The effect of different moisture levels on the enrichment of cow manure compost with iron and zinc metal scraps

Metal scrap is one of the most important materials in urban and especially industrial waste, which constitutes 70% of the total industrial waste. Metal scrap trade has been the main source of heavy metal production and pollution in the environment. In addition, metal scrap has properties of toxicity, pathogenicity, reliability in the environment and even in the body of living beings and corrosiveness. Therefore, by reusing these metals, environmental pollution will be prevented, and energy consumption and greenhouse gas emissions will be reduced. Also, metal scrap recycling can prevent the extraction of ore and raw materials and preserve non-renewable mineral resources. On the other hand, cow manure compost, as a material that is a product of cow manure composting, has a very variable composition, the range of its elements depends on several factors such as livestock feeding, type of keeping and processing of cow manure, type of composting. Organic fertilizers have high amounts of macronutrients such as nitrogen and potassium, but they are relatively poor in terms of micronutrients. Accordingly, mixing organic fertilizers with metal scraps of iron and zinc can lead to an increase in the concentration of these elements in the fertilizer by forming complexes and chelates in the fertilizer. Adding metal scraps of iron and zinc to organic fertilizers causes iron to be chelated by the organic parts of the fertilizer, such as fulvic acid and humic acid, which have low molecular weight. This process occurs as a result of the effect of organic materials in creating reductive conditions, which depends on factors such as acidity, oxidation and reduction potential, the amount and type of organic compounds, and the type of iron and zinc scraps. Furthermore, as a result of increasing moisture and creating saturated conditions, the amount of oxygen in the fertilizer decreases and is consumed by aerobic microorganisms to the extent that it reduces the redox potential and creates reductive conditions. In this situation, anaerobic microorganisms use oxidized substrates as electron acceptors and change the capacity of elements such as trivalent iron and tetravalent manganese to divalent iron and manganese by consuming protons. Organic fertilizers, having high organic matter, cause a further decrease of the redox potential and facilitate the aggravation of the reduction conditions and the change of the element capacity. The purpose of this research is the effect of different moisture levels on the enrichment of cow manure compost by using metal scraps of iron and zinc.

To study this research, a split-plot-factorial experiment was conducted in the basic design of completely randomized blocks with 3 replications. The main factor of the design was moisture, which included 3 moisture levels of saturation, half saturation, and quarter saturation, and the secondary factors included iron scrap and zinc scrap, each of which had 3 levels with percentages of 0, 0.2, and 2. Also, the treatments of sub-factors in 9 levels were as follows:1- iron particles 0% + zinc particles 0% (control), 2- iron particles 0% + zinc particles 0.2%, 3- iron particles 0% + zinc particles 2%, 4- iron particles + 0.2% 0% zinc particles, 5-2% iron particles + 0% zinc particles, 6-0.2% iron particles + 0.2% zinc particles, 7-0.2% iron particles + 2% zinc particles, 8-particles 2% iron + 0.2% zinc particles and 9-2% iron particles + 2% zinc particles.The cow compost manure used in this research consisted of cow manure and wheat straw. To prepare the compost, the cow manure produced was stored and decomposed for three months, then the manure that was relatively decomposed was composted using the Windrow method. After that, the produced compost was passed through a 2 mm sieve to remove extra materials such as gravel and wood pieces. To prepare the treatments, first, 100 grams of dry matter of cow manure compost was saturated with distilled water, the volume of distilled water was equal to 240 ml, and its weight humidity was equal to 200%. Accordingly, 120 ml of distilled water was used for the half-saturated moisture level (humidity equal to 100%) and 60 ml of distilled water was used for the quarter-saturated moisture level (humidity equal to 50%). Then iron and zinc scraps that had a metallic form were added to 100 grams of compost dry matter that was moistened in plastic containers. In the following, the samples were incubated for 60 days at a temperature of 25 ± 3 and and during this period, the weight moisture of the treatments was measured every 5 days and the treatments were moisturized to maintain the initial moisture. At the end of the incubation, the treatments were air-dried and their available iron and zinc concentrations were measured.

The results show that the amount of available iron in saturated moisture was higher than the other two moistures, which is probably due to the reduction of trivalent iron to divalent iron and the reduction of redox potential and, consequently, the increase in the solubility of iron metal compounds in fertilizer. On the other hand, no significant difference between available iron in the treatments was observed in half-saturated and quarter-saturated moistures, which can be attributed to the lack of reduction conditions and no change in iron capacity and its available amount. The results also indicated that with the use of iron scrap, the amount of available iron increases significantly, which increased with the use of more scrap i.e. 2% iron and the application of saturated moisture. Thus, in the treatment of 2% iron particles + 0% zinc particles, the highest amount of available iron was observed with the amount of 756.2 mg/kg, which increased by 335% compared to the control treatment at the same moisture. The addition of zinc scrap, either as a separate application or with simultaneous application with iron scrap, caused a decrease in the available concentration of iron. This result is probably due to the high content of zinc compared to iron and the competition of elements in the DTPA extract. From the obtained data, it can be concluded that the saturated moisture had an effect on the available concentration of zinc only in 2% zinc scrap treatments, and other treatments were not affected by the increase in moisture. The use of zinc scrap separately increased the available amount of zinc, qua in the saturation treatment of 0% iron particles + 2% zinc particles, it reached 4538.7 mg/kg. Probably due to the large supply of zinc scrap, the predominant form of which is zinc oxide, which has high solubility, and as a result, the interaction between the conditions of reduction and chelation of zinc element occurred. It was also found that the use of iron scrap increased the amount of available zinc either as a separate application or in interaction with zinc scrap. This is the case in the treatments of 0.2% iron particles + 2% zinc particles (5388.9 mg/kg) and 2% iron particles + 2% zinc particles (the highest amount of available zinc in fertilizer with 6363.2 mg/kg).

The results of this research indicate the positive effect of adding metal scraps of iron and zinc in saturated moisture to cow manure compost. The application of iron and zinc scraps separately caused a significant increase in the available amount of iron and zinc in the fertilizer. However, their simultaneous application caused a decrease in available iron and a further increase in available zinc in fertilizer. Due to the fact that metal scraps of iron and zinc have low solubility and on the other hand, organic fertilizers with high amounts of organic matters cause the formation of complexes and chelation, as well as increase the solubility of metal scraps, and as a result, it causes a significant increase in the amount of available iron and zinc, which This increases significantly in high moisture and reduction conditions. Therefore, the use of metal scraps of iron and zinc in saturated moisture has a significant effect on the enrichment of organic fertilizers with these elements and can be a good solution for the reuse of metal scraps.