| China is the largest producer of steel in the world with its annual crude steel output of near700Mt inrecent years. Slags are the byproducts of steel and iron industries, which account for15%-20%of globalsteel and iron production. However, the utilization of slag was only10%in China, which is far belowthat in developed countries. Disposing of these untreated slags not only requires plenty of land, but alsocauses many environmental problems. Slag contains large amounts of SiO2and CaO with smallamounts of micronutrients, it has been well documented that application of slag-based silicon (Si)fertilizer can not only improve the fertility of soils, but also enhance the growth and yield of crops andthe resistance to plant diseases. So, slag applied as Si fertilizer on paddy soil has benefits not only forrice health and growth, but also for economic and environmental issues. However, most slags containtrace amounts of heavy metals; this raises questions about the potential risks of heavy metals in slagleaching into environment. The objective of this paper was to realize steel or iron slag recycling bymeans of field application as Si-fertilizer with high efficiency and good safety. The research workmainly includes these aspects:(1) Choosing two different Si fertilizers through pot experiments toassess the effects of steel slag and iron slag applied at the same level of plant-available Si on rice growthand brown spot disease resistance and also to investigate the relationship between Si-mediatedultrastructural changes and brown spot disease infection in rice.(2) Collecting different types of slagsamples from ten steel and iron corporations in China examine phyto-available Si concentrations andthe total main chemical components, including total heavy metal concentrations of different slagmaterials. Furthermore, we aimed through BCR-and TCLP-extraction procedures to estimate thepotential mobility and leaching of metals.(3) Choosing three different steel slags to study Si-releasingcharacters from slags and their influencing factors through incubation experiments at both35oC and25oC.(1) In the pot experiment, we chose both steel and iron slags to investigate their effects on ricegrowth and disease resistance under greenhouse conditions. Scanning electron microscopy (SEM) andtransmission electron microscopy (TEM) were used to examine the effects of slags on ultrastructuralchanges in leaves of rice naturally infected with brown spot. Both Si fertilizers tested significantlyincreased rice growth and yield, and decreased brown spot incidence, with steel slag fertilizer showing astronger effect than iron slag fertilizer. Under the current experimental conditions, Si application at arate of400mg SiO2kg-1soil (available SiO2extracted by0.5M HCl) met the Si requirements of rice.SEM analysis showed that silicon application led to more pronounced cell silicification in rice leaves,more silica cells, and more pronounced and larger papilla as well. TEM analysis results showed thatmesophyll cells of silicon-untreated rice leaf were disorganized, with colonization of the fungus(Bipolaris oryzae), including chloroplast degradation and cell wall alterations. Silicon applicationpreserved mesophyll cells relatively intact and significantly increased the thickness of silicon layer. Itcan be concluded that applying Si fertilizer to Si-deficient paddy soil is necessary for both high riceyield and brown spot disease control. The immobile silicon deposited in host cell walls and papillae sites is the first physical barrier for fungal penetration and soluble Si in the cytoplasm enhancesphysiological or induced resistance to fungal colonization.(2) Steel and iron slags were collected from ten steel and iron corporations in China. The siliconphyto-availability and the main chemical components were tested. The results show that thephyto-available Si concentrations (extracted by0.5M HCl) of different slags varied widely, due todifferences in the total Si content and cooling process during slag formation. Water-cooling slagscontained more available Si than air-cooling steel slags. Slags could be divided into three classes. Thefirst class, with an available SiO2concentration greater than15%, including S3, S4, S6, S7, S8and S9represents high-grade materials for Si fertilizer. The second class, with an available SiO2concentrationbetween10%and15%, including S1, S11, S12, and S13could be considered as materials for Sifertilizer. The third class, with an available SiO2concentration is below10%(including S2, S5, S10andS12) has a low value for Si fertilizer.(3) We chose three different steel slags, including S1(in powder form, water-cooling), S2(ingranular form, water-cooling), S3(in granular form, air-cooling) with identically available Si content, tobe incubated in both soil suspension and distilled water media at35℃and25℃. Leaching solutionswere collected through centrifugation at regular intervals in order to study Si-releasing characters and itsimpacting factors. The results demonstrated that in soil suspension-incubation experiment, the rate of Sireleased from slags on the first day was mainly influenced by slag cooling process, but was impactedprincipally by temperature and grain size of slags as the experiment continued. The Si-releasingpercentage (the ratio of accumulation of Si releasing to available Si content) of S1, S2and S3were37.3%,30.3%and27.3%, respectively, after97-d-soil-incubation at35℃, compared to14.3%,7.86%and10.2%under25℃. However, in distilled water-incubation experiment, the amount of Si releasedfrom slags on the first day was mainly influenced by temperature, while afterwards it was affectedprincipally by grain size of slags and temperature but not by slag cooling process. The Si-releasingpercentage of S1, S2and S3was0.22%,0.16%and0.16%, respectively, after97-d-water-incubation at35℃, compared to0.17%,0.13%and0.14%at25℃.(4) Total chromium (Cr) concentrations in some slags far exceeded the Chinese Standard and theToxicity Characteristic Leaching Procedure (TCLP)-extractable concentrations of Cr in leachingsolutions of some slags exceeded the US EPA limit. The principal component analysis (PCA)demonstrated that the TCLP-extractable concentrations of Cr were not consistent with the total Crconcentrations in slags, but depended strongly on the European Community Bureau of Reference(BCR)-extractable chemical forms. No slags posed a significant risk of cadmium (Cd), lead (Pb),mercury (Hg) or arsenic (As) to agricultural ecosystems in which the slags are applied. |
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