Agent-assisted electrokinetic treatment of sewage sludge: Heavy metal removal effectiveness and nutrient content characteristics

Sustainable heavy metal removal from sewage sludge: A review of bioleaching and other emerging technologies

By 2050, global sewage sludge production is expected to increase by 51 %, rising from its current level of over 45 million tons of dry solids to nearly 68 million tons. This growth is primarily driven by population growth and the implementation of increasingly stringent environmental regulations. This increase in sewage sludge volume poses substantial challenges for sustainable management due to its complex composition. While sewage sludge contains valuable nutrients such as nitrogen (N), phosphorus (P), and potassium (K) that make it suitable for agriculture use, the presence of heavy metals (HMs), including cadmium (Cd), lead (Pb), mercury (Hg), chrome (Cr), copper (Cu), nickel (Ni) and zinc (Zn) creates significant barriers to its safe reuse. Inadequately treated sewage sludge, when repeatedly applied to agricultural soils, can lead to the accumulation of HMs, posing risks to long-term soil fertility, crop productivity, and broader environmental health. This review discusses various techniques for de-metallization of sewage sludge, including aerobic- and anaerobic bioleaching, chemical leaching, electrokinetic treatment, and supercritical fluid extraction. Among these techniques, anaerobic bioleaching is identified as the most environmentally sustainable option, as it offers a lower-energy, less chemically intensive approach to decrease HM content in the solid fraction of sewage sludge. This approach utilizes microbial activity under anaerobic conditions to solubilize and remove HMs, while minimizing nutrient loss and preserving the ecological integrity of the treated sewage sludge. Future research should prioritize the optimizing of anaerobic bioleaching processes to enhance both HM removal efficiency and nutrient retention. Additionally, integrating anaerobic bioleaching with air-assisted ultrasonication as a post treatment technology could further improve metal removal efficiency. This review aims to provide a comprehensive reference for researchers and practitioners seeking environmentally friendly solutions for HM removal from sewage sludge, ensuring its safe reuse in land applications and contributing to a circular agro-economy.

Chromium immobilization from wastewater via iron-modified hydrochar: Different iron fabricants and practicality assessment

The recycling of industrial solid by-products such as red mud (RM) has become an urgent priority, due to their large quantities and lack of reutilization methods can lead to resource wastage. In this work, RM was employed to fabricate green hydrochar (HC) to prepare zero-valent iron (ZVI) modified carbonous materials, and conventional iron salts (IS, FeCl3) was applied as comparison, fabricated HC labeled as RM/HC and IS/HC, respectively. The physicochemical properties of these HC were comprehensively characterized. Further, hexavalent chromium (Cr(VI)) removal performance was assessed (375.66 and 337.19 mg/g for RM/HC and IS/HC, respectively). The influence of dosage and initial pH were evaluated, while isotherms, kinetics, and thermodynamics analysis were also conducted, to mimic the surface interactions. The stability and recyclability of adsorbents also verified, while the practical feasibility was assessed by bok choy-planting experiment. This work revealed that RM can be used as a high value and green fabricant for HC the effective removal of chromium contaminants from the wastewater.

Free nitrous acid-assisted asymmetrical alternating current electrochemistry (FNA-AACE) for multi-heavy metals decontamination in waste activated sludge

Heavy metal contamination of waste activated sludge (WAS) is a key factor limiting the land application of sludge for nutrients recovery. This study proposes a novel free nitrous acid (FNA)-assisted asymmetrical alternating current electrochemistry (FNA-AACE) process to achieve high-efficiency decontamination of multi-heavy metals (Cd, Pb, and Fe) in WAS. The optimal operating conditions, the heavy metal removal performance of FNA-AACE, and the related mechanisms for maintaining the high performance were systematically investigated. During the FNA-AACE process, FNA treatment was optimal with an exposure time of 13 h at a pH of 2.9 and an FNA concentration of 0.6 mg/g TSS. Then the sludge was washed with EDTA in a recirculating leaching system under asymmetrical alternating current electrochemistry (AACE). The 6-h working and the following electrode cleaning were defined as a working circle of AACE. After three cycles of working-cleaning periods in AACE treatment, the cumulative removal efficiency of the toxic metals Cd and Pb reached over 97% and 93%, respectively, whilst that of Fe was greater than 65%. This surpasses most previously reported efficiencies and possesses a shorter treatment duration and sustainable EDTA circulation. The mechanism analysis suggested that FNA pretreatment provoked the migration of heavy metals for leaching enhancement, as well as reduced the demand for EDTA eluent concentration and increased conductivity, which can improve the AACE efficiency. Meanwhile, the AACE process absorbed the anionic chelates of heavy metals and reduced them to zero-valent particles on the electrode, regenerating the EDTA eluent and maintaining its high extraction efficiency for heavy metals. In addition, FNA-AACE could provide different electric field operation modes, allowing it to have flexibility for the real application processes. This proposed process is expected to be coupled with anaerobic digestion in wastewater treatment plants (WWTPs) for high efficiency of heavy metal decontamination, sludge reduction, and resource/energy recovery.

Facile preparation of a novel iminodisuccinate modified chitin and its excellent properties as a silver bioadsorbent and antibacterial agent

A novel iminodisuccinate modified chitin (ICH) was prepared using crab shells via a one-step facile procedure. The ICH with grafting degree of 1.46 and deacetylation degree of 47.68 % possessed maximum adsorption capacity of 2572.41 mg/g for silver ions (Ag(I)).The ICH also exhibited good selectivity and reusability. The adsorption followed better with the Freundlich isotherm model, while fitted well with both the Pseudo-first-order and Pseudo-second-order kinetics models. The characteristical results showed that the excellent Ag(I) adsorption capability of ICH should be attributed to both looser porous microstructure as well as additional functional groups-grafting molecular. Moreover, the Ag-loaded ICH (ICH-Ag) showed remarkable antibacterial properties against six typical pathogenic bacteria strains (Escherichia coli, Pseudomonas aeruginosa, Enterobacter aerogenes, Salmonella typhimurium, Staphylococcus aureus, and Listeria monocytogenes), with the corresponding 90 % minimal inhibitory concentrations ranged 0.426-0.685 mg/mL. Further study on the silver release, microcell morphology, and metagenomic analysis suggested that many Ag nanoparticles were formed after the Ag(I) adsorption, and the antibacterial mechanisms of the ICH-Ag involved both cell membranes destruction and intracellular metabolism disturbing. This research presented a coupling solution of crab shell wastes treatment with chitin-based bioadsorbents preparation, metal removal and recovery, as well as antibacterial agent production.