Iron-impregnated Novel Biochar for Reducing the Solubility of Nitrogen, Phosphorus and Heavy Metals in Broiler Litter-manured Soils: Characterization, Effectiveness and Mechanism

Abstract

Non-point-source pollution due to loss of phosphate (PO 4 3- -P), nitrate (NO 3 - -N), ammonium (NH 4 + -N), zinc (Zn) and copper (Cu) from agricultural-lands receiving broiler-litter (BL) has been a serious concern. Although biochar application for reducing such pollution is considered ecofriendly, its efficacy is limited by low-functionality. An attempt was made to engineer biochar for immobilizing these anionic (PO 4 3- -P, NO 3 - -N) and cationic (NH 4 + -N, Zn 2+ , Cu 2+ ) non-point-source pollutants in BL-manured soils. Engineered-biochar (FeBC) was synthesized by oxidizing pine-wood derived biochar with HNO 3 -H 2 SO 4 followed by iron (FeCl 3 .6H 2 O) impregnation and characterized using microscopic (SEM) and spectral (XRD and FTIR) techniques. The efficacy of FeBC was compared with that of water-washed-biochar (WBC) and inorganic-amendments (alum and ferrous-sulfate) in reducing the solubility of these pollutants in the BL-mixed Marlboro and Decatur soils. For each soil-type, BL was first mixed with the soil (5% w/w) and the mixtures were further amended with ameliorants (FeBC, WBC, alum, ferrous-sulfate) to formulate eight treatment-combinations. The treatments were incubated at constant moisture and temperature for 0, 7, 21, and 42 days. After each incubation, soils were analysed for total-dissolved P (TDP), dissolved-reactive P (DRP), soluble NH 4 + -N, NO 3 - -N, Zn, and Cu content. Results indicated that iron-impregnation led to increased-functionality of biochar and magnetite (Fe 3 O 4 ) coating on its surface. Among the ameliorants, engineered-biochar at 20% of applied-BL (FeBC2) could successfully immobilize all the studied anionic and cationic pollutants in BL-manured soils. Amending BL-mixed soils with FeBC2 resulted reduced-solubility of TDP (10.1-23.2%), DRP (8.82-18.7%), NH 4 + -N (14.8-18.4%), NO 3 - -N (3.14-12.2%), Cu (14.7-16.1%), and Zn (⁓20.9%) with respect to BL-mixed soils. Although inorganic amendments (alum and ferrous-sulfate) were highly effective in reducing the solubility of TDP (up to 79.1%), DRP (up to 90.0%), Zn (up to 71.5%) and Cu (up to 71.4%) in BL-manured soils; however, these amendments could not significantly reduce the solubility of NO 3 - -N and NH 4 + -N. The effectiveness of the WBC was the least. It could be inferred that the efficacy of FeBC was associated with its structural-attributes. Besides, Fe-induced immobilization of dissolved-organic-carbon (DOC) played important role in reducing Zn and Cu solubility in the BL-mixed soils amended with FeBC2. Engineered-biochar could be an effective amendment in immobilizing both the anionic and cationic pollutants in BL-treated soils.

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