## 1 Introduction
There are limited studies in the literature on the competitive sorption of heavy metals by biochars, most of them focusing on binary or tertiary systems. Investigated metals included Cd, Pb, Zn, Cr, Cu and As [1; 4; 6; 10; 12; 6]. A mesoporous biochar, prepared from Medulla Tetrapariacis, exhibited maximum adsorption capacity for Cu\({}^{2+}\) and Pb\({}^{2+}\) of 430.9 mg/g and 701.6 mg/g, respectively, in the presence of competing ions. Removal efficiencies of these ions were over 80% at a metal ion concentration of 200 mg/L. Sorption mechanisms were studied. The biochar was effective in the treatment of industrial wastewater [10]. Douglas fir biochar, produced by fast pyrolysis at 1000 \({}^{\circ}\)C and activated by KOH to increase its surface area, was studied for the adsorption of aqueous Cr\({}^{6+}\), Pb\({}^{2+}\) and Cd\({}^{2+}\) through batch tests. Sorption of all three heavy metals followed pseudo-second order kinetics and the Langmuir adsorption model. Sorption of these ions in the presence of various competitive ions was demonstrated [4]. In another work, three different types of biochars (rice husk, wood chip and wood mixture) were modified by acidic, alkaline, oxidic, manganese oxide and iron oxide impregnation. The properties of the modified biochars were analyzed and the adsorption characteristics of heavy metals Cd, Pb and Zn were evaluated under single and competitive conditions. The biochar which was modified by manganese oxide was more suitable, adsorbed an amount of heavy metals greater than 9.1 mg/g even under mixed conditions and the distribution coefficient followed the order Pb \(>\) Cd \(>\) Zn [12]. The adsorption capacity of cabbage waste was assessed as a biosorbent in systems with Cu\({}^{2+}\), Pb\({}^{2+}\), Zn\({}^{2+}\), or Cd\({}^{2+}\) ions. The principal functional groups in this waste material were carboxylic, hydroxyl and amine groups. The presence of competing ions suppressed the sorption of the target metal ions. Pb\({}^{2+}\) was adsorbed in larger amounts. In general, there was a linear inverse dependency between the sorption capacities and a number of different types of competitive metal ions [6]. A dairy manure-derived biochar presented removal capacities of above 486 mmol/kg for Pb, Cu, Zn and Cd metals. A rice husk biochar showed stronger competition than dairy manure biochar when the metals coexisted. When each metal was 1 mM in the multi-metal system, the removal efficiency of dairy manure biochar was reduced by 2-40.9%, whereas that of rice husk biochar by 38.4-100%. Pb was the least affected metal and Cd the most. The different mechanisms of adsorption were investigated [13]. Another research study evaluated the adsorption of heavy metals in mono- and multi-metal forms onto sesame straw biochar. In the mono-metal adsorption isotherm, the maximum adsorption capacities (mg/g) of metals were in the order of Pb (102) > Cd (86) > Cr (65) > Cu (55) > Zn (34) and in the multi-metal adsorption isotherm Pb (88) > Cu (40) > Cr (21) > Zn (7) > Cd (5). During multi-metal adsorption, it was found that Cd was easily exchanged and substituted by other metals [1]. The simultaneous immobilization of Sr, Mn, Pb and As heavy metals has not been reported so far. Previous investigations by the authors have demonstrated [19] that these metals coexisted in effluents from agricultural processing industries, sewage sludge and leachates from municipal solid waste deposition landfills. Therefore, the aim and the innovation of this work were to develop a cost-effective biochar sorbent for simultaneously removing Sr, Mn, Pb and As metals from multi-metal solutions.