Decontamination of Chrome Leather Waste and Its Modification for Use as a Commercial Fertilizer

Abstract

Chrome tanning is the most popular form of tannage worldwide due to the superior quality and versatility of the resultant leather compared to all other leather tanning materials. However, waste emanating from the production of chrome-tanned leather causes enormous disposal problems to human health and the environment due to the toxicity of chromium particularly in its hexavalent oxidation state. Besides, efforts to treat this type of leather waste either for chromium recovery, or for obtaining chrome-free collagen, have not been entirely successful for a number of reasons as investigated in this thesis. Thus, the current study assessed the amount of chrome leather waste generated by six selected tanneries operating in Kenya and mode of disposal of the waste. The assessment of leather waste generated in the six selected tanneries was done through a survey in which semi-structured questionnaires and key informant interviews were used as the instruments of data collection. Descriptive statistics were used to analyse the data collected in this study, and the analysis showed that 1,443,000kg of chromium-containing leather waste was generated by the 6 selected tanneries within a period of one month. This analysis further showed that out of the total amount of various types of leather solid wastes generated within one month in the selected tanneries, which was 2,112,560kg, 68.3% of this amount was actually chrome-tanned leather waste. The study also, established that the common methods of disposal of leather solid wastes at that time were; landfilling, open ground dumping and or incineration. In addition, the study developed a new eco-friendly and cost-effective method for dechroming leather waste and modified the waste to enable its utilization as a slow-release organic fertilizer. The new method involved detanning, chromium extraction and complexation of the remaining traces of chromium with potassium oxalate. The method was able to extract up to 99.9% of chromium from the waste in 24 hours. The dechromed waste was subjected to mild phosphoric acid hydrolysis to enhance controlled breakdown of peptide bonds in collagen to facilitate release of nitrogen and other nutrients, making them available for plant use. The collagen hydrolysate was modified further to make it more useful as a slow-release nitrogen fertilizer by treating it with 99% epichlorohydrin (EPICH). The modified collagen had 44% N, 21% P (as P2O5), 0.1% K (as K2O), 0.2% Mg, and 1.8% Ca, 27.0% TOC and C:N ratio of 0.61. The slow-release organic fertilizer was formulated using modified collagen and ground maize cobs as filler material. The maize cob based filler material was found to have high carbon to nitrogen ratio (C:N) with the following nutrient composition: 2.8% N, 14.04% P (as P2O5), 0.3% K (as K2O), 7.5% Mg, and 1.6% Ca, 38.12% TOC and C:N ratio of 13.61. The high C:N ratio contributed to the slow release of organic nitrogen into the soil in form of nutrients by slowing down the decomposition of the organic fertilizer to enable the release of nutrients at a rate that matches plant requirements. Besides, the filler contributed to the nutrient content of the new fertilizer. This study also, assessed the N mineralization rate of the new fertilizer formulation in a laboratory soil incubation experiment to determine the rate at which inorganic nitrogen is obtained by decomposition of organic matter and degradation of organic nitrogenous compounds in the organic fertilizer, making it bioavailable for use by plants following fertilizer application to the soil. The highest mineralization rate of organic N into NO3-N was observed during the 12th week, and the one of organic N into NH4+-N was observed during the 16th week of incubation. Results of single application of the new fertilizer formulation during planting (under greenhouse fertilizer trials) were subjected to ANOVA using Genstat 14th edition at 95% confidence interval (CI), and indicated that growth and productivity (yield) of kale and capsicum were comparable with the use of DAP and CAN conventional fertilizers during planting and top-dressing time, respectively. Application of the new fertilizer in growth and productivity of maize did not show

favourable results. However, use of different rates of this new fertilizer showed a significant difference (P < 0.05) on mean height of the maize stalk and chlorophyll content. The new fertilizer formulation (if applied at optimal rates), as found out in this study, was significantly better than DAP and CAN conventional fertilizers, particularly in terms of increasing chlorophyll content and number of leaves per capsicum plant. However, mean fruit yield and whole plant biomass for the capsicum crop under various fertilizer treatments did not show any significant difference (P > 0.05). The optimal rate of application for the new fertilizer formulation was 1238.50kg/ha fertilizer blended with 530.79kg/ha filler in enhancing growth and productivity of capsicum, while the optimal application rate of the new fertilizer toe enhance growth and productivity of kale was either 884.64kg/ha fertilizer blended with 884.64kg/ha filler or 1769.28kg/ha fertilizer without filler. The study concluded that chrome leather waste can efficiently be dechromed using the new method described in this study and the resulting chrome free waste can be modified using 99% EPICH to make a useful organic fertilizer. Large-scale production of the new organic fertilizer will reduce disposal to the environment of chrome leather waste that usually causes environmental pollution. The study recommends up-scaling and optimization of the new leather wastes dechroming and collagen modification methods described in this study for large-scale decontamination of chrome leather waste and commercial production of the new slow-release organic fertilizer. In addition, the study recommends addition of potassium rich supplements to the organic fertilizer to boost its potassium levels, as well as test the suitability of the new fertilizer for use to grow various other types of crops.