Insight into dewatering behavior and heavy metals transformation during waste activated sludge treatment by thermally-activated sodium persulfate oxidation combined with a skeleton builder-wheat straw biochar

Coupling sludge-based biochar and electrolysis for conditioning and dewatering of sewage sludge: Effect of char properties

The addition of sludge-based biochar during electrochemical pretreatment of sewage sludge, as an efficient hybrid technology, is potentially to be applied in sludge deep-dewatering. The chars functioned as conductors, catalysts and skeleton particles could enhance the sludge dewaterability and increase the calorific value of the dewatered sludge cake. However, the effect of synthesis conditions on the char properties and further on the dewatering performance is still unknown. Herein, the sludge-based particle electrodes (SPEs) under three main synthesis conditions, including liquid-solid ratio, pyrolysis temperature and time, were prepared. The sludge-based biochars (i.e., SPE-400, SPE-600, and SPE-800 pyrolyzed under 400, 600 and 800 °C, respectively) were characterized and utilized as three-dimensional electrodes during sludge electrolysis. The increased pyrolysis temperature (within 400-800 °C) resulted in the enrichment of metallic ions and increment of specific surface area and pore volume of SPE, which led to the increased catalysis and adsorption sites for viscous proteins (PNs). Particularly, the pores of SPE-800 provided more drainage channels as skeleton builders. Compared with raw sludge, the capillary suction time (CST) and the specific resistance of filtration (SRF) of the treated sludge with 3D-SPE-800 were reduced by 58.12% and 81.01%, respectively, but the net sludge solids yield (Y_N) was increased by 87.05%. The highest decrease of hydrophilic α-Helix content in PNs (from 9.93% to 7.30%) was observed when using SPE-800 as particle electrode, revealing the crucial role of char characteristics on protein reduction and subsequent dewatering enhancement. The synergistic effects of electrolysis and sludge-based biochar provided a new insight for a closed-loop pretreatment of sewage sludge in the wastewater treatment plant.

Autoclave and pulsed ultrasound cavitation based thermal activation of persulfate for regeneration of hydrogen titanate nanotubes as recyclable dye adsorbent

In the dye removal application, regeneration of hydrogen titanate nanotubes (HTN, H₂Ti₃O₇) has been achieved via thermal activation of persulfate anion (PS, S₂O₈^2-) by using the conventional hot plate technique which has limitations from the commercial perspective since it does not provide any precise control over the thermal generation process typically during the scale-up operation. To overcome this drawback, HTN have been synthesized via hydrothermal process which exhibit the methylene blue (MB) adsorption of 93% at the initial dye concentration and solution pH of 90 µM and 10 respectively. HTN have been regenerated via the thermal activation of PS by varying its initial concentration and regeneration temperature, within the range of 0.27-1 wt% and 40-80 °C, under the thermal conditions set by the autoclave and pulsed ultrasound (US) cavitation process. The results of recycling experiments suggest that the optimum values of initial PS concentration and temperature, for the regeneration of HTN under the autoclave conditions, are 1 wt% and 70 °C with the maximum MB adsorption of 92%, while, the corresponding values for the pulsed US cavitation process are 1 wt%, 80 °C, and 91% respectively. Thus, the regeneration and recycling of HTN have been successfully demonstrated by using the autoclave and pulsed US cavitation process. Under the optimum conditions, MB degradation involves the generation and attack of SO₄^•- for both the thermal generation techniques. The regeneration techniques developed here may be utilized in future during the scale-up operation and also for the regeneration of adsorbents besides HTN.

Carbon nanotube-based materials for persulfate activation to degrade organic contaminants: Properties, mechanisms and modification insights

Removal of harmful organic matters from environment has great environmental significance. Carbon nanotube (CNT) materials and their composites have been demonstrated to possess excellent catalytic activity towards persulfate (PS) activation for the degradation of organic contaminants. Herein, detailed information concerning the function, modification methods and relevant mechanisms of CNT in persulfate-based advanced oxidation processes (PS-AOPs) for organic pollutant elimination has been reviewed. The activation mechanism of PS by CNT might include radical and nonradical pathways and their synergistic effects. The common strategies to improve the stability and catalytic capability of CNT-based materials have also been put forward. Furthermore, their practical application potential compared with other catalysts has been described. Finally, the challenges faced by CNT in practical application are clearly highlighted. This review should be of value in promoting the research of PS activation by CNT-based materials for degradation of organic pollutants and the corresponding practical applications.

Applications of Biochar and Modified Biochar in Heavy Metal Contaminated Soil: A Descriptive Review

Given that the problem of contaminated soil continues to grow, the development of effective control and remediation measures has become imperative, especially for heavy-metal-contaminated soil. Biochar and modified biochar are eco-friendly and cost-effective remediation materials that are widely used in the remediation of contaminated soil. This review provides an overview of the different raw materials used in the preparation of biochar as well as the modification of biochar using various synthesis methods, highlighting their differences and providing recommendations for biochar and modified biochar as applied toward ameliorating pollution in soil contaminated by heavy metals. We also explore the effects of the physicochemical properties of raw materials, pyrolysis temperature, additives, and modification methods on the properties of the resulting biochar and modified biochar, and systematically present the types of soil and operating factors for repair. Moreover, the mechanisms involved in remediation of heavy-metal-contaminated soil by biochar and modified biochar are outlined in detail, and include adsorption, complexation, precipitation, ion exchange, and electrostatic attractions. Finally, the corresponding monitoring technologies after remediation are illustrated. Future directions for studies on biochar and modified biochar in the remediation of contaminated soil are also proposed to support the development of green environmental protection materials, simple preparation methods, and effective follow-up monitoring techniques.

Singlet oxygen-dominated activation of peroxymonosulfate by passion fruit shell derived biochar for catalytic degradation of tetracycline through a non-radical oxidation pathway

Waste-derived biochar has been emerged as promising catalysts to activate peroxymonosulfate (PMS) for the degradation of organic contaminants. Herein, passion fruit shell derived biochar (PFSC) was prepared by a one-pot pyrolysis method and used as a metal-free catalyst to activate PMS for the degradation of tetracycline hydrochloride (TC). The batch experiments indicated that the pyrolysis temperature could influence the efficiency of PFSC for the activation of PMS. In the PFSC-900 (prepared at 900 °C)/PMS system, the degradation rate of TC can reach 90.91%. The quenching test and electron paramagnetic resonance spectra revealed that the high catalytic performance of PFSC-900/PMS system was mainly attributed to the non-free radical reaction pathway containing a carbon bridge, and the TC degradation was controlled primarily by singlet oxygen-mediated oxidation. Moreover, the carboxyl group of ketones and the graphite-N atoms on PFSC-900 are the possible active sites of the non-free radical pathway including direct electron transfer or the formation of O₂^•-/¹O₂. This study not only shows a new type of biochar as an efficient catalyst for PMS activation but also provides a way of value-added reuse of passion fruit shell.

Effects of different biochars on physicochemical properties and immobilization of potentially toxic elements in soil - A geostatistical approach

The impact of different biochars (BCs) on the physicochemical properties and immobilization of potentially toxic elements (PTEs) in contaminated soil irrigated with industrial wastewater for the last three decades was studied. Furthermore, the efficacy of applied BCs in reducing geostatistical risks was also evaluated. For this purpose, BCs were prepared from green waste (Cynodon dactylon L.) for the first time at different pyrolysis temperature (400 °C, 600 °C and 800 °C), and amended the contaminated soil in pots with two different ratios of 2% and 5% (w/w) under controlled conditions. The BCs amended soil samples were analyzed after five months (equivalent to the life span of a wheat crop). The physicochemical impacts of applied BCs on the soil showed that the acidic soil was changed to basic. A tremendous increase in water holding capacity, cation exchange capacity, dissolved organic carbon, carbon, phosphorus and potassium contents was observed. The PTEs concentrations and geostatistical risks were significantly (p ≤ 0.05) decreased by all the BCs. Among them, BC prepared at 800 °C and applied at a ratio of 5% was showed the best effects by reducing the bioavailable concentrations of Cd, Pb, Cr, Ni, Cu, Mn, Fe, As, Co and Zn in 88%, 87%, 78%, 76%, 69%, 65%, 64%, 63%, 46% and 21%, respectively. Similarly, significant (p ≤ 0.05) reductions in geoaccumulation index, enrichment factor, contamination factor, and ecological risk were recorded. Therefore, BC prepared at 800 °C and applied at a ratio of 5% is recommended for soil remediation.

Insight into sludge dewatering by advanced oxidation using persulfate as oxidant and Fe2+ as activator: Performance, mechanism and extracellular polymers and heavy metals behaviors

This study established a Fe²⁺/persulfate oxidation system to dewater sludge in WWTPs. Dewatering performance, persulfate consumption and the variations of sludge pH, TN and TP during dewatering process were monitored. EPS and ζ-potential behaviors for ameliorating sludge dewatering was investigated. Transformation, leaching toxicity and environmental risk of heavy metals in sludge during dewatering were determined. Results demonstrated that after treated by Fe²⁺/persulfate oxidation system with 0.6 mmol/g-VS of persulfate at Fe²⁺/persulfate molar ratio 0.6, WC decreased to 53.5% and SCST increased to 4.15, which implied an excellent improvement of sludge dewatering. The fast persulfate consumption, the decrease of sludge pH and the increase of TN illustrated the positive effects of Fe²⁺ in activating persulfate and the decomposition of EPS by the activation products, SO₄^•- and ^•OH. Another product (Fe³⁺) generated during persulfate activation could decrease the content of phosphorus-containing matter (released from EPS decomposition) through the precipitation reaction with PO₄^3-. The decrease of TOC and UV-254 happened in HPO-A, HPO-N and TPI-A organic substance of EPS (mainly contained in TB-EPS fraction) indicated that the destruction of hydrophobic organic matter of EPS would stimulate the release of bound water, which was beneficial to dewater sludge. The largest protein loss in TB-EPS (from 24.5 to 10.7 mg/L) indicated that the effective decomposition of TB-EPS could significantly ameliorate sludge dewatering. The increase of ζ-potential indicated the degradation of organic matter in EPS with negative charge. To sum up, the destruction of protein-like substances in hydrophobic organic matter of TB-EPS was the main mechanism for improving sludge dewatering by Fe²⁺/persulfate oxidation system. 3D-EEM fluorescence spectroscopy analysis proved that these protein-like substances were mainly tryptophan protein and humic acid. Moreover, due to the disruption of EPS, the contents of heavy metals in sludge, and their leaching toxicity and environmental risk were reduced. Therefore, Fe²⁺/persulfate oxidation system has potential and application prospects to improve sludge dewatering and optimize sludge management in WWTPs.

Application of biochar from crop straw in asphalt modification

The objective of this study is to verify the feasibility of using biochar made from crop straw as a bitumen additive to improve some properties of bitumen. The differences between crop straw biochar prepared in a laboratory and commercial charcoal were investigated through scanning electron microscopy and laser particle size analyses. Furthermore, biochar-modified asphalt was prepared using the high-speed shear method, and the penetration, softening point, ductility at 15°C, and apparent viscosity of the asphalt binder with 6% biochar were measured at 120, 135, 150, 160, and 175°C. It was found that both the crop straw biochar and the commercial charcoal consist mainly of C, O, Si, and K, but the C content of crop straw biochar is slightly higher than that of commercial charcoal. The particle size of biochar is smaller than that of commercial charcoal, while the specific surface area is larger. It was determined that the addition of crop straw biochar significantly improved the high-temperature performance of asphalt, and that biochar and commercial charcoal have a similar influence on the high temperature performance of asphalt.

Insight into the improvement of dewatering performance of waste activated sludge and the corresponding mechanism by biochar-activated persulfate oxidation

A novel activator, corn biochar, was produced to activate persulfate to dewater waste activated sludge (WAS). Results demonstrated that the biochar-activated persulfate oxidation can effectively improve the dewatering performance of WAS. After treating WAS by biochar-activated persulfate oxidation (biochar dosage: 2.1 g/L, persulfate concentration: 7.5 mM) at the original WAS pH, standardized-capillary suction time (SCST) increased to 4.21 times and moisture content (MC) decreased to 43.4%, indicating an excellent performance of WAS dewatering. The decrease of residual persulfate with the increasing biochar dosage during WAS dewatering process illustrated that the role of persulfate in improving WAS dewatering was because of the biochar activation. The behaviors of extracellular polymers (EPS) proved that the protein in tightly bound EPS (TB-EPS) linked to WAS dewatering, and its content significantly reduced to 10.5 mg/g-volatile solids (VS) after WAS treatment. Three-dimensional fluorescence spectroscopy for EPS once again proved that the disintegration of tryptophan protein and humic acid (hydrophobic organic substances in EPS) was responsible for the improvement of WAS dewatering. To sum up, the biochar-activated persulfate oxidation was a feasible application in improving WAS dewatering.

The experimental optimization and comprehensive environmental risk assessment of heavy metals during the enhancement of sewage sludge dewaterability with ethanol and Fe(Ⅲ)-rice husk

The optimal concentrations of ethanol, Fe³⁺ and rice husk (RH) to enhance sludge dewaterability were determined by response surface methodology (RSM). Results showed the optimal concentrations of ethanol, Fe³⁺ and RH were 22.2 g/g DS, 239.9 mg/g DS and 348.9 mg/g DS, respectively, and the CST reduction efficiency reached 72.3%. The transformation behavior and mechanism of the heavy metals (HMs) during conditioning process were determined in terms of total HMs content, leaching tests, and fraction distribution. The environmental risk of HMs was quantitatively evaluated after conditioning in terms of bioavailability and ecotoxicity, potential ecological risks, and pollution levels. Results showed that the high ecological risk of HMs in raw sludge cake is primarily dominated by Cd and the use of Fe³⁺ alone negatively affected the immobilization of HMs and reduction of leaching toxicity. However, after repeated conditioning with Fe³⁺ and ethanol, the total HMs content reduction values in sludge cake were 75%, 93%, 100%, 91%, and 74% for Pb, Cr, Cd, Zn, and Cu, respectively. The potential ecological risk index (PERI) and geoaccumulation indicated low or no overall environmental risk after repeated conditioning. Particularly, the risk of Cd was reduced from high risk to low risk after repeated conditioning according to the PERI. Ethanol/Fe³⁺-RH can effectively reduce HMs risk from the sludge cake in the dewatering tests.

Role of extracellular polymeric substances in sludge dewatering under modified corn-core powder and sludge-based biochar pretreatments

Extracellular polymeric substances (EPS) which wrapped on sludge particles were deemed to hinder the outflowing of combined water in sludge system. The complex composition of EPS was the bottleneck for revealing its relationship with sludge dewaterability. In this study, a combined modified corn-core powder (MCCP) and sludge-based biochar (SBB) condition was executed to treat sludge for enhancing dehydration performance, and the concentration and the form distribution of organics in EPS, the variances of protein secondary structures were investigated. Correlation between the sludge dewaterability and EPS components were performed, found strong correlations among the net sludge solids yield (YN) and the specific resistance of filtration (SRF) (R = -0.923), Zeta potential (R = -0.971). Furthermore, the relationship between the secondary structures of protein and dehydration performance were strong related. With the optimal dosage of SBB and MCCP, aggregated strands and α -helix were released, indicated that the unfolding and despiralization in soluble EPS (S-EPS) were improved, disordered the sludge network, reduced the flowing resistance of bound water, finally enhancing sludge dewaterability.