This paper deals with marine plastic debris and its collection and recycling methods as one possible answer to the rising amount of plastic in marine environments. A novel approach is to use energy recovery, for example pyrolysis of marine plastic debris into high-energy products. Compared to other thermal processes, pyrolysis requires less technical effort and the end products can be stored or directly reused. In order to design such an onboard pyrolysis reactor, it is necessary to know more facts about the feedstock, especially the thermochemical behaviour and kinetic parameters. Therefore, a thermogravimetric analysis was carried out for three selected plastic sizes with a temperature range of 34–1000 °C. The results obtained from TGA showed the same curve shape for all samples: single stage degradation in the temperature region of 700–780 K with most of the total weight loss (95%). Small microplastics had an average activation energy of 320–325 kJ/mol.

Increasing the retention of nutrients by agricultural soils is of great interest to minimize losses of nutrients by leaching and/or surface runoff. Soil amendments play a role in nutrient retention by increasing the surface area and/or other chemical processes. Biochar (BC) is high carbon-containing by-product of pyrolysis of carbon-rich feedstocks to produce bioenergy. Biosolid is a by-product of wastewater treatment plant. Use of these by-products as amendments to agricultural soils is beneficial to improve soil properties, soil quality, and nutrient retention and enhance carbon sequestration. In this study, the adsorption of NH₄-N, P, and K by a sandy soil (Quincy fine sand (QFS)) and a silty clay loam soil (Warden silty loam (WSL)) with BC (0, 22.4, and 44.8 mg ha⁻¹) and biosolid (0 and 22.4 mg ha⁻¹) amendments were investigated. Adsorption of NH₄-N by the QFS soil increased with BC application at lower NH₄-N concentrations in equilibrium solution. For the WSL soil, NH₄-N adsorption peaked at 22.4 mg ha⁻¹ BC rate. Biosolid application increased NH₄-N adsorption by the WSL soil while decreased that in the QFS soil. Adsorption of P was greater by the WSL soil as compared to that by the QFS soil. Biosolid amendment significantly increased P adsorption capacity in both soils, while BC amendment had no significant effects. BC and biosolid amendments decreased K adsorption capacity by the WSL soil but had no effects on that by the QFS soil. Ca release with increasing addition of K was greater by the WSL soil as compared to that by the QFS soil. In both the soils, Ca release was not influenced by BC amendment while it increased with addition of biosolid. The fit of adsorption data for NH₄-N, P, and K across all treatments and in two soils was better with the Freundlich model than that with the Langmuir model. The nutrients retained by BC or biosolid amended soils are easily released, therefore are readily available for the root uptake in cropped soils.

Sludge from municipal wastewater treatment plants and organic fines from mechanical sorting of municipal solid waste (MSW) are two common widespread waste streams that are becoming increasingly difficult to utilise. Changing perceptions of risk in food production has limited the appeal of sludge use on agricultural land, and outlets via landfilling are diminishing rapidly. These factors have led to interest in thermal conversion technologies whose aim is to recover energy and nutrients from waste while reducing health and environmental risks associated with material re-use. Pyrolysis yields three output products: solid char, liquid oils and gas. Their relative distribution depends on process parameters which can be somewhat optimised depending on the end use of product. The potential of pyrolysis for the conversion of wastewater sludge (SS) and organic fines of MSW (OF) to a combustion gas and a carbon-rich char has been investigated. Pyrolysis of SS and OF was done using a laboratory fixed-bed reactor. Herein, the physical characterisation of the reactor is described, and results on pyrolysis yields are presented. Feedstock and chars have been characterised using standard laboratory methods, and the composition of pyrolysis gases was analysed using micro gas chromatography. Product distribution (char/liquid/gas) from the pyrolysis of sewage sludge and composted MSW fines at 700°C for 10 min were 45/26/29 and 53/14/33%, respectively. The combustible fractions of pyrolysis gases range from 36 to 54% for SS feedstock and 62 to 72% from OF. The corresponding lower heating value range of sampled gases were 11.8–19.1 and 18.2–21.0 MJ m⁻³, respectively.

A systematic study of the effect of nitrogen levels in the cultivation medium of Chlorella vulgaris microalgae grown in photobioreactor (PBR) on biomass productivity, biochemical and elemental composition, fatty acid profile, heating value (HHV), and composition of the algae-derived fast pyrolysis (bio-oil) is presented in this work. A relatively high biomass productivity and cell concentration (1.5 g of dry biomass per liter of cultivation medium and 120 × 10⁶ cells/ml, respectively) were achieved after 30 h of cultivation under N-rich medium. On the other hand, the highest lipid content (ca. 36 wt.% on dry biomass) was obtained under N-depletion cultivation conditions. The medium and low N levels favored also the increased concentration of the saturated and mono-unsaturated C16:0 and C18:1(n-9) fatty acids (FA) in the lipid/oil fraction, thus providing a raw lipid feedstock that can be more efficiently converted to high-quality biodiesel or green diesel (via hydrotreatment). In terms of overall lipid productivity, taking in consideration both the biomass concentration in the medium and the content of lipids on dry biomass, the most effective system was the N-rich one. The thermal (non-catalytic) pyrolysis of Chlorella vulgaris microalgae produced a highly complex bio-oil composition, including fatty acids, phenolics, ethers, ketones, etc., as well as aromatics, alkanes, and nitrogen compounds (pyrroles and amides), originating from the lipid, protein, and carbohydrate fractions of the microalgae. However, the catalytic fast pyrolysis using a highly acidic ZSM-5 zeolite, afforded a bio-oil enriched in mono-aromatics (BTX), reducing at the same time significantly oxygenated compounds such as phenolics, acids, ethers, and ketones. These effects were even more pronounced in the catalytic fast pyrolysis of Chlorella vulgaris residual biomass (after extraction of lipids), thus showing for the first time the potential of transforming this low value by-product towards high added value platform chemicals.

We quantified and investigated mechanisms for Cd²⁺ adsorption on biochars produced from plant residual and animal waste at various temperatures. Ten biochars were produced by pyrolysis of rice straw (RB) and swine manure (SB) at 300–700 °C and characterized. The Cd²⁺ adsorption isotherms, adsorption kinetics, and desorption characteristics were studied via a series of batch experiments, and Cd²⁺-loaded biochars were analyzed by SEM–EDS and XRD. The total Cd²⁺ adsorption capacity (Qc) increased with pyrolysis temperature for both biochars, however, rice straw-derived biochars had greater Qc than swine manure-derived biochars; hence, the biochar derived from rice straw at 700 °C (RB700) had the largest Qc, 64.4 mg g⁻¹, of all studied biochars. Cadmium adsorption mechanisms in this study involved precipitation with minerals (Qcₚ), cation exchange (Qcᵢ), complexation with surface functional groups (Qcₒ), and Cd-π interactions (Qcπ). Both the pyrolysis temperature and feedstock affected the quantitative contributions of the various adsorption mechanisms. The relative percent contributions to Qc for Cd²⁺ adsorption by RB and SB were 32.9–72.9% and 35.0–72.5% for Qcₚ, 21.7–50.9% and 20.4–43.3% for Qcᵢ, 2.2–14.8% and 1.4–18.8% for Qcₒ, and 1.4–3.1% and 3.0–5.8% for Qcπ, respectively. For biochars produced at higher pyrolysis temperatures, the contributions of Qcₚ and Qcπ to adsorption increased, while the contributions of Qcᵢ and Qcₒ decreased. Generally, Qcₚ dominated Cd²⁺ adsorption by high-temperature biochars (700 °C) (accounting for approximately 73% of Qc), and Qcᵢ was the most prominent mechanism for low-temperature biochars (400 °C) (accounting for 43.3–50.9% of Qc). Results suggested that biochar derived from rice straw is a promising adsorbent for the Cd²⁺ removal from wastewater and that the low-temperature biochars may outperform the high-temperature biochars for Cd²⁺ immobilization in acidic water or soils.

Biochar as a soil amendment has been reported to affect the content and availability of soil nutrients. In this study, we aimed to test whether the biochar addition to soils would change the availability and chemical fractionation of phosphate in soils. Two soils (Ultisol and Alfisol) were amended with five kinds of biochars at application rate of 0, 1, and 2% (w/w). After 3-month incubation, availability and chemical forms of P were measured to investigate the potential effect and role of biochar in improving P availability in soils. The biochars used here had a lager variation of P content, depending on their feedstocks. Compared to the untreated soils, application of biochars derived from deciduous tree leaves (DLB), reed (RB), and rice straw (RSB) significantly increased the pH of two soils. The total P content of biochar-amended soils was increased with the addition of biochars. However, only RSB exhibited a significant increase (p < 0.05) of total P content. Application of biochars significantly increased the NH₄Cl-extractable P content of two soils, indicating that biochars were able to increase the availability of phosphate in soils, but the amount of available P was dependent on biochar types. Ultisol and Alfisol amended with RSB (2% w/w) showed an increase in the P availability (0.5 M NaHCO₃-extractable P) by 46 and 39%, respectively. For strongly acidic Ultisol, addition of biochar significantly increased Al-P and Ca-P content, as well as decreased Fe-P content. The P desorption test indicated the release of P from soils increased with the addition of biochar. Results suggested that biochar would change the P sorption affinity of the soil and help to increase the availability of fixed P. The increase of P availability with biochar application was due to the pH change and direct P contribution from biochar. Our results concluded that biochar affected the availability, chemical forms, and sorption capability of phosphate in soil. The extent of biochar effects on soil P varied greatly with the type of feedstock of biochar and soil type.

The pore structure of biochar determines many biochar-induced environmental serves. In order to predict quantitatively, the environmental serves of biochar, it is very important to characterize the porosity and pore size distribution of biochar and to understand how biochar pore structure relates to the environmental serves. In this study, pore characteristics of biochars derived from different feedstocks were determined using nitrogen adsorption and the mercury intrusion porosimetry (MIP) methods. A great variation of pore characteristics in biochar was found, depending on feedstock material. The specific surface area (SSA) of biochars varied greatly, ranging from 1.06 to 70.22 m²/g. Total pore volume and porosity of biochars determined by the MIP method ranged from 1.28 to 3.68 cm³/g and from 57.8 to 79.7%, respectively. The pore size distribution of biochars had bimodal peaks in the range of 5–15 and 1.5–5 μm for the herbaceous plant and broad-leaf forest biochars, while coniferous forest biochar had two peaks at the pore sizes of 6–25 and 1.5–3 μm, respectively. Biochars had substantial storage pores (0.5–50 μm), accounting for about 85% of total pore volume, and small transmission and residual pores. The herbaceous plant biochars had larger volume of transmission pores (> 50 μm) than broad-leaf and coniferous forest biochar. Effects of pyrolysis conditions (temperature and residence time) on pore characteristics largely depended on feedstocks types. The difference in feedstocks would greatly affect pore characteristics of biochar, while the effect of pyrolysis conditions on biochar pore characteristics varied with biomass type. The detailed characterization of pore structure in biochars could effectively predict the potential impacts of biochars as soil amendment and pollutant sorbent.

In order to understand the influence of feedstock type on biochar adsorption of heavy metals, the adsorption characteristics of nickel (Ni²⁺), copper (Cu²⁺) and lead (Pb²⁺) onto biochars derived from mixed softwood and Miscanthus straw were compared. The biochars were produced from mixed softwood pellets (SWP) and Miscanthus straw pellets (MSP), at both 550 and 700 °C for each material, using a standardised production procedure recommended by the UK Biochar Research Centre. Kinetics analyses show that the adsorption of Ni²⁺ to all four biochars reached equilibrium within 5 min. The degree of Ni²⁺ removal for all four biochars remained nearly constant within initial pH values of 3–8, because the equilibrium pH values within this range were similar due to the buffering effect of the biochars. A sharp increase of Ni²⁺ removal percentage for all biochars at initial solution pH 8–10 was observed as the equilibrium pH also increased. MSP derived biochars generally had higher maximum adsorption capacities (Qₘₐₓ) for the three tested metals as compared with those from SWP, which was likely due to their higher degree of carbonisation during production. This study shows that feedstock type is a primary factor affecting the adsorption capacities of the tested biochars for heavy metals.

Biochar is recognized as an effective material for recovering excess nutrients, including phosphorus (P), from aqueous solutions. Practically, that benefits the environment through reducing P losses from biochar-amended soils; however, how salinity influences P sorption by biochar is poorly understood and there has been no direct comparison on P sorption capacity between biochars derived from different feedstock types under non-saline and saline conditions. In this study, biochars derived from wheat straw, hardwood, and willow wood were used to compare P sorption at three levels of electrical conductivity (EC) (0, 4, and 8 dS m⁻¹) to represent a wide range of salinity conditions. Phosphorus sorption by wheat straw and hardwood biochars increased as aqueous solution P concentration increased, with willow wood biochar exhibiting an opposite trend for P sorption. However, the pattern for P sorption became the same as the other biochars after the willow wood biochar was de-ashed with 1 M HCl and 0.05 M HF. Willow wood biochar had the highest P sorption (1.93 mg g⁻¹) followed by hardwood (1.20 mg g⁻¹) and wheat straw biochars (1.06 mg g⁻¹) in a 25 mg L⁻¹ P solution. Although the pH in the equilibrium solution was higher with willow wood biochar (~ 9.5) than with the other two biochars (~ 6.5), solution pH had no or minor effects on P sorption by willow wood biochar. The high sorption rate of P by willow wood biochar could be attributed to the higher concentrations of salt and other elements (i.e., Ca and Mg) in the biochar in comparison to that in wheat straw and hardwood biochars; the EC values were 2.27, 0.53, and 0.27 dS m⁻¹ for willow wood, wheat straw, and hardwood biochars, respectively. A portion of P desorbed from the willow wood biochar; and that desorption increased with the decreasing P concentration in the aqueous solution. Salinity in the aqueous solution influenced P sorption by hardwood and willow wood but not by wheat straw biochar. We conclude that the P sorption capacity of the studied biochars is dependent on the concentration of the soluble element in the biochar, which is dependent on the biochar type, as well as the salinity level in the aqueous solution.

Previous studies suggest that biochar has potential to benefit soil when used as an amendment, but only few studies have investigated how the different biochars affect the microbial activity of soil in a calcareous soil. Hence, to study the effect of the biochars obtained from wheat straw and cow manure and produced under different production conditions on two biological soil indicators, dehydrogenase activity and soil respiration, after 0, 60, and 120 days of incubation (DOI), an incubation experiment as a completely randomized design with factorial arrangement in three replicates was conducted in a calcareous soil. The results of the study showed that with increasing the pyrolysis temperature (300 and 500 °C) and pyrolysis residence times (1, 3, and 6 h) of biochars, regardless of feedstock source, the dehydrogenase activity and soil respiration decreased. Both maximum activity of dehydrogenase (20.93 μg TPF g⁻¹ 24 h⁻¹) and maximum soil respiration (0.26 mg CO₂ g⁻¹ 24 h⁻¹) were found in the biochar produced from wheat straw at 300 °C, and the residence time of 1 h at the level of 10 t ha⁻¹ and minimum of these soil biological traits was observed in control treatments (soil). Moreover, the maximum activity of dehydrogenase and soil respiration was observed in 60 DOI. Therefore, when applying biochar as an amendment for increasing microbial activity in calcareous soil, the production conditions of biochar, type of biochar, and long- and short-term effects of different biochars should be taken into consideration.