochar (P<0.05), and the abundance of soil ammonia-oxidizing gene (AOA, AOB) also significantly increased after application of SHC (P<0.05). This resulted in the enhanced efficiency of ammonia-oxidizing, which had an effect on the reduction of the NH4+-N concentrations in the floodwater. This study provides theoretical and experimental data support for the application of hydrochar in agro-environments with regard to ammonia volatilization reduction in paddy fields.

Application of Hydrochar Altered Soil Microbial Community Composition and the Molecular Structure of Native Soil Organic Carbon in a Paddy Soil.

The benefits and disadvantages of hydrochar incorporation into soil have been heavily researched. However, the effect of hydrochar application on the soil microbial communities and the molecular structure of native soil organic carbon (SOC) has not been thoroughly elucidated. This study conducted an incubation experiment at 25 °C for 135 days using a soil column with 0.5 and 1.5% hydrochar-amended paddy soil to explore the interconnections between changes in soil properties and microbial communities and shifts in native SOC structure using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) and NMR after hydrochar application. Hydrochar addition decreased the labile SOC fraction by 15.6-33.6% and increased the stable SOC fraction by 10.3-27.0%. These effects were significantly stronger for 1.5% hydrochar-treated soil. Additionally, hydrochar addition induced the native SOC with 1.0-3.0% more carbon and 6.0-13.0% higher molecular weight. The SOC in hydrochar-amended soil contained more aromatic compounds but fewer carbohydrates and lower polarity. This was resulted by a statistically significant reduction in Sphingobacterium, which was active in polycyclic aromatic hydrocarbon degradation, and an increase in Flavobacterium, Anaerolinea, Penicillium, and Acremonium, which were the efficient decomposers of labile SOC. These findings will help elucidate the potential influence of hydrochar on the carbon biogeochemical cycle in the soil.

Biowaste to treasure: Application of microbial-aged hydrochar in rice paddy could improve nitrogen use efficiency and rice grain free amino acids

Abstract Hydrochar is produced under hydrothermal carbonization, which can be considered as a cleaner process for black carbon production. Hydrochar shows good prospects in agro-environmental applications, but it is recommended to be ameliorated before application to improve its surface characteristics and reduce potential side-effects. This study, for the first time, proposed ameliorating poplar sawdust hydrochar via a microbial-aging process and applying it to rice paddy soil. In a soil-column experiment, sawdust hydrochar (SHC), microbial-aged SHC (M-SHC), and water-washed SHC (W-SHC) were added at different rates to examine their effects on rice growth and nitrogen usage efficiency (NUE). Additionally, the effects of hydrochar application on rice grain free amino acids (FAAs) were investigated. The study revealed that M-SHC had a positive effect on rice growth and increased the grain yield by 17.13%–22.99% compared with control. The NUE was also improved by M-SHC treatment. The positive effect of M-SHC on rice yield might be attributed to the reduced toxic biochar-derived dissolved organic matters, increased specific surface area, elevated hydrochar pH, and more nutrients released due to the microbial-aging process. Moreover, under M-SHC treatment, all 17 FAAs of grain were significantly higher than the control, by 46.26%. Both microbial-aging and water-washing amelioration of hydrochar had a positive effect on rice yield and FAAs, with the former exhibiting better prospects for application. This study will provide a new direction for cleaner recycling and utilization of biowaste.

Hydrochar-Amended Substrates for Production of Containerized Pine Tree Seedlings under Different Fertilization Regimes

There is a growing body of research that recognizes the potentials of biochar application in agricultural production systems. However, little is known about the effects of biochar, especially hydrochar, on production of containerized seedlings under nursery conditions. This study aimed to test the effects of hydrochar application on growth, quality, nutrient and heavy metal contents, and mycorrhizal association of containerized pine seedlings. The hydrochar used in this study was produced through hydrothermal carbonization of paper mill biosludge at 200 °C. Two forms of hydrochar (powder and pellet) were mixed with peat at ratios of 10% and 20% (v/v) under three levels of applied commercial fertilizer (nil, half and full rates). Application of hydrochar had positive or neutral effects on shoot biomass and stem diameter compared with control seedlings (without hydrochar) under tested fertilizer levels. Analysis of the natural logarithmic response ratios (LnRR) of quality index and nutrient and heavy metal uptake revealed that application of 20% (v/v) hydrochar powder or pellet with 50% fertilizer resulted in same quality pine seedlings with similar heavy metal (Cu, Ni, Pb, Zn and Cr) and nutrient (P, K, Ca and Mg) contents as untreated seedlings supplied with 100% fertilizer. Colonization percentage by ectomycorrhizae significantly increased when either forms of hydrochar were applied at a rate of 20% under unfertilized condition. The results of this study implied that application of proper rates of hydrochar from biosludge with adjusted levels of liquid fertilizer may reduce fertilizer requirements in pine nurseries.

Effect of Pyrochar and Hydrochar on Water Evaporation in Clayey Soil under Greenhouse Cultivation.

Greenhouse cultivation consumes large volumes of freshwater, and excessive irrigation induces environmental problems, such as nutrient leaching and secondary salinization. Pyrochar (biochar from high-temperature pyrolysis) is an effective soil amendment, and researches have shown that pyrochar application could maintain soil nutrient and enhance carbon sequestration. In addition to pyrochar from pyrolysis, hydrochar from hydrothermic carbonization is considered as a new type of biochar and has the advantages of low energy consumption and a high productive rate. However, the effect of these two biochars on water evaporation in clayey soils under a greenhouse system has seldom been studied. The relationship between water evaporation and biochar properties is still unknown. Thus, in the present study, water evaporation under pyrochar and hydrochar application were recorded. Results showed that both pyrochar and hydrochar application could inhibit water evaporation in clayey soil under greenhouse cultivation. Pyrochar showed a better inhibition effect compared with hydrochar. Correlation analysis indicated that the water evaporation rate was significantly positively correlated with bulk density of biochar (p < 0.05). Overall, application of pyrochar or hydrochar could both reduce soil bulk density and inhibit soil evaporation, and be available for greenhouse cultivation. However, the inhibition effect depends on the properties of the biochar.

Fate and distribution of nutrients and heavy metals during hydrothermal carbonization of sewage sludge with implication to land application

Hydrothermal carbonization has been considered effective to reduce sewage sludge volume and provide an opportunity to generate valuable byproducts for a wide range of potential applications. The fate and distribution of nutrients during hydrothermal carbonization of sewage sludge, along with that of heavy metals, is very important for realizing nutrient recovery and reuse by both hydrochar and liquid phase directly as soil amendments and/or organic fertilizer. In this study, we systematically investigated the migration and transformation of nutrients and heavy metals using chemical extractions and reaction severity (Ruyter model) methods at a temperature range of 120–300 °C for 30–180 min. With increasing reaction severity, carbon and nitrogen efficiency of hydrochar showed a liner and exponential decrease respectively, while phosphorus accumulated positively in hydrochar. Nitrogen species in hydrochar are mostly nitrogen-containing aromatic heterocycles whereas in liquid phase are predominantly ammonia-N and organic-N. The organic phosphorus in sewage sludge was transformed to inorganic species but non-apatite phosphorus showed an exponential reduction in hydrochar with increasing reaction severity, together with the decrease of orthophosphate in liquid phase. As for heavy metals, hydrothermal carbonization promoted decrease in direct bioavailable fractions (except arsenic) and increase in stable fraction (except cadmium and arsenic) in hydrochar as reaction severity elevated. Since hydrothermal effects led to redistribution of nutrients speciation and immobilization of heavy metals, non-farm land application of both hydrochar and liquid phase seems to be possible and unrestricted. This study provides fundamental knowledges for the construction of sludge management strategies to better nutrients recycling and reclamation.

Hydrothermal Carbonization of Various Paper Mill Sludges: An Observation of Solid Fuel Properties

Each year the pulp and paper industries generate enormous amounts of effluent treatment sludge. The sludge is made up of various fractions including primary, secondary, deinked, fiber rejects sludge, etc. The goal of this study was to evaluate the fuel properties of the hydrochars produced from various types of paper mill sludges (PMS) at 180 °C, 220 °C, and 260 °C. The hydrochars, as well as the raw feedstocks, were characterized by means of ultimate analysis, proximate analysis, moisture, ash, lignin, sugar, and higher heating value (HHVdaf) measurements. Finally, combustion indices of selected hydrochars were evaluated and compared with bituminous coal. The results showed that HHVdaf of hydrochar produced at 260 °C varied between 11.4 MJ/kg and 31.5 MJ/kg depending on the feedstock. This implies that the fuel application of hydrochar produced from PMS depends on the quality of feedstocks rather than the hydrothermal carbonization (HTC) temperature. The combustion indices also showed that when hydrochars are co-combusted with coal, they have similar combustion indices to that of coal alone. However, based on the energy and ash contents in the produced hydrochars, Primary and Secondary Sludge (PPS2) could be a viable option for co-combustion with coal in an existing coal-fired power plant.

Effects of temperature, time and acidity of hydrothermal carbonization on the hydrochar properties and nitrogen recovery from corn stover

Corn stover is hydrothermally carbonized at different temperatures (180 °C, 200 °C, 220 °C, 240 °C and 260 °C) for 4 h, for different residence times (1 h, 2 h, 4 h, 12 h and 24 h) at 220 °C, or by adding acid (1% and 2%) at 220 °C for 4 h. The yields, elemental analysis, functional group analysis, molecular composition and microstructure of those hydrochars are characterized, and the nitrogen recovery in the by-products is analysed to further understand the elemental transformation in the progress of hydrothermal carbonization. The results indicate that a higher temperature, longer residence time or acid addition can cause lower yields and simultaneously reduce the H/C (0.76–1.33) and O/C (0.19–0.68) ratios of the hydrochars by 13.1%–50.5% and 9.3–73.4% compared with feedstock, respectively. Py-GC/MS analyses showed that furan compounds decreasing or disappearing and phenolic compounds increasing significantly promotes the aromaticity of hydrochar during the hydrothermal carbonization, especially under severe conditions. The SEM images of hydrochars show that the formation of carbon spheres is the cause of changes in the surface morphology of hydrochar. The nitrogen recovery efficiency increased with the reaction severity, so that the liquid by-product from corn stovers by hydrothermal carbonization may be used as a fertilizer due to its abundant nutrients. This study provides an in-depth understanding of the corn stover-derived hydrochar, which is of great significance for the potential application of hydrochar.

Nonthermal air plasma dehydration of hydrochar improves its carbon sequestration potential and dissolved organic matter molecular characteristics

Labile organic compounds are associated with high environmental risk and are common on hydrochar surfaces. However, a comprehensive re-evaluation of hydrochar properties after the removal of labile compounds has long been overlooked. This study confirms that air-based nonthermal plasma can successfully modify hydrochar properties and change hydrochar's environmental benefits. NMR and FTIR results indicate that, aliphatic and alkyl structures are more reactive, while aromatic structures are highly resistant to the hydrochar modification process, leading to increased carbon sequestration potential and decreased dissolved organic matter (DOM). Van Krevelen diagram results indicate that dehydration controls the hydrochar modification process and leads to a decrease in oxygen content and O/C atomic ratio in the hydrochar; this weakens the ability of the hydrochar to immobilize hydrophobic organic pollutants (such as triclosan) due to the decrease in O‑alkyl C species within the hydrochar. Most importantly, air-based nonthermal plasma changes the structures of hydrochar associated DOM, and high molecular weight (>351 Da), and high degree of unsaturation and oxidation in the modified-hydrochar DOM compounds is observed. This study is therefore considered to have important implications for the carbon cycle and sustainable application of hydrochar.

Hydrochars from bamboo sawdust through acid assisted and two-stage hydrothermal carbonization for removal of two organics from aqueous solution

A series of bamboo hydrochars were prepared through acid-assisted and two-stage hydrothermal carbonization, characterized and evaluated for the adsorption of Congo red and 2-naphthol in aqueous solutions. The hydrochars have rough surfaces with BET surface areas of 6.77–57.74 m2/g and oxygen-rich functional groups. The additives in feed water revealed critical influences on the physical-chemical and adsorption properties of the hydrochars. The hydrochars can adsorb the two organics effectively, the highest adsorption capacities for Congo red and 2-naphthol are 90.51 and 72.93 mg/g, respectively, at 0.1 mg/mL and 298 K. The adsorption of the two organics on the selected hydrochars is a spontaneous and mainly physical adsorption process. 95% and 92% of the adsorption equilibrium could be accomplished in one hour for Congo red and 2-naphthol on the selected hydrochars, respectively. This study provides references for the production and application of hydrochars as efficient adsorbents in wastewater treatment.

Application of hydrochar and pyrochar to manure is not effective for mitigation of ammonia emissions from cattle slurry and poultry manure

Nitrogen (N) loss as ammonia (NH3) from agricultural systems is one of the major sources of atmospheric pollutants and is responsible for more than 50% of global NH3 emissions. Ammonia volatilization from animal manures may be altered by amendment with chars derived from pyrolysis (pyrochars) or hydrothermal carbonization (hydrochars) by providing exchange sites for ammonium (NH4+) or changing the pH of manure. Pyrochar and hydrochar differ in chemical and structural composition, specific surface area, and pH and therefore may affect NH3 volatilization differently. In a laboratory incubation experiment, we investigated the effect of pyrochar (pH 9.0) and hydrochar (pH 3.8) from Miscanthus on NH3 emission after addition to poultry manure and cattle slurry. We analyzed manure treatments with and without char addition and acidification and determined the effect of char addition on immobilization of manure-derived NH4+. Ammonia emission from pure poultry manure amounted 84% of the applied NH4+-N, while 67% of the applied NH4+-N was lost as NH3 from cattle slurry. Addition of pyrochar or hydrochar had no or only marginal effects on NH3 emissions except for a reduction in NH3 emissions by 19% due to hydrochar application to CS (p < 0.05), which seems to be primarily related to the char pH. Sorption of NH4+ by admixture of chars to manure was generally small: between 0.1- and 0.5-mg NH4+-N g−1 chars were sorbed. This corresponds to between 0.1 and 3.5% of the NH4+ applied, which obviously was not strong enough to reduce emissions of NH3. Overall, our results do not provide evidence that addition of pyrochar or hydrochar to cattle slurry and poultry manure is an effective measure to reduce NH3 volatilization.

Hydrothermal Carbonization of Municipal Woody and Herbaceous Prunings: Hydrochar Valorisation as Soil Amendment and Growth Medium for Horticulture

In this study, we investigate the suitability of hydrochar, produced at industrial scale by hydrothermal carbonization of municipal woody and herbaceous prunings, to be used as soil amendment and peat substitute in organic growth medium for horticulture. Fresh hydrochar and the products of two different hydrochar post-treatments (i.e., washing and aging) were compared in terms of potential phytotoxicity throughout physicochemical characterization and germination tests, performed with a sensitive species (Lactuca sativa). The results showed that the fresh hydrochar obtained from municipal green wastes complies with the Italian regulated parameters for the use as soil amendment. Moreover, hydrochar exhibits biological activity and a high content in organic C, Ca, and other micronutrients (Mg, Zn, Cu, Na, Cl). On the other hand, post-treatments are needed before application of hydrochar as peat substitute in potting mix, since appreciable phytotoxic effects on lettuce seed germination and radicle length of plantlets were observed (e.g., germination percentage of 56% and 54%, with 5 and 10 wt % of hydrochar in the blend, respectively). The inhibition of germination could be mainly attributed to the presence of polyphenols (tannins) and volatile fatty acids, which were most effectively removed through the aging post-treatment.

Preparation and Characterization of Activated Carbon from Hydrochar by Phosphoric Acid Activation and its Adsorption Performance in Prehydrolysis Liquor

Hydrochar was used to produce activated carbon with high BET surface area and large pore volume via phosphoric acid activation. The hydrochar described here can be obtained from hydrothermal carbonization of corn cob residue (CCR). Porous structure of activated carbons was characterized by nitrogen adsorption and scanning electron microscopy (SEM). Results showed that the specific surface area and total pore volume of activated carbon were increased to 2192 m2/g and 1.269 cm3/g, respectively, under conditions of 400 °C, 1 h, and an impregnation ratio of 3, from 5.69 m2/g and 0.136 cm3/g of the starting material. The chemical properties of hydrochar and activated carbons were further characterized by Fourier transform infrared spectroscopy (FT-IR), which confirmed the chemical transformation. Furthermore, the localized graphitic nature of the porous carbon was shown by the X-ray diffraction pattern. Thus, the adsorption capacity was enhanced for activated carbon in comparison with commercial carbon. The process of activated carbon preparation provided a high value-added application of hydrochar.

Hydrochars as peat substitute in growing media for organically grown potted herbs

Due to extensive media coverage of the “Terra Preta” soils found in the Amazon basin and the role of charcoal for their high fertility, carbonization of biomass is of increasing interest. In this context “biochar” made by pyrolysis and made by hydrothermal carbonization (HTC) have to be distinguished. The main advantage of the HTC process is the broader range of suitable raw materials because in addition to dry, also wet or water rich organic material can be used. Up to now research has focused on hydrochars as soil improvers at relative low application rates. In contrast, the aim of the current research was the use of hydrochars from woody landscaping material as well as from a mixture of woody landscaping material and grass as peat substitutes in growing media at 20-100% by volume. At 30% by volume, hydrochar significantly reduced growth of potted basil. At a higher percentage, growth after germination almost ceased. Subsequent experiments with chinese cabbage, lettuce and barley as well as an incubation test revealed that a strong immobilization of nitrogen is the main reason for reduced growth, but additionally both hydrochars seem to contain some phytotoxic substances. During co-composting of hydrochars with mature green waste compost nitrogen immobilization is reduced and phytotoxic substances are degraded within a few weeks. The results are in accordance with findings for the use of hydrochars as soil improvers, where often a loss of growth was caused directly after application of hydrochars, but some time later yield was increased. Risks and opportunities of hydrochars as peat substitutes in growing media are discussed.

A direct synthesis of adsorbable hydrochar by hydrothermal conversion of lignin

ABSTRACTHydrothermal conversion of lignosulfonate with or without formaldehyde as polymerization agent was conducted at 320°C. Compared with the hydrochar produced without formaldehyde addition (WF-hydrochar), the yield and surface area of the hydrochar produced with formaldehyde addition (F-hydrochar) increased 33.8 and 46.4%, respectively. The F-hydrochar showed much higher adsorption ability of Cu2+ than that of WF-hydrochar, while its adsorption ability of methyl orange was a little lower than that of WF-hydrochar. These discoveries explored a high yield of hydrochar production and a potential new application of hydrochar.

Hydrothermal carbonization of biomass from landscape management – Influence of process parameters on soil properties of hydrochars

Besides pyrolysis the technology of hydrothermal carbonization (HTC) is tested to produce hydrochars for soil improvement. The chemical and physical properties of the hydrochars mainly depend on the feedstock and the process parameters reaction time and process temperature. Systematic investigations on the influences of these process parameters on soil properties of hydrochars like water holding capacity (WHC) and cation exchange capacity (CEC) are missing.In this study, a rush-rich biomass was carbonized within defined HTC process conditions under variation of reaction time and process temperature to produce hydrochars. Analysis of WHC, CEC, the elemental composition and Fourier-transform infrared spectroscopy (FT-IR) were performed to evaluate the influence of HTC process conditions on the pedological hydrochar properties. The results indicated that at increasing reaction severity (reaction time and process temperature) WHC and CEC decreased as well as the elemental O/C ratio. The decrease of WHC and CEC is based on the decrease of the hydrochar surface polarity. However, even the lowest WHC and CEC of investigated hydrochars still exceeded those of pure quartz sand by factors of 5–10. An application of hydrochars produced at severe HTC conditions could improve WHC and CEC of sandy soils. This has to be investigated in further studies.

Effects of fresh and aged chars from pyrolysis and hydrothermal carbonization on nutrient sorption in agricultural soils

Abstract. Leaching of nutrients from agricultural soils causes major environmental problems that may be reduced with amendments of chars derived from pyrolysis (pyrochars) or hydrothermal carbonization (hydrochars). Chars are characterized by a high adsorption capacity – i.e. they may retain nutrients such as nitrate and ammonium. However, the physicochemical properties of the chars and hence their sorption capacity likely depend on feedstock and the production process. We investigated the nutrient retention capacity of pyrochars and hydrochars from three different feedstocks (digestates, Miscanthus, woodchips) mixed into different soil substrates (sandy loam and silty loam). Moreover, we investigated the influence of char degradation on its nutrient retention capacity using a 7-month in situ field incubation of pyrochar and hydrochar mixed into soils at three different field sites. Pyrochars showed the highest ability to retain nitrate, ammonium and phosphate, with pyrochar from woodchips being particularly efficient in nitrate adsorption. Ammonium adsorption of pyrochars was controlled by the soil type of the soil–char mixture. We found some ammonium retention on sandy soils, but no pyrochar effect or even ammonium leaching from the loamy soil. The phosphate retention capacity of pyrochars strongly depended on the pyrochar feedstock with large phosphate leaching from digestate-derived pyrochar and some adsorption capacity from woodchip-derived pyrochar. Application of hydrochars to agricultural soils caused small, and often not significant, effects on nutrient retention. In contrast, some hydrochars did increase the leaching of nutrients compared to the non-amended control soil. We found a surprisingly rapid loss of the chars' adsorption capacity after field application of the chars. For all sites and for hydrochar and pyrochar, the adsorption capacity was reduced by 60–80 % to less or no nitrate and ammonium adsorption. Thus, our results cast doubt on the efficiency of char applications to temperate zone soils to minimize nutrient losses via leaching.