Simultaneous removal of arsenic and toxic metals from contaminated soil: Laboratory development and pilot scale demonstration

Waste seaweed compost and rhizosphere bacteria Pseudomonas koreensis promote tomato seedlings growth by benefiting properties, enzyme activities and rhizosphere bacterial community in coastal saline soil of Yellow River Delta, China

This study investigated the effects of waste seaweed compost and rhizosphere bacteria Pseudomonas koreensis HCH2-3 on the tomato seedlings growth in coastal saline soils and chemical properties, enzyme activities, microbial communities of rhizosphere soil. Microcosmic experiment showed that the seaweed compost and rhizosphere bacteria (SC + HCH2-3) significantly alleviated the negative effects of salinity on the growth of tomato seedlings. SC + HCH2-3 amendment significantly increased the plant height and root fresh biomass of tomato seedling by 105.59% and 55.60% in the coastal saline soils, respectively. The soil properties and enzyme activities were also dramatically increased, indicating that the nutrient status of coastal saline soil was improved by SC + HCH2-3 amendment. In addition, Proteobacteria, Actinobacteriota and Firmicutes were the dominant phyla in the rhizosphere soil after adding seaweed compost and rhizosphere bacteria P. koreensis HCH2-3. The relative abundances of Massilia, Azospira, Pseudomonas and Bacillus increased in treatment SC + HCH2-3. Especially, the beneficial bacteria genera, such as Pseudomonas, Bacillus and Azospira, were significantly correlated with the increases of contents of total nitrogen, nitrate nitrogen and ammonium nitrogen in tomato rhizosphere soil samples. Consequently, adding waste seaweed compost and rhizosphere bacteria P. koreensis HCH2-3 into coastal saline soil was suggested as an effective method to relieve salt stress of tomato plants.

Concurrent sorption of antimony and lead by iron phosphate and its possible application for multi-oxyanion contaminated soil

Concurrent stabilization of oxyanions such as antimony (Sb), arsenic (As), and heavy metals including lead (Pb) and manganese (Mn) in contaminated soils is difficult because of their diverse chemical properties. Antimony and As are stabilized by sorption with iron oxides while heavy metals are stabilized by phosphate. Hence, iron phosphate can be used to simultaneously stabilize Sb and Pb. Therefore, this study aimed to evaluate the possibility of simultaneous stabilization of Sb and Pb using iron phosphate. A single and a mixed solution of Sb and Pb were reacted with synthesized iron phosphate. Contaminated soil by Sb, As, Mo, Cr, and Mn was treated with iron phosphate, and bioavailable metal concentrations were evaluated by extracting the soil with 0.05 M ammonium sulfate. In a single solution, Sb(III) and Sb(V) sorption rate ranged up to 97% and 65%, respectively. In a mixed metal solution, Sb sorption increased compared to the single solution and Pb removal reached more than 95% in all cases. The sorption of Sb increased as the pH decreased, but the Sb(III) sorption was less affected by the pH than Sb(V). In various pH ranges, Sb(III) and Sb(V) sorption rates increased by 26 ~ 32% and 38 ~ 68%, respectively, compared to the single solution. Especially, Sb(V) sorption significantly increased in the presence of Pb at lower pH because of the lower solubility of iron phosphate. In soil, iron phosphate slightly decreased bioavailable As, Cr, Mo, Sb, and Mn concentrations. Therefore, metalloids and metals can be simultaneously stabilized by iron phosphate both in solution and soil.

Removal of Toxic Metals from Sewage Sludge by Acid Hydrolysis Coupled with EDTA Washing in a Closed-Loop Process

Contamination with toxic metals prevents the use of sewage sludge (SS) as a soil fertilizer. Hydrodynamic cavitation, thermal microwaving, microwave-assisted alkaline, and acid hydrolysis coupled with ethylenediaminetetraacetate (EDTA) washing were tested as a method to remove toxic metals from SS. Acid hydrolysis coupled with EDTA washing was most effective and was used in a closed-loop process based on ReSoil technology. EDTA and process solutions were recycled at a pH gradient of 12.5-2, which was imposed by the addition of quicklime (CaO) and H2SO4. An average of 78%-Pb, 76%-Zn, 1%-Cu, and 17%-Cr were removed from SS in five consecutive batches. No wastewater was generated, only solid waste (40%). The EDTA lost in the process (42%) was resupplied in each batch. In a series of batches, the process solutions retained metal removal efficiency and quality. The treatment removed 70% and 23% of P and N, respectively, from SS and increased the leachability of Zn, Cu, Mn, and Fe in the washed SS by 11.7, 6.8, 1.4, and 5.2 times, respectively. Acid hydrolysis coupled with EDTA washing proved to be a technically feasible, closed-loop process but needs further development to reduce reagent, material, and nutrient loss and to reduce toxic emissions from the washed sludge.

Removal of toxic metals from sewage sludge by EDTA in a closed-loop washing process

Sewage sludge (SS) is a potential resource for P and other nutrients, but often contaminated with metals. SS containing Pb-102, Zn-968, Cu-267, Cr-101, Mn-222, and Fe-8374 mg kg^-1 was washed in a batch process for 1 h with a recycled washing solution containing 50 mmol L^-1 of recycled EDTA and 50 mmol L^-1 H₂SO₄, solid/liquid ratio 1/7 (w/V). After solid/liquid separation, the washed SS was further rinsed 3-times with cleansed recycled solutions. EDTA and process solutions were recycled/cleansed in a pH gradient of 12.5-2.0 imposed by addition of CaO and H₂SO₄ (the ReSoil® method). EDTA, recycled as ineffective Ca-EDTA, was activated by capturing Ca with H₂SO₄. The process was closed-looped, no wastewater was generated, solid waste was centrifuged away. 10 consecutive SS washing batches preserved the quality of the process solutions. Metals were mainly removed from the SS organic fraction, the average removal was Pb-35, Zn-59, Cu-60, Cr-19, Mn-25, and Fe-1%. Washing reduced the leachability of Cu and Cr from SS by 13.4 and 3.5 times, but increased the leachability of Pb, Zn, Mn, and Fe by 2.5, 3.8, 1.9, and 1.6 times, respectively. Metal concentrations in the leachates were below the limits stipulated as hazardous. The content of accessible P and K in washed SS decreased by 24 and 45%, and the total N decreased by 10%. Overall, the results prove the feasibility of the novel SS washing process.

Simultaneous Removal of Heavy Metals (Cu, Cd, Cr, Ni, Zn and Pb) from Aqueous Solutions Using Thermally Treated Romanian Zeolitic Volcanic Tuff

Increased concentrations of heavy metals in the environment are of public health concern, their removal from waters receiving considerable interest. The aim of this paper was to study the simultaneous adsorption of heavy metals (Cu, Cd, Cr, Ni, Zn and Pb) from aqueous solutions using the zeolitic volcanic tuffs as adsorbents. The effect of thermal treatment temperature, particle size and initial metal concentrations on the metal ions sorption was investigated. The selectivity of used zeolite for the adsorption of studied heavy metals followed the order: Pb > Cr > Cu > Zn > Cd > Ni. The removal efficiency of the heavy metals was strongly influenced by the particle sizes, the samples with smaller particle size (0−0.05 mm) being more efficient in heavy metals removal than those with larger particle size (1−3 mm). Generally, no relevant changes were observed in heavy metals removal efficiency for the treatment temperatures of 200 °C and 350 °C. Moreover, at a higher temperature (550 °C), a decrease in the removal efficiencies was observed. The Cd, Zn, Cu, Cr, Zn and Ni sorption was best described by Langmuir model according to the high values of correlation coefficient. The pseudo-first-order kinetic model presented the best correlation of the experimental data.