**1. Introduction**

Soils of arid and semiarid regions need the amendment of organic compounds due to being low in organic C content and as consequence unable to improve their physicochemical and biological properties and thus their yield of crops and their natural fertility [1, 2].

Organic amendments, such as farmyard manures, have been used as common organic manure for supplying nutrients to plants. After the second world war, there was a great interest in

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

using other organic wastes as organic fertilizer, farmyard manure has been used as a reference when sewage sludge or other organic materials have to be judged [3]. The sewage sludge and sludge compost amendment were ceased when it was found that soluble heavy metals in the soil and the total heavy metals in the crops were increasing when these wastes were applied [4]. However, sewage sludge and compost are the most widely used organic amendments, mainly by their high organic matter, N and P content, which are important nutrients for plant growth, their contribution to soil structure, and resistance to soils erosion [5–8]. On the other hand, application of sewage sludge as agricultural fertilizer is associated to numerous environmental and health problems such as those implicated by toxic metals, organic com‐ pounds, and possible other health problems related to pathogens [9–12].

The gradual increase in industrialization and urbanization in the last decades has created an enormous increase in volumes of wastes produced all over the world. Usually, these wastes are discharged to the environment [13], especially in countries which regulatory control is not strict.

The objective of the case study described in this chapter, together with methodology and literature review is to provide different scenarios of the role of organic waste in semi-arid soils poor in organic carbon, the effect of heavy metals on soil biogeochemical processes, their dispersion and mobility in soils, and the availability of heavy metals to plants.

#### **1.1. Semi-arid soils and tannery sludge**

Tannery sludge derives from a complex combination process where organic and inorganic materials become chemically bound to the protein of the hides and preserve it from deterio‐ ration. A significant number of operations within the tannery industry involve large amounts of water, chemicals, and energy leaving as waste large amounts of polluted water. These industrial effluents contain several types of chemicals such as dyes, levelling agents, acids, alkalis, phenols, carbonates, alcohols, cyanide, and heavy metals, among others [14].Byproducts generated during leather manufacturing are usually rich in proteic matter and organic substances, thus it is a potential resource that can be used as fertilizer in agriculture production. This leather processing waste is an attractive disposal for soil amendment as it has proven to improve the physical properties of soil and supply organic matter and plant nutrients [15–19].

The use of these waste in semi-arid soils as organic fertilizer of plants could be an alternative disposal method and simultaneously it will resolve the environmental risks presented when they are abandoned to open sky.

Under natural environmental conditions, chromium is present in either the trivalent Cr(III) or the hexavalent Cr(VI) [20, 21]. The effect of Cr on health have been widely studied [22]. Cr(VI) is about 300 times more toxic than Cr(III).Health effects of Cr have been reported in lung cancer, and birth defects [23, 24].Cr(III) have relatively low toxicity and are easily precipitated and immobilized; however, Cr(VI) is toxic, water soluble, and highly mobile, and can then be transported into the surrounding surface soil and ground water [25, 26].Tannery sludge contains both trivalent (CrIII) and hexavalent (CrVI) chromium. There is little information about the biogeochemical conditions affecting solubility of heavy metals in arid and semi-arid soils including soil pH, Eh, and dissolved organic carbon contents [27]. The information generated in this study is critically important for assessing the benefits or potential risks of using tannery waste to treat semi-arid soils for re-forestation.

### **1.2. Mine tailings amended with organic wastes**

Mine tailing disposal sites from either inactive or abandoned mines are common in arid and semiarid regions throughout the mine region around the world. These tailings have been stored outside and have contaminated local ecosystems and harmed the nearby populations [28]. Today, areas containing mine tailings can be found in urban and agricultural zones and mine tailing storage after the closure of mining operations is becoming increasingly problem‐ atic in the arid and semiarid region because of wind erosion. These areas are a source of air pollution giving off particles [29]. Short-term exposure to mine tailing particles can lead to illness, while long-term exposure may lead to premature death in adults and children [30, 31].
