## 1 Introduction  
Water is a vital resource for living organisms and the whole ecosystem and its pollution has detrimental health, environmental and economic effects. Among the various organic and inorganic pollutants, such as metalloids, tants and synthetic compounds, heavy metals have become a major environmental problem worldwide, due to their toxicity, persistence and bio-accumulation in aquatic, human and animal organisms [1, 2]. Strontium, manganese lead and arsenic are discharged from various industries such as ceramic and firecracker, coal mining, petroleum refining, metal plating, glass and battery manufacturing, production of dyes, fertilizers, pesticides, as well as agricultural processing and municipal solid waste management [3, 4, 5], whereas manganese is a micronutrient for plants, animals and humans at trace levels [6, 7].Their presence in surface and groundwaters can inhibit the growth of aqueous organisms and their final uptake by humans even at very low concentrations 0.01-0.02 mg/L [2, 4, 6] can cause serious diseases including cancer, encephalopathy, hypertension, liver and kidney damage, immunological, neurological and mental abnormalities, etc. Therefore, effective methods for the removal of these heavy metals from water bodies are necessary for ecosystem protection. A wide range of methods have been applied to eliminate Sr, Mn, Pb and As heavy metals from contaminated waters, such as solvent extraction, reverse osmosis, chemical oxidation and coagulation, precipitation, membrane separation, ion-exchange and adsorption [3, 6, 8, 9]. Adsorption has become highly favored in recent years due to its high performance, simple operation, minimal consumption of chemicals, high recovery and low cost [2, 3, 10]. Various types of materials have been used for the sorption of heavy metals, including principally active carbon, but also carbonaceous nanofibers, graphene oxide, clays, zeolites, synthetic polymers and various composites [2; 9; 11]. However, the high cost and complexity of preparation and regeneration of these materials restrict their application in large scales. A promising green material, biochar, which has been widely used for the improvement of soil fertility, reduction of greenhouse gas emissions and carbon sequestration, has been proven successful in the immobilization of heavy metals. Produced from the pyrolysis of a variety of agricultural, forest, animal and other organic wastes, it could act as a sustainable low-cost adsorbent [10; 12]. The high adsorption capacity of biochar is attributed to its unique properties, such as large surface area and porosity, cation exchange capacity and negatively charged functional groups on its surface [4; 7; 8; 9; 12; 13; 14]. A wide variety of waste biomass materials have been used as feedstocks for biochar production. Biochars from agricultural and forest residues, such as sawdust, rice husk, date palm, corn cob, sugarcane bagasse and coconut shell [7; 12; 15; 16; 17], have been found to immobilize Sr, Mn, Pb and As with maximum adsorption capacities 0.6-0.74 mg/g for Sr, 2.4-3.6 mg/g for Mn, 3.6-43.4 mg/g for Pb and 0.8-7.1 mg/g for As, respectively. Biochars derived from animal waste, such as chicken manure and swine sludge, have been reported [3; 8; 18] to sorb significant amounts of Sr, Mn, Pb and As, ranging between 2.8 mg/g and 75 mg/g. In order to improve the adsorption capability of biochars for heavy metals, limited by their physicochemical properties and/or the pyrolysis conditions applied, numerous modification techniques were developed, involving further physical or chemical treatment [3; 4; 14; 17; 18]. Heavy metals do not exist as a single species in the environment and often coexist in wastewaters. Their adsorption depends not only on biochar characteristics, certain conditions during the process and the nature of each metal, but also on their competitive behavior for solid sorption sites. Their mobility is known to be influenced by mutual and other environmental components interactions.