Pentachlorophenol dechlorination with zero valent iron: a Raman and GCMS study of the complex role of surficial iron oxides

An iron-biochar composite from co-pyrolysis of incinerated sewage sludge ash and peanut shell for arsenic removal: Role of silica

Modification of biochar by low-cost iron sources has gained increasing attention to improve pollutants removal performance and reduce production costs compared to conventional chemical modifications. While such iron sources generally have complex compositions, their effects on properties of the iron-biochar composite are not well investigated. This study produced an iron-biochar (RBC) composite from co-pyrolysis of incinerated sewage sludge ash (ISSA) and peanut shell, and examined the role of silica with widespread existence in ISSA and other low-cost iron sources on properties of the iron-biochar composite relevant to As(III)/As(V) removal. Silica was found to react with iron during the pyrolysis process at 850 °C and formed iron silicon at the expense of producing zero valent iron and Fe₃O₄ which resulted in a poorer removal efficacy for As(III) and As(V) compared to the iron-biochar (FBC) made from pure Fe₂O₃ and peanut shell. Moreover, a high leaching of reactive silica from RBC was observed which affected the formation of corrosion products of ZVI and competed with arsenic for active adsorption sites. Despite this, RBC still exhibited a maximum adsorption capacity of 17.44 and 57.56 mg/g towards As(III) and As(V) respectively at pH 3.0. Overall, this study provides an interesting insight into upcycling ISSA into useful media for sorptive removal of arsenic from aqueous solutions.

ZIF-8-modified Au-Ag/Si nanoporous pillar array for active capture and ultrasensitive SERS-based detection of pentachlorophenol

A novel SERS substrate based on a zeolitic imidazolate framework-8 (ZIF-8) film-modified Au-Ag/Si nanoporous pillar array (ZIF-8/Au-Ag/Si-NPA) was successfully fabricated for pentachlorophenol (PCP) detection. The Au-Ag/Si-NPA was synthesized through immersion plating and replacement reaction on the Si-NPA, which was prepared by the hydrothermal etching. The ZIF-8 film was coated via layer-by-layer growth technique. The ZIF-8 film is nanoporous and its thickness can be controlled by varying the growing number, which can significantly influence the SERS performance of the substrate. The substrate with optimal ZIF-8 thickness exhibited an excellent SERS response to PCP molecules. The SERS enhancement factor reached up to 1.8 × 10⁷ and the detection limit was down to 10^-13 M. Moreover, the substrate showed good uniformity with a relative standard deviation (RSD) of 8.7% and good selectivity. The PCP detection is hardly interfered by the coexisting organic compounds. The high SERS performance may be due to the enrichment effect of the ZIF-8 film. The ZIF-8 film could capture and enrich the trace PCP molecules by electrostatic interaction between the negatively charged PCP^- and the positively charged ZIF-8. This work suggests that the ZIF-8/Au-Ag/Si-NPA substrate has potential application in SERS analysis of the polar organic pollutant detection in environmental media.

Effect of O2, Ni0 coatings, and iron oxide phases on pentachlorophenol dechlorination by zero-valent iron

This study explores the zero-valent iron (ZVI) dechlorination of pentachlorophenol (PCP) and its dependence on the dissolved oxygen (O₂), presence/formation of iron oxides, and presence of nickel metal on the ZVI surface. Compared to the anoxic system, PCP dechlorination was slower in the presence of O₂, which is a potential competitive electron acceptor. Despite O₂ presence, Ni⁰ deposited on the ZVI surfaces catalyzed the hydrogenation reactions and enhanced the PCP dechlorination by Ni-coated ZVI bimetal (Nic/Fe). The presence of O₂ led to the formation of passivating oxides (maghemite, hematite, lepidocrocite, ferrihydrite) on the ZVI and Nic/Fe bimetallic surfaces. These passive oxides resulted in greater PCP incorporation (sorption, co-precipitation, and/or physical entrapment with the oxides) and decreased PCP dechlorination in the oxic systems compared to the anoxic systems. As received ZVI comprised of a wustite film, and in the presence of O₂, only ≈ 17% PCP dechlorination observed after 25 days of exposure with tetrachlorophenol being detected as the end product. Wustite remained as the predominant oxide on as received ZVI during the 25 days of reaction with PCP under oxic and anoxic conditions. ZVI acid-pretreatment resulted in the replacement of wustite with magnetite and enhanced PCP degradation (e.g. ≈ 52% of the initial PCP dechlorinated after 25 days under oxic condition) with accumulation of mixtures of tetra-, tri-, and dichlorophenols. When the acid-washed ZVI was rinsed in NiSO₄/H₂SO₄ solution, Ni⁰ deposited on the ZVI surface and all the wustite were replaced with magnetite. After 25 days of exposure to the Nic/Fe, ≈ 78% and 97% PCP dechlorination occurred under oxic and anoxic conditions, respectively, producing predominantly phenol. Wustite and magnetite are respectively electrically insulating and conducting oxides and influenced the dechlorination and H₂ production. In conclusion, this study clearly demonstrates that the dissolved oxygen present in the aqueous solution decreases the PCP dechlorination and increases the PCP incorporation when using ZVI and Nic/Fe bimetallic systems. The findings provide novel insights towards deciphering and optimizing the performance of complex ZVI and bimetallic systems for PCP dechlorination in the presence of O₂.