## 1 Introduction  
Arsenic is a harmful metalloid, and long-term exposure to high arsenic can easily cause chronic poisoning or skin cancer [1]. In arid and semi-arid areas such as Inner Mongolia, Shanxi and Henan, there native high arsenic groundwater and soil arsenic pollution [2]. According to the 2014 National Soil Pollution Survey Communique, arsenic is one of the main soil pollutants, with an excess rate of 2.7% [3]. Arsenic mostly exists in soil in the form of oxygen-containing anions such as arsenate (AsO4\({}^{3-}\)) and arsenite (AsO3\({}^{3-}\)), showing different environmental behaviors. Arsenic generally remains and accumulates in soil in the form of arsenides such as iron (Fe-As), aluminum (Al-As), calcium (Ca-As), water-soluble (AsO4\({}^{3-}\) and AsO3\({}^{3-}\)), exchangeable and residue (O-As) [4]. Among them, water-soluble arsenide and exchangeable arsenide can be absorbed by crops, so they are generally regarded as soil effective arsenic indicators. While residue As is the most stable in soil and is not easily migrated and transformed to be absorbed by crops, it is generally considered to convert the effective arsenic in the soil into residual arsenic when remediating soil arsenic pollution, so as to reduce the risk of arsenic exposure in crops [5]. Therefore, it is of great significance to explore a passivation material that can greatly reduce the effective arsenic content in soil for soil arsenic pollution remediation. Compared with electric remediation, heat treatment and chemical leaching technology, soil passivation remediation in solidification/stabilization technology has the advantages of low economic cost, simple operation, significant passivation effect, easy recognition by farmers and suitability for large-scale promotion, so it is the most widely used [6]. The principle of soil passivation remediation is to reduce the effective arsenic content in the soil by adding a passivator to the soil, which is manifested by the water-soluble and exchangeable arsenic in the soil being fixed by the passivator into a residual state. There are many types of passivators, which can be divided into organic materials such as biochar, livestock and poultry manure, and inorganic materials such as clay minerals and iron-manganese compounds [7; 8; 9]. [PERSON] [10] used rice husks and sawdust agricultural wastes as raw materials to prepare biochars, and further modified with KMnO4 solution and applied them to arsenic-contaminated soil. Through column leaching experiments, they found that this type of modified biochar can reduce the mobility of As in soil, and also observed that the passivation effect was positively correlated with the addition ratio. [PERSON] [11] used two soil amendments, oyster shell waste and biochar, to carry out passivation tests in highly arsenic-contaminated soil, and both oyster shell waste and biochar effectively reduced the leaching capacity of arsenic in acidic soil, and reduced the exchangeable arsenic content from 105.8 mg/kg to 54.0 mg/kg. [PERSON] [12] applied ordinary peat charcoal and iron-modified carbon with a soil mass fraction of 20% to potted plants, and found that the content of water-soluble arsenic in soil decreased by 2.6-19.6% and 3.9-19.0%, respectively, indicating that iron modification had little effect on the passivation ability of biochar. MnO2 has strong oxidation and affinity for As, and it is easy to oxidize As(III) to As(V) with lower toxicity, and biochar then adsorbs As(V) [13]. [PERSON] [14] used permanganate and biochar as raw materials to prepare manganese-oxide-modified biochar composites via pyrolysis and studied their adsorption characteristics in red soil, and found that manganese oxides improved the hydrogen/carbon ratio (H/C), oxygen/carbon ratio (O/C), surface hydrophilicity and surface adsorption capacity of biochar, indicating that manganese oxides play a vital role in arsenic removal.