Copper and chromium removal from industrial sludge by a biosurfactant-based washing agent and subsequent recovery by iron oxide nanoparticles.

Industrial wastewater treatment generates sludge with high concentrations of metals and coagulants, which can cause environmental problems. This study developed a sequential sludge washing and metal recovery process for industrial sludge containing > 4500 mg/kg Cu and > 5000 mg/kg Cr. The washing agent was formulated by mixing glycolipid, lipopeptide, and phospholipid biosurfactants from Weissella cibaria PN3 and Brevibacterium casei NK8 with a chelating agent, ethylenediaminetetraacetic acid (EDTA). These biosurfactants contained various functional groups for capturing metals. The optimized formulation by the central composite design had low surface tension and contained relatively small micelles. Comparable Cu and Cr removal efficiencies of 37.8% and 38.4%, respectively, were obtained after washing the sludge by shaking with a sonication process at a 1:4 solid-to-liquid ratio. The zeta potential analysis indicated the bonding of metal ions on the surface of biosurfactant micelles. When 100 g/L iron oxide nanoparticles were applied to the washing agent without pH adjustment, 83% Cu and 100% Cr were recovered. In addition, X-ray diffraction and X-ray absorption spectroscopy of the nanoparticles showed the oxidation of nanoparticles, the reduction of Cr(V) to the less toxic Cr(III), and the absorption of Cu. The recovered metals could be further recycled, which will be beneficial for the circular economy.

Denitrification of nitrate in regeneration waste brine using hybrid cation exchanger supported nanoscale zero-valent iron with/without palladium nanoparticles.

The Sustainable Development Goals require that reducing waste is a priority. This work described the application of an innovative zero-waste hybrid ion exchange nanotechnology that concurrently removed nitrate and induced denitrification to ammonia, with the ability to generate fertilizer for the agriculture sector from the recycled by-products. Herein, hybrid cation exchanger-supported zero-valent iron (Fe0), and bimetallic Fe0/Pd nanoparticles (HCIX-Fe0 and HCIX-Fe0/Pd) were synthesized and successfully validated for denitrification of nitrate in spent waste brine that contained nitrate. The kinetics of nitrate catalysis by both HCIX-Fe0 and HCIX-Fe0/Pd were compared and presented by six kinetic models, namely, zero-order, pseudo first- and second-order reaction, pseudo first- and second-order adsorption, and Elovich. HCIX-Fe0/Pd displayed a higher kinetic value than HCIX-Fe0, with k1 of 0.0019 and 0.0026 min-1, respectively. Nitrate was predominantly catalysed to NH4+ at a ratio of ammonia to other nitrogen compounds of around 80:20. Although HCIX-Fe0/Pd showed slightly better (14%) kinetic results, it was determined as unfavourable for real-life application due to low selectivity toward N2 gas and the need to use H2 gas. Based on practicability, the HCIX-Fe0 was further validated. The effect of salt (using NaCl) and the role of initial pH conditions were optimized and discussed. The recovery of nitrate removal was also calculated, and a recovery range of 91.42-99.14% was obtained for three consecutive runs. The sustainable, novel, zero waste hybrid ion exchange nanotechnology using the combination of two fixed-bed columns containing nitrate-selective resin for nitrate removal and novel HCIX-Fe0 for nitrate reduction to NH4+ may be a promising sustainable solution toward the goal of discharging zero nitrate waste to the environment.